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The heat-dissipating capacity of a surface having micropillar wick structures, which resembles the evaporator section of a vapor chamber, is mainly limited by the liquid flow rate through the porous structure (permeability) and the capillary pressure gradient. The efficacy of a regular vapor chamber is determined from two parameters, namely, the dry-out heat flux and temperature of the evaporator surface. These two parameters possess a counter relation to each other. The work described herein introduces and evaluates the performance of a novel idea of electro-osmosis-aided thin-film evaporation from a micropillar array structure. This study is conducted using a discretized approach that is validated against the thin-film evaporation model and additionally the electro-osmotic flow model with pre-existing pressure gradient conditions. The unique feature of this approach is that it results in an increment in the magnitude of dry-out heat flux without significantly changing the surface temperature, wherein the increase in permeability is due to the addition of electro-osmotic flow. This comprehensive model considers various geometries, zeta potentials, and extremal electric fields and establishes the beneficial effects of the application of an external electric field. The results are used to predict the sensitivity and the dependence of the dry-out heat flux and the evaporator surface temperature on these parameters. For a host of electro-osmotic parameters considered herein, a maximum increment of up to 320% in the dry-out heat flux is observed for an external electric field of 10

This work demonstrates thermally programmable dynamic capillarity in exclusively engineered nanochannels functionalized by grafted smart elastomeric layers onto their inner surfaces. Tunable control of the capillarity is observed over the temperature window of 25-31 °C, deciphering the possibility of a sevenfold alteration in the rate of capillary flow. A simple theory explains the confluence of viscous and capillary interactions as mediated by the non-trivial interplay of the substrate wettability, confinement-induced surface layering of molecules, and thermally activated modulation of surface tension, to bring out this intriguing effect. The technology is demonstrated to be completely reconfigurable over the intended spatial and temporal regimes, via selective grafting of the channel surface and preferential choice of the activation temperature. Such favorable features as opposed to more complex yet non-reconfigurable flow manipulation strategies previously reported are likely to open up new possibilities of highly precise controlled nanofluidic manipulation of temperature-sensitive biological samples and chemical species on-demand, for applications ranging from biomedical technologies to energy harvesting and water purification.

We quantitatively investigate the role of voltage fluctuation in terms of different waveforms on the electrodeposition dynamics and morphology for varying electrolyte concentrations. Dependent on the electrolyte concentration, a wide range of morphologies ranging from highly branched dendrites to comparatively closed packed electrodeposits has been captured. We mechanistically map the deposition dynamics by image analysis and demonstrate the highly porous dendritic dynamics to be independent of external perturbation. Additionally, comparatively closed packed morphological features show significant sensitivity toward the frequency and nature of the waveforms. The results provide fundamental insights into the correlation between the time scales of voltage fluctuation and growth dynamics. We comprehensively analyze the effect of the waveform nature on the average deposition height and show sinusoidal fluctuation to be preferred over square and pulse for metal batteries for lower deposition heights.

It is well known that the wettability of a droplet on a solid substrate can be modified by the application of an electric field. The phenomenon of electrowetting along with the associated physics of droplet shape change and dynamics has traditionally been studied at the micro-scale leading to exciting applications. The present work is undertaken to explore the physics of electrowetting actuation of droplet movement at the molecular level. Molecular simulations are performed to obtain the dynamic spreading of the droplet under the action of a radially symmetric electric field on a silica substrate. The dynamic behavior of the contact diameter is found to be qualitatively similar to that observed at the laboratory scale. Further simulations of droplet actuation across an array of electrodes illustrated the dynamics of the center of mass, which is then used to estimate the contact line friction and compared with the predictions from a reduced-order model. A scaling analysis is used to probe the physics of the problem correlating the contact line friction coefficient and the droplet velocity after actuation. The results and understanding elicited from the fundamental approach have the potential to guide the development of quick and precise control of nano-sized droplets and may prove to be pivotal in the development of future nanofluidic systems, nanomanufacturing methodologies, and high-resolution optoelectronic devices.

Hypothesis Dried blood droplet morphology may potentially serve as an alternative biomarker for several patho-physiological conditions. The deviant properties of the red blood cells and the abnormal composition of diseased samples are hypothesized to manifest through unique cell-cell and cell-substrate interactions leading to different morphological patterns. Identifying distinctive morphological trait from a large sample size and proposing confirmatory explanations are necessary to establish the signatory pattern as a potential biomarker to differentiate healthy and diseased samples. Experiments Comprehensive experimental investigation was undertaken to identify the signatory dried blood droplet patterns. The corresponding image based analysis was in turn used to differentiate the blood samples with a specific haematological disorder “Thalassaemia” from healthy ones. Relevant theoretical analysis explored the role of cell-surface and cell-cell interactions pertinent to the formation of the distinct dried patterns. Findings The differences observed in the dried blood patterns, specifically the radial crack lengths, were found to eventuate from the differences in the overall interaction energies of the system. A first-generation theoretical analysis, with the mean field approximation, also confirmed similar outcome and justified the role of the different physico-chemical properties of red blood cells in diseased samples resulting in shorter radial cracks.

Diabetes, a chronic condition, is one of the prevalent afflictions of the 21st century, and if left unchecked, this ailment could lead to severe life-threatening complications. A widely accepted methodology for monitoring diabetes is the estimation of the glucose and ketone contents in the body-fluids, viz. blood, urine, etc. Additionally, certain conditions such as starvation, and following a protein rich diet (e.g., keto-diet) could also lead to significant changes in the ketone content, thereby resulting in false-positive diagnosis. Hence, a precise, portable, and on-demand procedure for the rapid and combined estimation of glucose and ketone in the bodily-fluids is of utmost importance. To that end, paper-based analytical devices (μPADs) are promising tools, owing to their multitudinous advantages, and compatibility with biofluids. Although, numerous researchers have contributed substantially in the fundamental investigation, design, and fabrication of μPADs for various applications, a combined platform capable of rapid, accurate and on-demand glucose and ketone detection, that is easy to fabricate, is still relatively unexplored. Moreover, the flow dynamics of an analyte, in combination with enzyme-catalysed (for glucose) and uncatalyzed reactions (for ketone), within a porous paper matrix is also vaguely understood. Herein, we present a facile laser-printing based fabrication of colorimetric sensors on a filter paper, for rapid, and non-invasive estimation of glucose and ketone contents in urine. The urine sample, upon being deposited in a particular expanse, is wicked through the paper matrix, and reacts with specific reagents in the designated zone(s), giving rise to a final color, concomitant with the glucose or ketone content in the sample. The device design enables the liquid to be wicked into the porous matrix in a way that would concentrate the colored product in a dedicated detection zone, thereby augmenting the feasibility for accurate colorimetric detection. Furthermore, we present for the first time, a detailed dynamic model of the flow-field in a variable cross-section paper device using the Richards’ equation, while also considering the species transport and reaction kinetics within the porous media. The results of the numerical simulation agree well with those observed experimentally, thereby validating the present model. Finally, we also developed a web and desktop-based application that would enable the user to upload the images of the colored zones to provide an accurate estimate of the glucose and ketone content in the sample. We believe that our model, in combination with the proposed fabrication methodology, and the in-house developed app., would enable rapid and reliable fabrication of μPADs for various fundamental investigations, and applications pertaining to affordable health-care monitoring.Graphical Abstract

Electrokinetics of deformable interfaces holds the key in developing bio-mimetic micro-devices for probing microvascular physiology and performing in vitro bio-analytical procedures. While such systems have been analyzed in the literature with the aid of a set of simplifying assumptions to some extent, implications of axial modulations in the surface potential and interfacial slip, otherwise ominous artifacts of the coupling between intrinsic electro-chemistry and substrate wettability in practical realms of surface engineering, remain grossly unaddressed in the same context. Here, we bring out the interplay of the wall compliance, periodically modulated surface potential, and patterned interfacial slip that is itself coupled with the interfacial electrochemistry, toward altering the internal hydrodynamics of a deformable microfluidic channel. Manifested in terms of a perturbed pressure field, this essentially leads to an alteration in the load bearing capacity of the concerned electro-mechanical system. By exploiting this unique coupling of the interfacial electro-mechanics, hydrodynamics, and substrate elasticity, we further demonstrate the plausibility of a significant augmentation in the load bearing capacity of the resulting system, over favorable parametric regimes. This opens up emerging possibilities of developing novel electro-kinetically modulated lubricated systems with giant augmentations in their load bearing performance.

A Rose petal is well known for a unique form of hydrophobicity, exhibiting simultaneously high apparent equilibrium contact angle (θ*) as well as high adhesion that hinders the rolling off of droplets. This behaviour is fundamentally different than that commonly encountered in other bio-mimetic superhydrophobic surfaces (like a lotus leaf) that have air entrapped in the solid-liquid contact zone (Cassie Baxter wetting state), thereby exhibiting low adhesion. Based on laser scanning confocal microscopy as well as underwater in-situ atomic force microscopy, we obtain high resolution image of the water-substrate contact region (∼0.9–20 μm) which is significantly higher than that reported so far in the literature. We clearly demonstrate that this unique "Petal effect" can indeed be attributed to the Cassie impregnating wetting state, thereby resolving the prevailing ambiguity on this topic. We have also replicated the structure of the actual rose petals using soft lithography on cross-linked polydimethylsiloxane (PDMS, Sylgard 184) and show that both θ* as well as the adhesive properties of the replicated surface to be nearly identical to that of an actual petal. This implies that the force of adhesion depends on the wetting state and the area of contact. Incidentally, the negative replica of the petal, which is obtained as an intermediate during the replication process, exhibits slightly higher adhesion and identical θ*, as compared to the actual rose petal and its positive replica. However, the prevalent wetting state on the negative replica turns out to be Cassie like due to the presence of entrapped air. Both the negative and the positive replica of the Rose petal can potentially be used as biomimetically fabricated sticky hydrophobic surface.

In healthcare practice, the sedimentation rate of red blood cells (erythrocytes) is a widely used clinical parameter for screening of several ailments such as stroke, infectious diseases, and malignancy. In a traditional pathological setting, the total time taken for evaluating this parameter varies typically from 1 to 2 h. Furthermore, the volume of human blood to be drawn for each test, following a gold standard laboratory technique (alternatively known as the Westergren method), varies from 4 to 5 ml. Circumventing the above constraints, here we propose a rapid (∼1 min) and highly energy efficient method for the simultaneous determination of hematocrit and erythrocyte sedimentation rate (ESR) on a microfluidic chip, deploying electrically driven spreading of a tiny drop of blood sample (∼8 μl). Our unique approach estimates these parameters by correlating the same with the time taken by the droplet to spread over a given radius, reproducing the results from more elaborate laboratory settings to a satisfactory extent. Our novel methodology is equally applicable for determining higher ranges of ESR such as high concentration of bilirubin and samples corresponding to patients with anemia and patients with some severe inflammation. Furthermore, the minimal fabrication steps involved in the process, along with the rapidity and inexpensiveness of the test, render the suitability of the strategy in extreme point-of-care settings.

Enhancing solute dispersion in electrically actuated flows has always been a challenging proposition, as attributed to the inherent uniformity of the flow field in the absence of surface patterns. Over the years, researchers have focused their attention towards circumventing this limitation, by employing several fluidic and geometric modulations. However, the corresponding improvements in solute dispersion often turn out to be inconsequential. Here we reveal that by exploiting the interplay between an externally imposed temperature gradient, subsequent electrical charge redistribution and ionic motion, coupled with the rheological complexities of the fluid, one can achieve enhancement of up to one order of magnitude of solute dispersion in a pressure-driven flow of an electrolyte solution. Our results demonstrate that the complex coupling between thermal, electrical, hydrodynamic and rheological parameters over small scales, responsible for such exclusive phenomenon, can be utilized in designing novel thermally actuated microfluidic and bio-microfluidic devices with favourable solute separation and dispersion characteristics.

Hypothesis Viscoelastic liquids could be used as potential substrates in the microfluidics paradigm. The theoretical and experimental investigation of an evaporating aqueous droplet, over a viscoelastic liquid substrate, could provide a fundamental perspective of the complex interplay amongst capillarity, viscosity, and elasticity, resulting in a wide array of intriguing dynamics, which could be important in several microscale processes. Experiments The evaporation dynamics of a water droplet atop an un-crosslinked polydimethylsiloxane film (polymeric liquid substrate) are examined using an optical goniometer and a laser scanning confocal microscopy, to discern the interfaces. The recorded videos were analyzed to estimate the contact angles, velocities, and other parameters of relevance. Findings The viscoelasticity of the film, in conjunction with evaporation, triggered a self-propulsion in the droplet, leading to crumpling of the polymeric film, and finally culminating in the encapsulation of the water drop by the polymer. The evaporation caused a dynamic variation in both the radius and contact angle of the droplet. The physics of the hitherto unreported phenomena is explained via the development of a semi-analytical model, considering all the relevant forces. We postulate that this symbiotic and self-sustained dynamics would pave the path towards the comprehension of micro-swimmers and surface encapsulation, to name a few.

Pattern formation during evaporation of biofluids has numerous biomedical applications, e.g., in disease identification. The drying of a bidisperse colloidal droplet involves formation of coffee ring patterns owing to the deposition of constituent particles. In the present study, we examine the distinctly different pattern formations during the drying of a colloidal solution depending on the nature of the constituent proteins. The pattern formations of two oppositely charged proteins, namely HSA and lysozyme, have been studied in the presence of fluorescence polystyrene beads of two different sizes (providing better image contrast for further analysis). The variation of pattern formation has been studied by varying the concentrations of the proteins as well as the particles. Furthermore, using image analysis, the patterns are segmented into different regions for quantification. To explain the variations in the patterns, we delve into the interplay of the interactions, especially the capillary and the DLVO forces (between the particles and the substrate). The developed methodology based on the coffee ring effect may be used to identify individual proteins.

The formation of specific patterns during evaporation of biofluids e.g. blood, serum etc. can be analyzed for quick identification of several diseases. Human Serum Albumin (HSA) is frequently used as the model protein to study amyloid fibril formation that is responsible for numerous neurodegenerative diseases like Parkinson’s disease, Alzheimer’s disease etc. In the present study the fibril growth of HSA is characterized by analyzing the drying patterns of the protein solution. The fibril formation of HSA is confirmed by spectrofluorimetry and the β-sheet content is quantified by far-UV Circular Dichroism spectrometry. Examinations of the drying patterns of HSA, in presence of fluorescent nanoparticles, reveals distinctive pattern transformations with increase in the β-sheet content visualized through fluorescence microscopy and FESEM. The fluorescence images of the dried droplet are analyzed using image processing by the color thresholding method. A rapid quantification methodology of the β-sheet content of HSA and the fraction of unfolded protein has been developed by comparing the results of the image analysis with those of the far-UV CD data.

Microscopic investigations of any abnormality associated with erythrocyte/red blood cell morphology constitute an important segment of the age-old peripheral smear test. Though the test is conducted on a glass slide, the effect of glass and similar other solid substrates on erythrocyte morphology remained majorly unexplored. In the first of its kind investigation, we have outlined the effect of varying the substrate surface potential on erythrocyte morphology. Such a substrate induced phenomenon has been quantified for two distinctly different drying configurations (droplets and film) upon systematically varying the cell concentration. Experimental results and supporting theoretical analysis unambiguously show the surface potential of the solid substrate to be the most influential parameter in the process of morphological alteration. The findings of the present investigation may be utilized to formulate an error-free protocol for the baseline peripheral smear test of hematological diagnosis.

Understanding the dynamics of blood flow in physiologically relevant confinements turns out to be an outstanding proposition in biomedical research. Despite the large number of studies being reported to theoretically elucidate the dynamics of red blood cells (RBCs) in confined geometries, in vitro experimental studies unveiling the implications of the collective dynamics of red blood cells in physiologically relevant bio-mimetic microfluidic channels remain elusive. Here, we investigate the implications of complex dynamvic interactions between the whole blood and a deformable channel wall fabricated using a hydrogel matrix. For a range of flow rates, we map the trajectories of the RBCs for varying levels of softness of the microchannel wall. We compare these scenarios with the reference cases of rigid polydimethylsiloxane (PDMS) channels. Our results reveal that the smallest channels investigated herein exhibit the most intricate interactions between the collective dynamics of the RBC and the wall flexibility, attributable to confinement-induced hydrodynamic interactions in the presence of spatially varying shear rates. These results may open up new paradigms in conceptual understanding of in vivo dynamics of blood flow through simple in vitro experiments on a simple microfluidic platform.

A facile methodology to create a wrinkled surface with a tailored topography is presented herein. The dependency of the elasticity of poly(dimethyl)siloxane (PDMS) on the curing temperature has been exploited to obtain a substrate with an elasticity gradient. The temperature gradient across the length of PDMS is created by a novel set-up consisting of a metal and insulator connected to a heater and the highest usable (no degradation of PDMS) temperature gradient is used. The time-dependent temperature distributions along the substrate are measured and the underlying physics of the dependence of the PDMS elasticity on the curing temperature is addressed. The PDMS substrate with the elasticity gradient is first stretched and subsequently oxidized by oxygen plasma. Upon relaxation, an ordered wrinkled surface with continuously varying wavelength and amplitude along the length of PDMS is obtained. The extent of hydrophobicity recovery of this plasma oxidized PDMS with varying elasticity has been studied. The change in the wavelength and amplitude of the regular patterns on the substrate can be controlled by varying operational parameters like applied pre-strain, plasma power and the heater temperature. It has been found that the spatial distributions of the topography and the hydrophobicity collectively decide the resultant wettability of the substrate. Such surfaces with gradients in the substructure dimensions demonstrate different wetting characteristics that may lead to a wide gamut of applications including droplet movement, cell adhesion and proliferation, diffraction grating etc.

The effect of viscous dissipation in forced convection of power-law fluids through microchannels of different cross-sectional geometries is studied numerically over the ranges of power-law index, 0.8 ≤ n ≤ 1.2 and Brinkman number, 0.001 ≤ Br ≤ 0.1 while keeping Peclet number constant at Pe = 10. Two types of thermal boundary conditions, namely, uniform wall temperature (T2) and uniform heat flux (H2), have been employed at the microchannel wall and the results of the temperature fields are expressed in terms of Nusselt number. The interplay between the fluid rheology and viscous dissipation effect gives rise to significant alteration in the net convective transport and thus can be beneficial in the thermal design of biofluidic devices.

This paper reports the results of our studies on the performance of thin film evaporation, mostly used in the evaporator of an ultra-thin vapor chamber using micropillar arrays. The dry-out heat flux and evaporator surface temperature have been used as the performance metrics. A numerical model has been formulated to solve for the fluid flow and phase change heat transfer considering the capillary pumping action (wicking action) using a cell-by-cell forward approach and validated with results reported in the literature. The solutions of the model equation are then utilized to study the effects of geometry and arrangement of the micropillars within the vapor chamber and to gain further insights into the interplay of the various forces. It is seen that these variations strongly influence the pumping action as well as the shape of the liquid vapor interface thereby impacting the pressure field, flow rate and the dry-out heat flux. A conical shape of the micropillar (having the same volume as that of other shapes) has a significant impact on the dry-out heat flux e.g., reducing the top diameter from 10 to 8 μm results in an 18% reduction. For a rectangular arrangement of the micropillars, the performance is quite sensitive to the transverse and axial spacing. In the range of 30–40 μm of axial and transverse spacing, the dry out heat flux can be altered by as much as 20%. Higher transverse pitch with lower axial pitch is found to be a better combination for a given overall geometry and number of pillars.

In order to demonstrate the influence of the surface chirality of the nanoparticles on amyloid fibrillation, the inhibiting effectiveness of the chiral gold nanoparticles, synthesized using the two enantiomeric forms (i.e., d- and l-) of glutamic acid, toward the fibrillation of human serum albumin (HSA) has been investigated. Here the enantiomers of glutamic acid are used as both reducing and stabilizing/capping agents. It is found that the surface chirality is the only major difference between the d-glutamic acid mediated gold (DGAu) and l-glutamic acid mediated gold (LGAu) nanoparticles. The fibrillation process has been monitored using various biophysical techniques, e.g., turbidity assay, Thioflavin T fluorescence kinetics, Congo red binding study, circular dichroism spectroscopy, fluorescence microscopy, and transmission electron microscopy. The experimental results illustrate that DGAu is more effective in inhibiting the formation of HSA fibrils than LGAu. Furthermore, the differential inhibiting e...

The effect of oscillation induced by a frequency-dependent alternating current (AC) electric field to dissociate preformed amyloid fibrils has been investigated. An electrowetting-on-dielectric type setup has been used to apply the AC field of varying frequencies on preformed fibrils of human serum albumin (HSA). The disintegration potency has been monitored by a combination of spectroscopic and microscopic techniques. The experimental results suggest that the frequency of the applied AC field plays a crucial role in the disruption of preformed HSA fibrils. The extent of stress generated inside the droplet due to the application of the AC field at different frequencies has been monitored as a function of the input frequency of the applied AC voltage. This has been accomplished by assessing the morphology deformation of the oscillating HSA fibril droplets. The shape deformation of the oscillating droplets is characterized using image analysis by measuring the dynamic changes in the shape dependent parameters such as contact angle and droplet footprint radius and the amplitude. It is suggested that the cumulative effects of the stress generated inside the HSA fibril droplets due to the shape deformation induced hydrodynamic flows and the torque induced by the intrinsic electric dipoles of protein due to their continuous periodic realignment in presence of the AC electric field results in the destruction of the fibrillar species.

The formation and deposition of amyloid fibrils have been linked to the pathogenesis of numerous debilitating neurodegenerative disorders. Serum albumins serve as good model proteins for understanding the molecular mechanisms of protein aggregation and fibril formation. Graphene-based nanotherapeutics appear to be promising candidates for designing inhibitors of protein fibrillation. The inhibitory effect of graphene oxide (GO) nanoparticles on the fibrillation of human serum albumin (HSA) in an in vitro mixed solvent system has been investigated. The methods used include ThT fluorescence, ANS binding, Trp fluorescence, circular dichroism, fluorescence microscopy, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. It was observed that GO inhibits HSA fibrillation and forms agglomerates with β-sheet rich prefibrillar species. Binding of GO prevents the formation of mature fibrils with characteristic cross-β sheet but does not promote refolding to the native state.

The capillary driven flow of water inside a microchannel with altered wettabilities is experimentally investigated and modeled theoretically. The surfaces of the PDMS made microchannel are exposed to oxygen plasma, rendering the surfaces increasingly hydrophilic, which provides the driving force for the flow. The plasma treated surfaces are characterized using topography and phase imaging of AFM scanning, as well as nano-indentation, to correlate the distinct structural changes to the hydrodynamic profiles of the advancing meniscus. The experimental results are further analyzed using a newly proposed slip velocity model. The aim is to obtain a qualitative relationship between the surface properties and the flow parameters, namely the advancing meniscus velocity and pressure drop inside the channel. The insights are of fundamental importance in diverse fields, such as enhanced oil recovery, microfluidic devices, cell separation, and pathology. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4616–4627, 2017

AC electrowetting is used to induce oscillation in a nanofluid droplet with significant changes in the shape dependent parameters at an optimized frequency. The presence of the synthesized silver nanoparticles not only enhances the wetting characteristics of the resulting nanofluid droplets, but leads to the augmentation of their heat extraction capability from a hot spot. The low frequency AC electrowetting induced droplet shape deformation generates surface waves and associated internal flow inside the droplet. This augments the convective heat transfer process resulting in additional evaporative cooling. The imposed internal flow and mixing also contribute to the reuse of the residual (after evaporation) nanoparticles to recreate the nanofluid droplet on the substrate. The generated surface waves are characterized using image analysis of the oscillating droplets in terms of their amplitude, frequency and damping. A model based on Stoke's drift phenomenon is used to analyze the results indicating augmented heat transfer in the low frequency regime.

Dielectric breakdown puts an operating limit to the applied electric field in electrowetting-based applications. Herein, we report that the electrical properties relevant to electrowetting for a widely used dielectric, PDMS, can be substantially enhanced upon the incorporation of graphene oxide (GO) fillers into the PDMS polymer matrix. The GO-PDMS composite is characterized by higher values of the dielectric constant, higher breakdown electric field, and lower actuation voltages due to enhanced interfacial polarization. The improvement of these properties with the increase in concentration of GO, is explored till the ‘percolation threshold’. This newly developed composite as the dielectric, droplet actuation and electric field-induced droplet transportation over a fabricated triangular electrode system are performed. The results are compared with the literature data on PDMS and other dielectrics to establish the superior qualities of the dielectric for rapid transportation of droplets at significantly lower voltages with potential for new applications.

Droplet impact dynamics on super-hydrophobic micro-structured surfaces is vital in a multitude of processes such as spray coating, cooling, and inkjet printing etc. Majority of the research has been focused on the rebound phenomena while partial rebound is relatively underreported. Recent studies indicate that partial rebound is encountered where transition of wetting state may take place on super-hydrophobic surfaces. Herein, we report that satellite drops are formed during partial rebound of droplets even on hydrophobic surfaces with unidirectional topography created using a mask-less and stamp-less fabrication process. The simultaneous capillary driven retraction along the wrinkles and the strong contact line pinning across these anisotropic substrates leads to formation of satellite drops on hydrophobic surfaces. Excess rebound energy has been evaluated as functions of contact angle, contact angle hysteresis and the maximum spreading diameter to explain partial rebound on such surfaces with potential applications in the design of novel, functional surfaces.

The improvements in the dynamic performance of droplet movement over a newly designed and fabricated triangular coplanar electrode system utilizing the principle of electrowetting on dielectric (EWOD) is extensively probed and compared with results available in the literature. The process is further explored by developing a model, taking into account the pertinent driving and retarding forces, the role of the effective contact line length and an in-situ evaluation of the contact line friction coefficient. Furthermore, the efficiency of the fabricated digital microfluidic platform is quantified and compared in terms of driving force and velocities.

Electrically driven dynamic spreading of drops on soft solids is of fundamental importance in a plethora of applications ranging from bio-medical diagnostics to liquid lenses and optoelectronics. However, strategies reported in this regard are challenged by the fact that the spreading gets significantly arrested due to viscoelastic dissipation at the three phase contact line. Circumventing these limits, here we bring out a possibility of substantial augmentation in the rate of electro-spreading on a soft matrix by deploying nano-scale fluidic suspensions of optimal volume fraction. We attribute these findings to a consequent increment in the electrical stresses toward combating the viscoelastic dissipation in the interfacial layer. We also present a simple scaling theory that unveils the manner in which the nano-suspension alters the spreading dynamics of a droplet, effectively by changing the final equilibrium contact angle. These findings open up new possibilities of using nano-fluids of optimal concentration toward modulating the dynamic spreading of a drop on a deformable substrate, a paradigm hitherto remaining unexplored.

The amyloid β-peptide fragment comprising residues 25-35 (Aβ25-35 ) is known to be the most toxic fragment of the full length Aβ peptide which undergoes fibrillation very rapidly. In the present work, we have investigated the effects of the micellar environment (cationic, anionic, and nonionic) on preformed Aβ25-35 fibrils. The amyloid fibrils have been prepared and characterized by several biophysical and microscopic techniques. Effects of cationic dodecyl trimethyl ammonium bromide (DTAB), cetyl trimethylammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS), and nonionic polyoxyethyleneoctyl phenyl ether (Triton X-100 or TX) on fibrils have been studied by Thioflavin T fluorescence, UV-vis spectroscopy based turbidity assay and microscopic analyses. Interestingly, DTAB and SDS micelles were observed to disintegrate prepared fibrils to some extent irrespective of their charges. CTAB micelles were found to break down the fibrillar assembly to a greater extent. On the other hand, the nonionic surfactant TX was found to trigger the fibrillation process. The presence of a longer hydrophobic tail in case of CTAB is assumed to be a reason for its higher fibril disaggregating efficacy, the premise of their formation being largely attributed to hydrophobic interactions. Proteins 2016; 84:1213-1223. © 2016 Wiley Periodicals, Inc.

Protein and peptide aggregation/fibrillation is reported to be responsible for several neurological disorders. Fibrillation of the amyloid β-peptide fragment (25–35) which is a biologically active region of the full length peptide, has been observed to be significantly inhibited in presence of the two dimensional nanomaterial graphene oxide (GO). Fibrillation and inhibition of the Aβ25–35 peptide by GO has been performed at 37 °C at physiological pH (pH 7.4). The inhibition process is monitored by ThioflavinT fluorescence (ThT), circular dichroism spectroscopy, matrix assisted laser desorption/ionization mass spectrometry, dynamic light scattering experiments etc. The soluble fraction of the protein is quantified by the BCA assay. Microscopic techniques are used to study the morphology of the fibrils formed. GO is observed to inhibit the fibrillation even at very low concentrations and is amplified with increase in concentration of GO. ThT kinetic data fitted well with a sigmoidal curve and shows that GO is able to lengthen the lag phase of the fibrillation process. It appears that surface adsorption of protein on the nanomaterial prevents the monomers to come together. It is speculated that the presence of both polar and non-polar moieties in GO interact strongly with the hydrophobic and hydrophilic residues of the Aβ25–35 peptide monomer units, thus preventing further aggregation.

To understand the pharmacokinetics of administered nanomaterials, it is essential to examine the stability and biological activity of proteins by investigating the physicochemical characteristics of the protein–nanoparticle bioconjugate. In this work, the mechanistic detail of the interaction between human serum albumin (HSA) and silver nanoparticles synthesized using nontoxic and biodegradable chitosan as a reducing and stabilizing agent, have been investigated at the nanobio interface. A combination of spectroscopic, calorimetric, and microscopic techniques have been employed to monitor the interaction process. The results illustrate that the chitosan-mediated silver nanoparticles spontaneously bind to HSA without appreciable conformational changes of the protein. Furthermore the potential of the nanoparticles to inhibit the formation of HSA amyloid-like fibrils, in vitro, has been analyzed using thioflavin T fluorescence, circular dichroism, fluorescence microscopy, and transmission electron microscopy. The experimental observations indicate that interactions between HSA and chitosan-based silver nanoparticles have led to appreciable reduction in amyloid fibril formation. Additionally, cytotoxicity and hemolytic assays are performed to ensure the biocompatibility of the nanoparticles within the application limit.

We bring out unique aspects of the pulsatile flow of a blood analog fluid (Xanthan gum solution) in a biomimetic microfluidic channel. Pressure waveforms that mimic biologically consistent pulsations are applied on physiologically relevant cylindrical microchannels fabricated using polydimethylsiloxane. The in vivo features of the relevant waveforms like peak amplitude and dicrotic notch are reproduced in vitro. The deformation profiles exhibit viscoelastic behavior toward the end of each cycle. Further, the time-varying velocity profiles are critically analyzed. The local hydrodynamics within the microchannel is found to be more significantly affected by pressure waveform rather than the actual wall deformation and the velocity profile. These results are likely to bear far-reaching implications for assessing micro-circulatory dynamics in lab on a chip based microfluidic platforms that to a large extent replicate physiologically relevant conditions.

Here, we report the intriguing movements of an extended liquid meniscus on a silicon substrate under the influence of sinusoidal alternating current (AC) voltages at different operating frequencies. As opposed to droplet electrowetting, wherein the droplet spreads and experiences oscillations at the free surface, the application of AC voltage to a thin liquid film results in distinct and uniform dewetting, in conjunction with augmented wetting. Image analyzing interferometry is used for the precise measurement of the film thickness profile and other associated parameters. We postulate that the classic Young-Lippmann equation fails to explain the dynamics of an extended meniscus and evince that the dynamics of film displacement could be successfully explained by considering the product of the applied electric field and its gradient, as opposed to the existing consideration of a square dependence on the applied voltage. The physics of the hitherto unreported phenomena is elucidated by developing a mathematical model, taking into consideration all of the germane forces governing the dynamics of the thin liquid film. We affirm that the present study would serve as a fundamental background for a fascinating mode of liquid actuation, with inherent application potential in several existing and novel microfluidic systems.

The balance of intermolecular and surface forces plays a critical role in the transport phenomena near the contact line region of an extended meniscus in several technologically important processes. Externally applied fields can alter the equilibrium and stability of the meniscus with concomitant effects on its shape and spreading characteristics and may even lead to an oscillation. This feature article provides a detailed account of the present and past efforts in exploring the behavior of curved thin liquid films subjected to mild thermal perturbations, heat input, and electrical and magnetic fields for pure as well as colloidal suspensions, including the effects of particle charge and polarity. The shape-dependent intermolecular force field has been evaluated in situ by a nonobtrusive optical technique utilizing the interference phenomena and subsequent image processing. The critical role of disjoining pressure is identified along with the determination of the Hamaker constant. The spatial and temporal variations of the capillary forces are evaluated for the advancing and receding menisci. The Maxwell-stress-induced enhanced spreading during electrowetting, at relatively low voltages, and that due to the application of a magnetic field are discussed with respect to their distinctly different characteristics and application potentials. The use of the augmented Young-Laplace equation elicited additional insights into the fundamental physics for flow in ultrathin liquid films.

In this work, we have fabricated physiologically relevant polydimethylsiloxane microfluidic phantoms to investigate the fluid-structure interaction that arises from the interaction between a non-Newtonian fluid and the deformable wall. A shear thinning fluid (Xanthan gum solution) is used as the blood analog fluid. We have systematically analyzed the steady flow characteristics of the microfluidic phantom using pressure drop, deformation, and flow visualization using micro-PIV (Particle Image Velocimetry) to identify the intricate aspects of the pressure as well as the velocity field. A simple mathematical formulation is introduced to evaluate the flow induced deformation. These results will aid in the design and development of deformable microfluidic systems and provide a deeper understanding of the fluid-structure interaction in microchannels with special emphasis on biomimetic in-vitro models for lab-on-a-chip applications.

Morphology of erythrocyte membrane has been recognized as an alternative biomarker of several patho-physiological states. Numerous attempts have been made to upgrade the existing method of primitive manual counting, particularly exploring the light scattering properties of erythrocyte. All the techniques are at best semi-empirical and heavily rely on the effectiveness of the statistical correlations. Precisely, this is due to the lack of a non-empirical scale of the so-called “morphological scores”. In this article, fractal dimension of erythrocyte membrane has been used to formulate a suitable scoring scale. Subsequently, the rapid experimental output of flow-cytometry has been functionally related to the mean morphological quantifier of the whole cell population via an optimum neural network model (R2 = 0.98). Moreover, the fractal dimension has been further demonstrated to be an important parameter in early detection of an abnormal patho-physiological state, even without any noticeable poikilocytic transformation in micrometric domain.

Electrowetting on dielectric (EWOD) on unidirectional microstructured surfaces has recently evoked significant interest as they can modulate the effect of electrowetting, and can thus find applications in directional wetting in microfluidic systems. However, the dependency of such EW phenomenon on their initial state of wetting and anisotropy is far from being well understood. The current study addresses the initial wetting states and their implication on the anisotropic electrowetting using a wrinkled EWOD platform. Herein we demonstrate a facile stampless and maskless structure generation technique to fabricate wrinkles of varying topography. Further, we have demonstrated alteration in the interfacial wetting conditions by modulating the wrinkle topography, and its effect on the droplet behavior during electrowetting. The capillary wicking-assisted electrowetting on these wrinkled surfaces is in specific direction dictated by the ordered wrinkles and prompts enhanced spreading of the droplet. We also demonstrate that while the enhancement of unidirectional electrowetting is stronger in conformal wetting state surfaces, composite wetting state surfaces depict a reversal in anisotropy.

Analytical solution and mathematical forms of various expressions

Major contribution to the technological evolution in the semiconductor industry can be attributed to the development of the micro-fabrication techniques that enabled reduction in the chip area with an increase in storage or memory. Not just the semiconductor industry, but miniaturization has also led to the concept of a functional laboratory on a single chip, commonly known as “lab-on-a-chip” devices. These platforms have been realized lately for carrying out chemical and biological processes and are touted to be the devices of the future.

Electrically actuated transport dynamics of colloidal droplets, on a hydrophobic dielectric film covering an array of electrodes, is studied here. Specifically, the effects of the size and electrical properties (zeta-potential) of the colloidal particles on such transport characteristics are investigated. For the colloidal droplets, the application of an electrical voltage leads to additional attenuation of the local dielectric-droplet interfacial tension. This is due to the electrically triggered enhanced colloidal particle adsorption at the dielectric-droplet interface, in the immediate vicinity of the droplet three-phase contact line (TPCL). The extent of such interfacial particle adsorption, and hence, the extent of the consequential reduction in the interfacial tension, is dictated by the combined effects of the three-phase contact line spreading, particle size, the interfacial electrostatic interaction between the colloidal particles (if charged) and the charged dielectric surface above the activated electrode, and the interparticle electrostatic repulsion. The electrical driving force of varying magnitude, stemming from this altered solid-liquid interfacial tension gradient in the presence of the colloidal particles, culminates in different droplet transport velocity and droplet transfer frequency for different colloidal droplets. We substantiate the inferences from our experimental results by a quasi-steady state force balance model for colloidal droplet transport. We believe that the present work will provide an accurate framework for determining the optimal design and operational parameters for digital microfluidic chips handling colloidal droplets, as encountered in a plethora of applications.

Droplet motion on a surface with chemical energy induced wettability gradient has been simulated using molecular dynamics (MD) simulation to highlight the underlying physics of molecular movement near the solid-liquid interface including the contact line friction. The simulations mimic experiments in a comprehensive manner wherein microsized droplets are propelled by the surface wettability gradient against forces opposed to motion. The liquid-wall Lennard-Jones interaction parameter and the substrate temperature are varied to explore their effects on the three-phase contact line friction coefficient. The contact line friction is observed to be a strong function of temperature at atomistic scales, confirming their experimentally observed inverse functionality. Additionally, the MD simulation results are successfully compared with those from an analytical model for self-propelled droplet motion on gradient surfaces.

The effect of axial flow variation on the hydrodynamic (Taylor–Aris) dispersion in a patterned microchannel consisting of periodic pillars and gaps is characterized for limiting Cassie–Baxter and Wenzel state configurations. The entry effects of flow for subsequent gaps and pillars are crucial as the flow tends towards plug flow over the gaps due to free slip at the air–liquid interface for the Cassie state or increased cross section for the Wenzel state and fully-developed parabolic flow over the pillars. The study includes analyzing the solute concentration distribution as a function of Peclet number based on the gap length, using CFD simulation and quantifying the dispersion based on Aris method of moments. For the Cassie State, simulations predict a narrower solute concentration distribution compared to flow in smooth channels. Detailed analysis reveal varied dispersion characteristics with Peclet number: similar dispersion with a time lag for low Peclet numbers and a reduced dispersion for high Peclet Number due to enhanced effect of slip. For the Wenzel state, a considerable loss of solute occurs due to the presence of dead zones within the gaps resulting in a significant increase in dispersion. The changed dispersion characteristics may have profound implications in microfluidic applications involving mixing and separation e.g., in liquid chromatography.

The effect of surface wettability on the dynamics of crack formation and their characteristics are examined during the drying of aqueous colloidal droplets (1 μL volume) containing nanoparticles (53 nm mean particle diameter, 1 w/w %). Thin colloidal films, formed during drying, rupture as a result of the evaporation-induced capillary pressure and exhibit microscopic cracks. The crack initiation and propagation velocity as well as the number of cracks are experimentally evaluated for substrates of varying wettability and correlated to their wetting nature. Atomic force and scanning electron microscopy are used to examine the region in the proximity of the crack including the particle arrangements near the fracture zone. The altered substrate-particle Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions, as a consequence of the changed wettability, are theoretically evaluated and found to be consistent with the experimental observations. The resistance of the film to cracking is found to depend significantly on the substrate surface energy and quantified by the critical stress intensity factor, evaluated by analyzing images obtained from confocal microscopy. The results indicate the possibility of controlling crack dynamics and morphology by tuning the substrate wettability.

The performance of a specially fabricated micro heat pipe etched on a silicon wafer is analyzed to evaluate its cooling potential. Triangular microgrooves are etched on a silicon wafer via a lithographic process with two reservoirs at the two ends. Anodic bonding and electrochemical spark erosion techniques are used to seal the microgrooves with a thin Pyrex glass and to create holes for vacuum connections and coolant insertion, respectively. The cooling potential of the micro heat pipe is evaluated by accurately measuring the temperature distributions along the channel length at different values of the applied heat load. A mathematical model is proposed and numerically solved to evaluate the axial variation of the radius of curvature of the coolant film to examine the cooling potential of the on-chip micro heat pipe system.