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Study of nucleation and growth dynamic events of cubic-phase ice crystals at TiO2–water nanointerface.

Surfaces with extreme wettability (too low, superhydrophobic; too high, superhydrophilic) have attracted considerable attention over the past two decades. Titanium dioxide (TiO2) has been one of the most popular components for generating superhydrophobic/hydrophilic coatings. Combining TiO2 with ethanol and a commercial fluoroacrylic copolymer dispersion, known as PMC, can produce coatings with water contact angles approaching 170°. Another property of interest for this specific TiO2 formulation is its photocatalytic behavior, which causes the contact angle of water to be gradually reduced with rising timed exposure to UV light. While this formulation has been employed in many studies, there exists no quantitative guidance to determine or tune the contact angle (and thus wettability) with the composition of the coating and UV exposure time. In this article, machine learning models are employed to predict the required UV exposure time for any specified TiO2/PMC coating composition to attain a certain wettability (UV-reduced contact angle). For that purpose, eight different coating compositions were applied to glass slides and exposed to UV light for different time intervals. The collected contact-angle data was supplied to different regression models to designate the best method to predict the required UV exposure time for a prespecified wettability. Two types of machine learning models were used: (1) parametric and (2) nonparametric. The results showed a nonlinear behavior between the coating formulation and its contact angle attained after timed UV exposure. Nonparametric methods showed high accuracy and stability with general regression neural network (GRNN) performing best with an accuracy of 0.971, 0.977, and 0.933 on the test, train, and unseen data set, respectively. The present study not only provides quantitative guidance for producing coatings of specified wettability, but also presents a generalized methodology that could be employed for other functional coatings in technological applications requiring precise fluid/surface interactions.

The interaction of rising gas bubbles with submerged air-repelling or air-attracting surfaces is relevant to various technological applications that rely on gas-microvolume handling or removal. This work demonstrates how submerged metal meshes with super air-attracting/repelling properties can be employed to manipulate microvolumes of air, rising buoyantly in the form of bubbles in water. Superaerophobic meshes are observed to selectively allow the passage of air bubbles depending on the mesh pore size, the bubble volume-equivalent diameter, and the bubble impact velocity on the mesh. On the other hand, superaerophilic meshes reduce or amplify the volume captured from a train of incoming bubbles. Finally, a spatial wettability pattern on the mesh is used to control the size of the outgoing bubble, and an empirical relation is formulated to predict the released gas volume. The study demonstrates how porous materials with controlled wettability can be used to precisely modulate and control the outcome of bubble/mesh interactions.

Water is ubiquitous in natural systems, ranging from the vast oceans to the nanocapillaries in the earth crust or cellular organelles. In bulk or in intimate contact with solid surfaces, water molecules arrange themselves according to their hydrogen (H) bonding, which critically affects their short- and long-range molecular structures. Formation of H-bonds among water molecules designates the energy levels of certain nonbonding molecular orbitals of water, which are quantifiable by spectroscopic techniques. While the molecular architecture of water in nanoenclosures is of particular interest to both science and industry, it requires fine spectroscopic probes with nanometer spatial resolution and sub-eV energy sensitivity. Graphene liquid cells (GLCs), which feature opposing closely spaced sheets of hydrophobic graphene, facilitate high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) measurements of attoliter water volumes encapsulated tightly in the GLC nanovessels. We perform in situ TEM and EELS analysis of water encased in thin GLCs exposed to room and cryogenic temperatures to examine the nanoscale arrangement of the contained water molecules. Simultaneous quantification of GLC thickness leads to the conclusion that H-bonding strengthens under increased water confinement. The present results demonstrate the feasibility of nanoscale chemical characterization of aqueous fluids trapped in GLC nanovessels and offer insights on water molecule arrangement under high-confinement conditions.

Liquid-jet impact on porous, relatively thin solids has a variety of applications in heat transfer, filtration, liquid-fuel atomization, incontinence products, and solid-substrate erosion, among others. Many prior studies focused on liquid-jet impact on impermeable substrates, and some have investigated the hydraulic jump phenomenon. In the present work, the liquid jet strikes a superhydrophobic, permeable, metal mesh orthogonally, and the radial spreading and throughflow of the liquid are characterized. The prebreakthrough hydraulic jump, the breakthrough velocity, and the postbreakthrough spatial distributions of the liquid are investigated by varying the liquid properties (density, surface tension, and viscosity) and the openness of the metal mesh. The hydraulic jump radius in the prebreakthrough regime increases with jet velocity and is independent of the liquid properties and mesh geometry (pore size, wire diameter and pitch). The breakthrough velocity increases with surface tension of the liquid and decreases with the mesh opening diameter and liquid viscosity. A simple analytical model predicts the jet breakthrough velocity; its predictions are in accordance with the experimental observations. In the postbreakthrough regime, as the jet velocity increases, the liquid flow rate penetrating the mesh shows an initially steep increase, followed by a plateau, which is attributed to a Cassie-Baxter-to-Wenzel transition at the impact area of the mesh.

Spreading of liquid droplets on wettability-confined paths has attracted considerable attention in the past decade. On the other hand, the inverse scenario of a gas bubble spreading on a submerged, wettability-confined track has rarely been studied. In the present work, an experimental investigation of the spreading of millimetric gas bubbles on horizontally submerged, textured, wettability-confined tracks is carried out. The width of the track is kept fixed along its entire length, and the spreading behavior of a gas bubble, dispensed at one end of the track, is studied. The effects of varying track width, bubble diameter, and ambient liquid are investigated. Post-contact, the gas bubble spreads along the track at a linear rate with time, while remaining pinned at its back end; the recorded spreading speed is O(0.5 m/s). An inertio-capillary force balance describes the experimentally observed spreading dynamics with excellent agreement.

In the modern construction industry, large gypsum (CaSO4·2H2O) boards are manufactured through a two-step procedure, which features the heating of fine gypsum powders to form the intermediate plast...

Porous substrates have the ability to transport liquids not only laterally on their open surfaces but also transversally through their thickness. Directionality of the fluid transport can be achieved through spatial wettability patterning of these substrates. Different designs of wettability patterns are implemented herein to attain different schemes (modes) of three-dimensional transport in a high-density paper towel, which acts as a thin porous matrix directing the fluid. All schemes facilitate precise transport of metered liquid microvolumes (dispensed as droplets) on the surface and through the substrate. One selected mode features lateral fluid transport along the bottom surface of the substrate, with the top surface remaining dry, except at the initial droplet dispension point. This configuration is investigated in further detail, and an analytical model is developed to predict the temporal variation of the penetrating drop shape. The analysis and respective measurements agree within the experimental error limits, thus confirming the model's ability to account for the main transport mechanisms.

Liquid jet impingement on porous materials is particularly important in many applications of heat transfer, filtration, or in incontinence products. Generally, it is desired that the liquid not penetrate the substrate at or near the point of jet impact, but rather be distributed over a wider area before reaching the back side. A facile wettability-patterning technique is presented, whereby a water jet impinging orthogonally on a wettability-patterned nonwoven substrate is distributed on the top surface and through the porous matrix, and ultimately dispensed from prespecified points underneath the sample. A systematic approach is adopted to identify the optimum design that allows for a uniform distribution of the liquid on horizontally mounted substrates of ∼50 cm2 area, with minimal or no spilling over the sample edges at jet flow rates exceeding 1 L/min. The effect of the location of jet impingement on liquid distribution is also studied, and the design is observed to perform well even under offset jet i...

Spontaneous pumpless transport of droplets on wettability-confined tracks is important for various applications, such as rapid transport and mixing of fluid droplets, enhanced dropwise condensation, biomedical devices, and so forth. Recent studies have shown that on an open surface, a superhydrophilic track of diverging width, laid on a superhydrophobic background, facilitates the transport of water from the narrower end to the wider end at unprecedented rates (up to 40 cm/s) without external actuation. The spreading behavior on such surfaces, however, has only been characterized for water. Keeping in mind that such designs play a key role for a diverse range of applications, such as handling organic liquids and in point-of-care devices, the importance of characterizing the spreading behavior of viscous liquids on such surfaces cannot be overemphasized. In the present work, the spreading behavior on the aforementioned wettability-patterned diverging tracks was observed for fluids of different viscosities. Two dimensionless variables were identified, and a comprehensive relationship was obtained. Three distinct temporal regimes of droplet spreading were established: I), a Washburn-type slow spreading, II) a much faster Laplace pressure-driven spreading, and III), a sluggish density-augmented Tanner-type film spreading. The results offer design guidance for tracks that can pumplessly manage fluids of various viscosities and surface tensions.

The impact of liquid droplets on permeable substrates is important for a number of applications, such as fog collection, liquid atomization, and interaction of liquids with filters and textiles. When a water droplet impacts a wettable mesh, it penetrates the mesh easily with a part of the liquid remaining pinned. On the other hand, when striking a superhydrophobic mesh, part of the water droplet may penetrate and detach from the parent droplet, depending upon the impact velocity and the relative length scales of the droplet and the mesh. In most cases, the remaining droplet would rebound from the top of the superhydrophobic mesh. In this work, we study the impact of a water droplet on a wettability-patterned mesh, with the droplet centrally impacting the wettability-contrast line between the superhydrophobic and superhydrophilic semi-infinite domains. Half of the droplet seeing the superhydrophobic domain responds to it in a fashion that differs from the half hitting the superhydrophilic mesh side. This creates a wide range of post-impact scenarios, depending on the impact conditions and the relative characteristics of the droplet and the mesh. The difference in mesh wettability leads to a net unbalanced surface-tension force that makes the droplet rebound with a horizontal momentum component directed from the non-wettable to the wettable side. Some part of the droplet may even detach during such directional rebounding (i.e., vectoring). Along with the experimental results, a simplified analytical model is presented, which differentiates the cases of detachment or no detachment during vectoring.

The directed transport of liquids at the nanoscale is of great importance for nanotechnology applications ranging from water filtration to the cooling of electronics and precision medicine. Here we demonstrate such unidirectional, pumpless transport of water nanodroplets on graphene sheets patterned with hydrophilic/phobic areas inspired by natural systems. We find that spatially varying patterning of the graphene surfaces can lead to water transport at ultrafast velocities, far exceeding macroscale estimates. We perform extensive molecular dynamics simulations to show that such high transport velocities (O(102 m/s)) are due to differences of the advancing and receding contact angles of the moving droplet. This contact angle hysteresis and the ensuing transport depend on the surface pattern and the droplet size. We present a scaling law for the driving capillary and resisting friction forces on the water droplet and use it to predict nanodroplet trajectories on a wedge-patterned graphene sheet. The present results demonstrate that graphene with spatially variable wettability is a potent material for fast and precise transport of nanodroplets with significant potential for directed nanoscale liquid transport and precision drug delivery.

Frost formation is omnipresent when suitable environmental conditions are met. A good portion of research on combating frost formation has revolved around the passive properties of superhydrophobic (SHPO) and slippery lubricant-impregnated porous (SLIP) surfaces. Despite much progress, the need for surfaces that can effectively combat frost formation over prolonged periods still remains. In this work, we report, for the first time, the use of electrically conductive SHPO/SLIP surfaces for active mitigation of frost formation. First, we demonstrate the failure of these surfaces to passively avert prolonged (several hours) frosting. Next, we make use of their electroconductive property for active Joule heating, which results in the removal of any formed frost. We study the role of the impregnating lubricant in the heat transfer across the interface, the surface, and the ambient. We show that, even though the thermal properties of the impregnating lubricant may vary drastically, the lubricant type does not noticeably affect the defrosting behavior of the surface. We attribute this outcome to the dominant thermal resistance of the thick frost layer formed on the cooled surface. We support this claim by drawing parallels between the present system and heat transfer through a one-dimensional (1D) composite medium, and solving the appropriate transient transport equations. Lastly, we propose periodic thermal defrosting for averting frost formation altogether. This methodology utilizes the coating's passive repellent capabilities, while eliminating the dominant effect of thick deposited frost layers. The periodic heating approach takes advantage of lubricants with higher thermal conductivities, which effectively enhance heat transfer through the porous multiphase surface that forms the first line of defense against frosting.

An acrylic emulsion artists’ paint containing chlorinated copper phthalocyanine pigment was modified with variable-size multilayer graphene (exfoliated graphite) to induce low electrical resistance; composite films were spray-cast on common printing paper, heat-cured, and subsequently polished under mild compression, to produce highly conductive paper. The mechanically robust conductive paint showed excellent adhesion to the underlying paper, as determined by Taber abrasion and tape peel tests, which displayed no adhesive failure under the test conditions studied. The conductivity of the paper substrates were tuned by changing the concentration and the size of the multilayer graphene particles. Detailed conductivity measurements showed stable Ohmic current–voltage behavior. The optimum graphene-in-paint formulations resulted in sheet resistances of the order of 10 Ω/sq. Standard electrostatic force microscopy measurements showed uniform surface electric field gradient distribution strongly correlating with the surface topography. Similarly, scanning Kelvin probe microscopy measurements indicated stable work functions close to 5 eV, comparable to highly-ordered pyrolytic graphite. Furthermore, Kelvin probe measurements were more sensitive to surface charges related to copper phthalocyanine domains, which are known to have semiconducting properties. Finally, the conductive papers were also tested in the 0.50–0.75 terahertz frequency range for electromagnetic interference shielding (EMI) characterization and displayed quasi-metallic shielding performance.

Marine oil spills seriously endanger sea ecosystems and coastal environments, resulting in a loss of energy resources. Environmental and economic demands emphasize the need for new methods of effectively separating oil-water mixtures, while collecting oil content at the same time. A new surface-tension-driven, gravity-assisted, one-step, oil-water separation method is presented for sustained filtration and collection of oil from a floating spill. A benchtop prototype oil collection device uses selective-wettability (superhydrophobic and superoleophilic) stainless steel mesh that attracts the floating oil, simultaneously separating it from water and collecting it in a container, requiring no preseparation pumping or pouring. The collection efficiencies for oils with wide ranging kinematic viscosities (0.32-70.4 cSt at 40 °C) are above 94%, including motor oil and heavy mineral oil. The prototype device showed high stability and functionality over repeated use, and can be easily scaled for efficient cleanup of large oil spills on seawater. In addition, a brief consolidation of separation requirements for oil-water mixtures of various oil densities is presented to demonstrate the versatility of the material system developed herein.

Thermal Brownian motors (TBMs) are nanoscale machines that exploit thermal fluctuations to provide useful work. We introduce a TBM-based nanopump which enables continuous water flow through a carbon nanotube (CNT) by imposing an axial thermal gradient along its surface. We impose spatial asymmetry along the CNT by immobilizing certain points on its surface. We study the performance of this molecular motor using molecular dynamics (MD) simulations. From the MD trajectories, we compute the net water flow and the induced velocity profiles for various imposed thermal gradients. We find that spatial asymmetry modifies the vibrational modes of the CNT induced by the thermal gradient, resulting in a net water flow against the thermal gradient. Moreover, the kinetic energy associated with the thermal oscillations rectifies the Brownian motion of the water molecules, driving the flow in a preferred direction. For imposed thermal gradients of 0.5-3.3 K/nm, we observe continuous net flow with average velocities up to 5 m/s inside CNTs with diameters of 0.94, 1.4, and 2.0 nm. The results indicate that the CNT-based asymmetric thermal motor can provide a controllable and robust system for delivery of continuous water flow with potential applications in integrated nanofluidic devices.

Dropwise condensation (DWC) heat transfer depends strongly on the maximum diameter (Dmax) of condensate droplets departing from the condenser surface. This study presents a facile technique implemented to gain control of Dmax in DWC within vapor/air atmospheres. We demonstrate how this approach can enhance the corresponding heat transfer rate by harnessing the capillary forces in the removal of the condensate from the surface. We examine various hydrophilic-superhydrophilic patterns, which, respectively, sustain and combine DWC and filmwise condensation on the substrate. The material system uses laser-patterned masking and chemical etching to achieve the desired wettability contrast and does not employ any hydrophobizing agent. By applying alternating straight parallel strips of hydrophilic (contact angle ∼78°) mirror-finish aluminum and superhydrophilic regions (etched aluminum) on the condensing surface, we show that the average maximum droplet size on the less-wettable domains is nearly 42% of the width of the corresponding strips. An overall improvement in the condensate collection rate, up to 19% (as compared to the control case of DWC on mirror-finish aluminum) was achieved by using an interdigitated superhydrophilic track pattern (on the mirror-finish hydrophilic surface) inspired by the vein network of plant leaves. The bioinspired interdigitated pattern is found to outperform the straight hydrophilic-superhydrophilic pattern design, particularly under higher humidity conditions in the presence of noncondensable gases (NCG), a condition that is more challenging for maintaining sustained DWC.

Many important applications in fluid management could benefit from unidirectional transport through porous media via a simple, large-area, low-cost coating treatment; in essence, a fluid diode demonstrated herein for water using common cellulosic paper substrates. In electronics, the diode is an electrical component with asymmetric current transfer characteristics. A light (, ACS Applied Materials & Interfaces, 6 (15), ISSN:1944-8244, ISSN:1944-8252

Electromagnetic interference (EMI) shielding reduces coupling between signals, crosstalk among electrical components, noise in cables and communication systems, etc. With the increasing speed of terahertz (THz) electronic circuits and systems, THz EMI shielding is becoming increasingly more important. We review recent and pioneering studies on shielding property–structure characterization and applications of carbon nanocomposite materials in the THz frequency domain. The theory of EMI shielding by nanocomposite materials is summarized first. A description of relevant fabrication methods follows, along with structural characterization details. THz probing and characterization of carbon nanofillers and their composites as EMI shielding and attenuation materials is presented next. Finally, the application of these materials in quasi-optical THz components, including polarizers and potentially mesh filters, as well as related manufacturing techniques are reviewed and discussed. Specific examples are presented in some detail to introduce the reader to this exciting and rapidly evolving technological area.

We demonstrate the fabrication of free-standing polymeric nanocomposite films, which present magnetic and electrically conductive anisotropic properties. Magnetically functionalized carbon nanofibers are dispersed in a polymeric solution and, upon casting under a weak external magnetic field, are easily oriented and permanently assembled in a head-to-tail orientation in the polymer film during solvent evaporation. Magnetic and conductive property studies reveal that the resulting films have a high degree of anisotropy in both cases, thus allowing their use in functional complex devices. As a proof of concept, we demonstrate the potential application of these films as flexible THz polarizers. The detailed study shows that very high attenuation values per unit film thickness and fiber mass concentration are achieved, paving thus the way for cost-effective fabrication of substrate-free systems that have advantage over conventional devices realized so far.

Low-cost, large-area, superhydrophobic coating treatments are of high value to technological applications requiring efficient liquid repellency. While many applications are envisioned, only few are realizable in practice due to either the high cost or low durability of such treatments. Recently, spray deposition of polymer–particle dispersions has been demonstrated as an excellent means for producing low-cost, large-area, durable, superhydrophobic composite coatings/films; however, such dispersions generally contain harsh or volatile solvents, which are required for solution processing of polymers as well as for dispersing hydrophobic nanoparticles, thus inhibiting scalability due to the increased cost in chemical handling and environmental safety concerns. Moreover, such coatings usually contain fluoropolymers due to their inherent low surface energy, a requirement for superhydrophobicity, but concerns over their biopersistence has provided an impetus for eliminating these chemicals. For spray coating, the former problem can be overcome by replacing organic solvents with water, but this situation seems paradoxical: Producing a highly water-repellent coating from an aqueous dispersion. We report a water-based, nonfluorinated dispersion for the formation of superhydrophobic composite coatings applied by spray on a variety of substrates. We stabilize hydrophobic components (i.e., polymer, nanoparticles) in water, by utilizing chemicals containing acid functional groups (i.e., acrylic acid) that can become ionized in aqueous environments under proper pH control (pH > 7). The functional polymer utilized in this study is a copolymer of ethylene and acrylic acid, while the particle filler is exfoliated graphite nanoplatelet (xGnP), which contains functional groups at its periphery. Once spray deposited and dried, the components become insoluble in water, thus promoting liquid repellency. Such coatings can find a wide range of applications due to their benign processing nature as well as the variety of substrates on which they can be deposited., ACS Applied Materials & Interfaces, 5 (24), ISSN:1944-8244, ISSN:1944-8252

A solution-based, large-area coating procedure is developed to produce conductive polymer composite films consisting of hollow-core carbon nanofibers (CNFs) and a fluoroacrylic co-polymer available as a water-based dispersion. CNFs (100 nm dia., length ∼130 μm) were dispersed by sonication in a formic acid/acetone co-solvent system, which enabled colloidal stability and direct blending of the CNFs and aqueous fluoroacrylic dispersions in the absence of surfactants. The dispersions were sprayed on smooth and microtextured surfaces, thus forming conformal coatings after drying. Nanostructured composite films of different degrees of oil and water repellency were fabricated by varying the concentration of CNFs. The effect of substrate texture and CNF content on oil/water repellency was studied. Water and oil static contact angles (CAs) ranged from 98° to 164° and from 61° to 164°, respectively. Some coatings with the highest water/oil CAs displayed self-cleaning behavior (droplet roll-off angles

Rheological behavior of concentrated suspensions of chemical vapor deposition carbon nanotubes in uniaxial elongation and simple shear is studied experimentally and theoretically. Nanotubes are suspended in viscous host liquids—castor oil or its blends with n-decane. The elongational measurements are performed by analyzing self-thinning (due to surface tension effect) liquid threads of nanotube suspensions. A quasi-one-dimensional model is used to describe the self-thinning process, whereas corrections accounting for thread nonuniformity and necking are introduced a posteriori. The effects of nanotube concentration and aspect ratio, viscosity of the suspending liquid, and initial diameter of the self-thinning thread in uniaxial elongation are elucidated. The results for uniaxial elongation are compared with those for simple shear. The correspondence in the results of the shear and elongational measurements is addressed and interpreted. The results conform to the Herschel–Bulkley rheological constitutive equation (i.e., power law fluids with yield stress). However, the yield stress in elongation is about 40% higher than in simple shear flow, which suggests that the original Herschel–Bulkley model need modification with the yield stress being a function of the second invariant of the deviatoric stress tensor. The present effort is the first to study capillary self-thinning of Herschel–Bulkley liquids, which are exemplified here by suspensions of carbon nanotubes.

A mild-temperature, nonchemical technique is used to produce a nanohybrid multifunctional (electro-conducting and magnetic) powder material by intercalating iron oxide nanoparticles in large aspect ratio, open-ended, hollow-core carbon nanofibers (CNFs). Single-crystal, superparamagnetic Fe3O4 nanoparticles (10 nm average diameter) filled the CNF internal cavity (diameter

We present a method to fill 2−5-nm-diameter channels of closed multiwalled carbon nanotubes (MWNT) with an aqueous fluid and perform in situ high-resolution observations of fluid dynamic behavior in this confined system. Transmission electron microscope (TEM) observations confirm the successful filling of two types of MWNTs and reveal disordered gas/liquid interfaces contrasting the smooth curved menisci visualized previously in MWNT with diameter above 10 nm. Electron energy loss spectroscopy (EELS) and energy dispersive spectrometry (EDS) analyses, along with TEM simulation, indicate the presence of water in MWNT. A wet−dry transition on the nanometer scale is also demonstrated by means of external heating. The results suggest that when ultrathin channels such as carbon nanotubes contain water, fluid mobility is greatly retarded compared to that on the macroscale. The present findings pose new challenges for modeling and device development work in this area.

A novel approach in creating circuit electrodes with features as fine as 100 μm is demonstrated using a single 38 μm diameter orifice, piezoelectrically driven print head to deposit metallic nanoparticle suspensions. The suspensions consist of gold particles of ∼20 nm diameter suspended in toluene solvent. The amount of gold nanoparticles present in the suspension is 30% wt. Inductor and capacitor electrode patterns are deposited onto a glass substrate and thermally processed at 300°C for 15 minutes to drive off the solvent and allow the nanoparticles to sinter, thereby yielding a conductive path with a resistivity of O(10−7 ) Ω m.Copyright © 2003 by ASME

A transmission electron microscopy examination of hydrothermally produced carbon nanofibres/nanotubes with outer diameter 50–200 nm suggests that the tube walls are inclined with respect to the tube axis. The apex angles are in the range 8°–16°. The structure of these tubes and their growth mechanism can be described by a conical-scroll model. The conical-scroll structure enables functionalization of both inner and outer tube surfaces. The outer wall of these nanofibres is shown to be covered by a hair-like layer, with a characteristic length of about 0.5 nm. Electron energy loss spectroscopy suggests that these 'hairs' are functional groups containing oxygen and carbon. The presence of these groups on the tube surface can account for the reported hydrophilic character of these tubes.

This paper combines a theoretical model with experimental measurements to elucidate the role of key operating parameters affecting solder microdroplet deposition in the electronics manufacturing industry. The experimental investigation is used to evaluate the final deposit (bump) shapes and trends predicted by the model. The effects of substrate temperature, material composition, layer thickness, and thermal contact resistance (including surface oxidation) are delineated. Solder-deposit shape comparisons between experiments and modeling suggest that the value of thermal contact resistance may change with process parameters, and is probably dependent on the solder phase. It is established that inferences regarding the overall shape or solidification times of solder bumps using limited modeling trends should be made only after careful consideration of the substrate composition, accurate representation of the thermal contact resistance, and adequate resolution of the fluid dynamical oscillatory motion and its effects on solidification rates. It is shown that modeling tools can be used in conjunction with experiments to promote our fundamental understanding of the transport processes in the complex solder jetting technology.

Carbon nanotube (CNT) membranes hold the promise of extraordinary fast water transport for applications such as energy efficient filtration and molecular level drug delivery. However, experiments and computations have reported flow rate enhancements over continuum hydrodynamics that contradict each other by orders of magnitude. We perform large scale molecular dynamics simulations emulating for the first time the micrometer thick CNTs membranes used in experiments. We find transport enhancement rates that are length dependent due to entrance and exit losses but asymptote to 2 orders of magnitude over the continuum predictions. These rates are far below those reported experimentally. The results suggest that the reported superfast water transport rates cannot be attributed to interactions of water with pristine CNTs alone.

A novel method of smoothing surfaces with nanoscale roughness is described, based on the Kelvin effect. The problem of vapor redistribution in cylindrical channels and over rough planar walls with nanoscale texture is posed and solved analytically. Vapor deposition (condensation) on the walls initially produces a deposit emulating the surface landscape. After a saturated state at the deposit surface is reached, the Kelvin effect should result in higher vapor pressure/ concentration near the convex sections of the wall and in lower vapor pressure/ concentration near the concave sections. As a result, local vapor fluxes should arise directed from the locally convex to the locally concave regions. Accordingly, the deposited layer at the wall should vaporize (or sublimate) at the convex sections due to depletion and vapor should condense at the concave sections, thus causing smoothing of physical surface unevenness. This mechanism of smoothing of nanoscale roughness has not been considered in detail or used before, even though the basic physics of the Kelvin effect is well known. In the present work, the smoothing kinetics is predicted and the characteristic timescales are calculated in the general case of axisymmetric and non-axisymmetric perturbations of the cylindrical channel walls, as well as for planar surfaces. In addition, experimental data are presented to show that the theoretically motivated approach is also practically realizable.

Ice formation can have catastrophic consequences for human activity on the ground and in the air. Here we investigate water freezing delays on untreated and coated surfaces ranging from hydrophilic to superhydrophobic and use these delays to evaluate icephobicity. Supercooled water microdroplets are inkjet-deposited and coalesce until spontaneous freezing of the accumulated mass occurs. Surfaces with nanometer-scale roughness and higher wettability display unexpectedly long freezing delays, at least 1 order of magnitude longer than typical superhydrophobic surfaces with larger hierarchical roughness and low wettability. Directly related to the main focus on heterogeneous nucleation and freezing delay of supercooled water droplets, the observed ensuing crystallization process consisted of two distinct phases: one very rapid recalescent partial solidification phase and a subsequent slower phase. Observations of the droplet collision process employed for the continuous liquid mass accumulation up to the point of ice formation reveal a previously unseen atmospheric-pressure, subfreezing-temperature regime for liquid-on-liquid bounce. On the basis of the entropy reduction of water near a solid surface, we formulate a modification to the classical heterogeneous nucleation theory, which predicts the observed freezing delay trends. Our results bring to question recent emphasis on super water-repellent surface formulations for ice formation retardation and suggest that anti-icing design must optimize the competing influences of both wettability and roughness., Langmuir, 27 (6), ISSN:0743-7463, ISSN:1520-5827

Super-repellent nanostructured composite coatings applied over large areas by spray and subsequent thermal treatment are reported. Solution blending of poly(vinylidene fluoride) and poly(ethyl 2-cyanoacrylate) is implemented to formulate filler particle dispersions used to apply these coatings. The wettability of these coatings is manipulated using hydrophobic poly(tetrafluoroethylene) and hydrophilic zinc oxide particle fillers or their combination. The resulting coatings feature contact angles up to 164 degrees for water and 154 degrees for a water and isopropyl alcohol mixture (9:1 weight ratio; surface tension approximately 40 mN/m). A self-cleaning ability is revealed by droplet roll-off angles below 10 degrees . The results show that the fillers affect the coating surface energy and surface roughness, in turn influencing the wettability of the coatings.

Thermosensitive hydrogels are materials which globally shrink/swell in water when the surrounding temperature crosses the lower critical solution temperature (LCST). We demonstrate here a novel class of cross-linked polymeric materials, which do not shrink/swell in water globally, but nevertheless reveal a hydrogel-like, stimuli-responsive behavior. In particular, they demonstate a positive thermosensitive release of the embedded fluorescent dye significantly modulated when temperature crosses the LCST. Using staining with copper, transmission electron microscopy and energy dispersive X-ray analysis, we show that this effect is associated with nanogel "raisins" dispersed in such materials (e.g., polymer nanofibers). Shrinkage of individual nanogel "raisins" at elevated temperatures increases nanoporosity via increased exposure of the existing nanopores to water, or formation of new nanopores/nanocracks in the overstretched polymer matrix in the vicinity of shrinking nanogel "raisins". As a result, the release rate of the embedded dye from the nanofibers increases at elevated temperatures. We suggest that similar functional materials with embedded nanogel "raisins" will find applications in nanofluidics and as drug carriers for controlled drug release.

A model is developed from the general aerosol dynamic equation, using a bimodal integral formulation that includes particle formation and growth by coagulation in the free molecular regime. The particle inception mode accounts for the introduction of newly formed particles which, through coagulative collisions with one another, constitute the source of the particles in the growth mode. A numerical solution for the system of the first three moments of the particle volume distribution function is discussed, under the assumption of a logarithmic-normal behavior of the two modes of the size distribution function. The bimodal integral solution is subject to a detailed comparison with the MAEROS sectional model for the case of an aerosol that undergoes free molecular coagulation occurring simultaneously with particle formation by a Gaussian source pulse, under flamelike conditions.

This work examines the release of a model water-soluble compound from electrospun polymer nanofiber assemblies. Such release attracts attention in relation to biomedical applications, such as controlled drug delivery. It is also important for stem cell attachment and differentiation on biocompatible electrospun nanofiber scaffolds containing growth factors, which have been encapsulated by means of electrospinning. Typically, the release mechanism has been attributed to solid-state diffusion of the encapsulated compound from the fibers into the surrounding aqueous bath. Under this assumption, a 100% release of the encapsulated compound is expected in a certain (long) time. The present work focuses on certain cases where complete release does not happen, which suggests that solid-state diffusion may not be the primary mechanism at play. We show that in such cases the release rate can be explained by desorption of the embedded compound from nanopores in the fibers or from the outer surface of the fibers in contact with the water bath. After release, the water-soluble compound rapidly diffuses in water, whereas the release rate is determined by the limiting desorption stage. A model system of Rhodamine 610 chloride fluorescent dye embedded in electrospun monolithic poly(methylmethacrylate) (PMMA) or poly(caprolactone) (PCL) nanofibers, in nanofibers electrospun from PMMA/PCL blends, or in core-shell PMMA/PCL nanofibers is studied. Both the experimental results and theory point at the above mentioned desorption-related mechanism, and the predicted characteristic time, release rate, and effective diffusion coefficient agree fairly well with the experimental data. A practically important outcome of this surface release mechanism is that only the compound on the fiber and pore surfaces can be released, whereas the material encapsulated in the bulk cannot be freed within the time scales characteristic of the present experiments (days to months). Consequently, in such cases, complete release is impossible. We also demonstrate how the release rate can be manipulated by the polymer content and molecular weight affecting nanoporosity and the desorption enthalpy, as well as by the nanofiber structure (monolithic fibers, fibers from polymer blends, and core-shell fibers). In particular, it is shown that, by manipulating the above parameters, release times from tens of hours to months can be attained.

A room-temperature, open-air method is devised to selectively intercalate relatively low-molecular-weight polymers (approximately 10-100 kDa) from dilute, volatile solutions into open-end, as-grown, wettable carbon nanotubes with 50-100 nm diameters. The method relies on a novel self-sustained diffusion mechanism driving polymers from dilute volatile solutions into carbon nanotubes and concentrating them there. Relatively low-molecular-weight polymers, such as poly(ethylene oxide) (PEO, 600 kDa) and poly(caprolactone) (PCL, 80 kDa), were encapsulated in graphitic nanotubes as confirmed by transmission electron microscopy, which revealed morphologies characteristic of mixtures in nanoconfinements affected by intermolecular forces. Whereas relatively small, flexible polymer molecules can conform to enter these nanotubes, larger macromolecules (approximately 1000 kDa) remain outside. The selective nature of this process is useful for filling nanotubes with polymers and could also be valuable in capping nanotubes.

Co-electrospinning is ideally suited for fabricating continuous fibers encasing materials within a polymer sleeve, but requires relatively complex coannular nozzles. A single-nozzle co-electrospinning technique is demonstrated using blends of poly(methyl methacrylate) (PMMA)/polyacrylonitrile (PAN) solutions in dimethylformamide (DMF). The as-spun fibers have outer diameters in the range of 0.5-5 microm and possess a core-shell structure similar to that attained via coannular nozzles. The technique relies on the precipitation of PMMA solution droplets, which become trapped at the base of the Taylor cone issuing the PAN solution jet from its tip. A theoretical analysis shows that the outer shell flow is sufficiently strong to stretch the inner droplet into the Taylor cone, thus forming a core-shell jet. The method seems attractive for technological applications involving macroscopically long and radially inhomogeneous or hollow nano/micro fibers.

Surfaces patterned with alternating (binary) superhydrophobic-superhydrophilic regions can be found naturally, offering a bio-inspired template for efficient fluid collection and management technologies. We describe a simple wet-processing, thermal treatment method to produce such patterns, starting with inherently superhydrophobic polysilsesquioxane-silica composite coatings prepared by spray casting nanoparticle dispersions. Such coatings become superhydrophilic after localized thermal treatment by means of laser irradiation or open-air flame exposure. When laser processed, the films are patternable down to ∼100 μm scales. The dispersions consist of hydrophobic fumed silica (HFS) and methylsilsesquioxane resin, which are dispersed in isopropanol and deposited onto various substrates (glass, quartz, aluminum, copper, and stainless steel). The coatings are characterized by advancing, receding, and sessile contact angle measurements before and after thermal treatment to delineate the effects of HFS filler concentration and thermal treatment on coating wettability. SEM, XPS and TGA measurements reveal the effects of thermal treatment on surface chemistry and texture. The thermally induced wettability shift from superhydrophobic to superhydrophilic is interpreted with the Cassie-Baxter wetting theory. Several micropatterned wettability surfaces demonstrate potential in pool boiling heat transfer enhancement, capillarity-driven liquid transport in open surface-tension-confined channels (e.g., lab-on-a-chip), and select surface coating applications relying on wettability gradients. Advantages of the present approach include the inherent stability and inertness of the organosilane-based coatings, which can be applied on many types of surfaces (glass, metals, etc.) with ease. The present method is also scalable to large areas, thus being attractive for industrial coating applications.

A wide array of technological applications requires localized high-rate delivery of dissolved compounds (in particular, biological ones), which can be achieved by forcing the solutions or suspensions of such compounds through nano or microtubes and their bundled assemblies. Using a water-soluble compound, the fluorescent dye Rhodamine 610 chloride, frequently used as a model drug release compound, it is shown that deposit buildup on the inner walls of the delivery channels and its adverse consequences pose a severe challenge to implementing pressure-driven long-term fluidic delivery through nano and microcapillaries, even in the case of such homogeneous solutions. Pressure-driven delivery (3-6 bar) of homogeneous dye solutions through macroscopically-long (approximately 1 cm) carbon nano and microtubes with inner diameters in the range 100 nm-1 microm and their bundled parallel assemblies is studied experimentally and theoretically. It is shown that the flow delivery gradually shifts from fast convection-dominated (unobstructed) to slow jammed convection, and ultimately to diffusion-limited transport through a porous deposit. The jamming/clogging phenomena appear to be rather generic: they were observed in a wide concentration range for two fluorescent dyes in carbon nano and microtubes, as well as in comparable transparent glass microcapillaries. The aim of the present work is to study the physics of jamming, rather than the chemical reasons for the affinity of dye molecules to the tube walls.

Description de l'affinement de l'echantillonnage thermophoretique permettant une amelioration de la qualite des informations. Les mesures sont effectuees sur une flamme a diffusion d'ethylene laminaire et asymetrique