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The process of localized cleaning of silver sulphide from the silver surface has been investigated in this work. An artificial black encrustation was generated on the silver surface and its laser cleaning was performed using an excimer laser (KrF based) working at a wavelength of 248 nm and pulse width 25 ns. Laser process parameters i.e. laser fluence and number of pulses were used to perform the experiment and after different optical, microstructural, elemental, microhardness and surface topography analysis of the laser ablated zone was performed. The parametric window selected for the excimer laser cleaning were laser fluence (200-400 mJ/cm2), repetition rate 4 Hz and number of pulses (60-300). It was observed that excimer laser is very effective in the cleaning of black encrustation having 6 wt. % of sulphur from the surface without any significant removal of the silver substrate. The fluence value of 260.48 mJ/cm2 was observed as above threshold value for removal of sulphide from the encrusted surface while a fluence of 384.85 mJ/cm2 and 220 pulses showed the maximum removal of sulphur encrustation (0.38 wt. % of sulphur remaining). Formation of the peripheral rim was observed to occur above 280.17 mJ/cm2 of fluence value. The topography of the ablated surface showed a removal of 5-10 um of the sulphide layer from the surface whereas the thickness of the sulphide coating was around 8-10 um. Microhardness value showed a change of microhardness of the silver surface from 75.12 HV0.5 (uncoated parent silver) to 96.07 HV0.5 (centre of the ablated zone).Finally, it was concluded that excimer laser cleaning is a novel method for localized removal of silver sulphide encrustation from silver surface., Comment: 20 pages

Axial gradients in wall elasticity may have significant implications in the deformation and flow characteristics of a narrow fluidic conduit, bearing far-reaching consequences in physiology and bio-engineering. Here, we present a theoretical and experimental framework for fluid-structure interactions in microfluidic channels with axial gradients in wall elasticity, in an effort to arrive at a potential conceptual foundation for in vitro study of mirovascular physiology. Towards this, we bring out the static deformation and steady flow characteristics of a circular microchannel made of polydimethylsiloxane (PDMS) bulk, considering imposed gradients in the substrate elasticity. In particular, we study two kinds of elasticity variations - a uniformly soft (or hard) channel with a central strip that is hard (or soft), and, increasing elasticity along the length of the channel. The former kind yields a centrally constricted (or expanded) deformed profile in response to the flow. The latter kind leads to increasingly bulged channel radius from inlet to outlet in response to flow. We also formulate an analytical model capturing the essential physics of the underlying elastohydrodynamic interactions. The theoretical predictions match favourably with the experimental observations and are also in line with reported results on stenosis in mice. The present framework, thus, holds the potential for acting as a fundamental design basis towards developing in vitro models for micro-circulation, capable of capturing exclusive artefacts of healthy and diseased conditions.

This paper represents a numerical and experimental investigation of fluids mixing in straight and serpentine micro-channels. These microchannels are fabricated with the help of soft lithography technique. The width of straight and serpentine micro-channels after fabrication is obtained around 335 and 282 µm respectively. Numerical simulations of fluids mixing in micro-channels are performed by solving momentum, continuity and mass diffusion equations. Mixing performance has been evaluated in three average inlet velocities such as 0.001, 0.007 and 0.0167 m/s. The numerical results show that in serpentine micro-channel, an effective mixing has been achieved in almost all the flow conditions whereas in straight channel that is achieved for 0.001 m/s velocity only. Water and water-KMnO4 solution are used as working fluids in this study. The experimental results show that the mixing efficiency of around 95.5, 70.20, and 48.54% has been achieved for straight channel at inlet velocities 0.001, 0.007 and 0.0167 m/s respectively. However, in serpentine micro-channel, the mixing efficiency has been achieved around 97.12, 95.10, and 65.21% at inlet velocities 0.007, 0.0167 and 0.03 m/s respectively. Moreover, the experimental results are compared with the numerical results where it has been observed that the results are quite identical to each other.

This study demonstrates controlled maneuvering of polystyrene particles in a simple microfluidic device by exploiting the underlying electrokinetic phenomena, primarily dielectrophoresis (DEP). The device mainly uses negative DEP force to trap micrometer-sized particles near the positive electrode in a low-conductivity suspension, but also exploits positive DEP force if the conductivity increases. This device furthermore shows flexibility in selective yet localized trapping with low electric field requirements, laying a foundation for flexible preconcentrators in low-cost bioanalytical detection systems for $e.g.$ medical diagnostics.

Enhancing the sensitivity of colorimetric detection in paper-devices is a quintessential step in achieving frugal diagnosis. Here, we demonstrate an effective way of improving the detection sensitivity of paper-based devices, as mediated by electro-kinetic mechanisms. By directly employing blood plasma, we investigate the electro-kinetic clustering of glucose, a neutral molecule in paper devices. Under the influence of uniform electric field, dispersed glucose gets accumulated in the paper strips. Due to the combination of electroosmotic flow and electrophoretic migration, we achieve two-fold increase in the colour intensity for both normal and diabetic samples. This approach is robust and possesses better sensitivity than conventional colorimetric assays and can be easily extended to other body fluid based diagnosis. These results may turn out to be of profound importance in improving the quality of pathological diagnosis in low-cost paper-based point-of-care devices deployed in resourcelimited settings

Diffusion of colored dye on water saturated paper substrates has been traditionally exploited with great skill by renowned water color artists. The same physics finds more recent practical applications in paper based diagnostic devices deploying chemicals that react with a bodily fluid yielding colorimetric signals for disease detection. During spontaneous imbibition through the tortuous pathways of a porous electrolyte saturated paper matrix, a dye molecule undergoes diffusion in a complex network of pores. The advancing front forms a strongly correlated interface that propagates diffusively but with an enhanced effective diffusivity. We measure this effective diffusivity and show that it is several orders of magnitude greater than the free solution diffusivity and has a significant dependence on the solution pH and salt concentration in the background electrolyte. We attribute this to electrically mediated interfacial interactions between the ionic species in the liquid dye and spontaneous surface charges developed at porous interfaces, and introduce a simple theory to explain this phenomenon., 14

The present study deals with the effect of surfactants on the cross-stream migration of droplets in a confined fluidic environment, both experimentally and theoretically. Presence of an imposed flow induces droplet deformation and disturbs the equilibrium that results in subsequent surfactant redistribution along the interface. This further creates a gradient in surface tension, thus generating a Marangoni stress that significantly alters the droplet dynamics. On subsequent experimental investigation, it is found that presence of surfactants reduces the cross-stream migration velocity of the droplet. High-speed photography is utilized to visualize the transport of droplets in a microfluidic channel. It is shown that the channel confinement significantly enhances the surfactant-induced retardation of the droplet. In addition, a larger surfactant concentration is found to induce a greater reduction in cross-stream migration velocity of the droplet, the effect of which is reduced when the initial transverse position of the droplet is shifted closer to the channel centerline. To support our experimental results, an asymptotic approach is adopted to solve the flow field in the presence of bulk-insoluble surfactants and under the assumption of small shape deformation. A good match between our theoretical prediction and the experimental results is obtained. The present analysis provides us with a wide scope of application towards various droplet-based microfluidic as well as medical diagnostic devices where manipulation of droplet trajectory is a major issue.

We exploit the combinatorial advantage of electrokinetics and tortuosity of a cellulose-based paper network on laboratory grade filter paper for the development of a simple, inexpensive, yet extremely robust (shows constant performance for 12 days) 'paper-and-pencil'-based device for energy harvesting applications. We successfully achieve harvesting of a maximum output power of ∼640 pW in a single channel, while the same is significantly improved (by ∼100 times) with the use of a multichannel microfluidic array (maximum of up to 20 channels). Furthermore, we also provide theoretical insights into the observed phenomenon and show that the experimentally predicted trends agree well with our theoretical calculations. Thus, we envisage that such ultra-low cost devices may turn out to be extremely useful in energizing analytical microdevices in resource limited settings, for instance, in extreme point of care diagnostic applications.

We investigate electroosmotic flow of two immiscible viscoelastic fluids in a parallel plate microchannel. Contrary to traditional analysis, the effect of the depletion layer is incorporated near the walls, thereby capturing the complex coupling between rheology and electrokinetics. Toward ensuring realistic prediction, we show the dependence of electroosmotic flow rate on the solution pH and polymer concentration of the complex fluid. In order to assess our theoretical predictions, we have further performed experiments on electroosmosis of an aqueous solution of polyacrylamide (PAAm). Our analysis reveals that neglecting the existence of a depletion layer would result in grossly incorrect predictions of the electroosmotic transport of such fluids. These findings are likely to be of importance in understanding electroosmotically driven transport of complex fluids, including biological fluids, in confined microfluidic environments.