arXiv subjects

Suman Chakraborty

Publications and source records attributed to Suman Chakraborty.

At least 73 records · Page 4Linked to original sources

arXiv2018

Electric field- induced Pinch-off of a migrating compound droplet in confined micro channel

The present study looks into the pinch-off dynamics of a compound droplet, which is suspended in another fluid in a parallel plate microchannel. The droplet is subjected to a transverse electric field in the presence of an imposed pressure driven flow. For the present study, a leaky dielectric model have been taken into consideration. When a concentric compound droplet migrates in a pressure driven flow, the inner droplet shifts from the concentric position and forms a eccentric configuration that finally leads to the rupture of the outer shell. The present investigation have uncovered that the temporal evolution of droplet eccentricity as well as the kinetics of the thinning of the outer droplet are markedly influenced by the strength of the electric field as well as the electric properties of the system. The present study also shows that the conversion of different modes of droplet pinch-off mode , such as the equatorial cap breaking-off or the hole-puncturing mode can be attained by altering the electric field strength and its electrical properties. Finally the present study depicts that these factors also alter the pinch-off time as well as its location on the outer interface. Therefore the outcomes of the present study offers an effective means of modulating the morphology of compound droplets in a confined channel by applying an electric field.

physics.flu-dyn↗

arXiv2018

Pitcher Plant Inspired Biomimetic Liquid Infused Slippery Surface Using Taro Leaf

Bio-inspired anti-wetting surfaces, such as lotus leaf or pitcher plant, have led to the development of stable liquid infused slippery surfaces for various scientific applications. The present work demonstrates the use of biomimetic superhydrophobic surface (inspired from taro leaf) for fabrication of a stable air-liquid film. The taro leaf replica is fabricated on PDMS using two step soft-molding technique, where microstructures of the replicated surface are used to retain silicone oil layer to form a stable slippery surface. The fabricated surface exhibits a low contact angle hysteresis (CAH) of 2°, with sliding angle (SA) of 1.1°, which further affirms the super-slippery nature of the surface. Furthermore, it also shows excellent self-repairing ability, thermal stability and long-term durability of the oil coating against high shear rates, high impact droplets etc. We thus envisage that the present method of fabrication of slippery surface is simple and economical, and thus useful for various applications such as water drag reduction, self-cleaning, anti-fogging, anti-fouling etc.

cond-mat.soft↗

arXiv2018

Interfacial viscosity-induced suppression of lateral migration of a surfactant-laden droplet in a non-isothermal Poiseuille flow

Understanding and modulating the cross-stream motion of a surfactant-coated droplet in pressure driven flow has great implications in many practical applications. A combination of interfacial viscosity and Marangoni stress acting over a surfactant-coated droplet in pressure driven flow offers greater flexibility to modulate the cross-stream motion of it. Despite the intense theoretical and numerical research towards manipulating the surfactant-laden Newtonian droplets in Poiseuille flow, the experimental investigations are seldom explored. Herein, we report our study on understanding the influence of interfacial viscosity on the cross-stream motion of a surfactant-coated Newtonian droplet in both isothermal and non-isothermal Poiseuille flow from a theoretical as well as an experimental perspective. A theoretical model has been developed to understand the effect of interfacial viscosity on the lateral migration of a droplet under the assumptions of no shape deformation and negligible fluid inertia or thermal convection. Theoretical analysis is performed under two limiting conditions: (i) when the transport of surfactants is dominated by surface-diffusion and (ii) when the transportation of surfactants is dominated by surface-convection. Our theoretical analysis shows that both the dilatational as well as the shear surface viscosities suppress the lateral migration velocity of the droplet. Experiments have been performed to validate the theoretically predicted droplet trajectories and to understand the influence of channel confinement on the lateral migration of the droplet. It has been observed from the experiments that the droplet travels faster towards the centerline of the flow in a highly confined domain. The results presented in this study could provide new vistas in designing and analyzing various droplet-based microfluidic, biomedical and bio-microfluidic devices.

physics.flu-dyn↗

arXiv2018

Controlled Electrokinetic Particle Manipulation using Paper-and-Pencil Microfluidics

Dielectrophoresis is a very promising technique for particle manipulation on a chip. In this study, we demonstrate a controlled mannuvering of polystryrene particles on a simple paper-and-pencil based device by exploiting the underlying electrokinetics with primary contribution from dielectrophoretic (DEP) forces. On contrary to other reported DEP devices, the present configuration does not demand a shophitcated laboratory module for creating a non-uniform electric field, which is essential requirement in DEP settings. We demonstrate positive dielectrophoresis (pDEP) to trap 1 um size polystyrene particle for low-conductivity suspending medium, at an applied field strength of 100 V/cm. In addition, the switching of the trapping direction (positive to negative dielectrophoresis) can be simply achieved by manipulating the conductivity of the media. We further bring out an optimum range of pH for effective particle trapping. These results have significant implications towards designing cell-on-a-chip based point of care diagnostic devices for resource limited settings.

cond-mat.soft↗

arXiv2018

Electrowetting of a nano-suspension on a soft solid

The wetting of solid surfaces can be manoeuvred by altering the energy balance at the interfacial region. While electric field acts favourably to spread a droplet on a rigid surface, this tendency may be significantly suppressed over soft surfaces, considering a part of the interfacial energy being utilized to deform the solid elastically by capillary forces. Here, we bring out a unique mechanism by virtue of which addition of nano-particles to the droplet brings in a favourable recovery of the electro-spreading characteristics of a soft surface, by realizing an alteration in the effective dielectric constant of the interfacial region. Our experiments further demonstrate that this mechanism ceases to be operative beyond a threshold volume fraction of the particle suspension, due to inevitable contact line pinning. We also develop a theory to explain our experimental observations. Our findings provide a non-contact mechanism for meniscus spreading and droplet control, bearing far-reaching implications in biology and engineering.

cond-mat.soft↗

arXiv2018

An electrokinetic route to giant augmentation in load bearing capacity of compliant microfluidic channels

The performances of lubricated systems widely used in natural, biological, and artificial settings are traditionally dictated by their load bearing capacities. Here we unveil that, by exploiting a unique coupling between interfacial electro-mechanics, hydrodynamics and substrate compliance, it is plausible to realize a massive augmentation in the load bearing capacities ofcompliant microfluidic channels. Our analysis demonstrates that the interplay between wettability and charge modulation in association with the solution chemistry and surface compliance results in this remarkable phenomenon. These results are likely to open up novel design paradigms of augmenting the load bearing capacities of miniaturized bio-mimetic units through the realization of a symmetry breaking phenomenon triggered by asymmetries in electromechanical and hydrodynamic transport over interfacial scales.

physics.flu-dyn↗

arXiv2018

Rotational Instabilities in Stratified Miscible Layers

Instability of stratified multi-phase flow in a rotating platform becomes important because of a potential role in micro-mixing and micro-machines. Centrifugal actuation can play an important role in driving the flow and Coriolis force can enhance the mixing in a short span by destabilizing the flow. In this study, we focus on the impact of the Coriolis force on a rotating viscosity-stratified flow with a thin diffusive mixed layer between two fluid layers. Modal stability analysis is used to estimate the critical parameters, namely Rotation number, Reynolds number, and wave number, which are responsible to modulate the instability mechanism for different viscosity contrasts. Present study explores competing influences of rotational forces against the viscous and inertial forces. Correspondingly, rotational direction (clockwise/anticlockwise) shows a significant effect on the spatio-temporal instability mechanism and anticlockwise rotation promotes the instability. Usually, miscible viscosity stratified flow with respect to streamwise disturbance becomes more unstable for a thinner mixed layer. On the contrary, our numerical computation confirms a completely contrasting scenario, considering Coriolis force driven instability of a miscible system on account of spanwise disturbances. Possible physical mechanisms for the same are discussed in terms of base flow and energy fluctuation among perturbed and base flow. Comparison of two and three-dimensional instability is done to give a clear-cut idea about the linear instability of the flow system considered herein. Velocity and viscosity perturbation distributions display a critical bonding between the vortices near and away from mixed layer, which may be responsible for the variation of instability with respect to viscosity ratio and rotational direction.

physics.flu-dyn↗

arXiv2018

Electrothermally induced flow in AC EWOD droplet

We show a novel way of generating electrohydrodynamic motion inside a droplet under AC electrowetting. Using a very low applied voltage (10<V) strong electrothermal motion can be generated in the droplet. Alternating current electrothermal (ACET) actuation originates due to imposed Joule heating into the fluid body via applied electric field. A parametric study based on operating parameters and geometrical parameter is conducted experimentally and numerically. With the operating ranges of parameters, high fluid velocity of the order of mm/s can be generated inside the droplet. Further improvement in fluid motion can be anticipated through optimization technique. The investigations revealed in the analysis can be used as useful tool in the design of efficient drop-based micromixer.

physics.flu-dyn↗

arXiv2018

Alternating Current Electrothermal Flow for Energy Efficient Thermal Management of Microprocessor Hot Spots

The paper presents numerical results on an innovative concept of an efficient, site-specific cooling technology for microprocessor hot spots using Alternating Current Electrothermal Flow (ACET). ACET deploys an electrokinetic transport mechanism without requiring any external prime mover and can be shown to be highly effective in reducing hot spot temperatures below their allowable limits. The proposed technique leverages the heat source(s) itself to drive the fluid in the ACET cooling mechanism, thereby making this a highly energy efficient active technique. Our parametric analyses present the optimal range of fluid properties, viz. electrical conductivity where the cooling efficiency is maximum. Further, the impact of geometrical parameters as well as input voltages has been characterized. Our results show a reduction of hot spot temperature by more than 30% for flow through a channel of 600 microns under appropriate conditions at an applied AC voltage of 10-12 V. Comparison with other micro-cooler technologies show the proposed technique to be more energy efficient in its range of operation. These results establish the potential of the ACET cooling and can open up a new avenue to be explored for thermal management of miniaturized devices and systems.

physics.app-ph↗

arXiv2018

Directional free-surface flows on a microfluidic chip with interplay of electrical and thermal fields

Free-surface electrokinetic flows have been attracting increasing attention from the research community over recent times, as attributable to their diverse fields of applications ranging from fluid mixing, particle manipulation to bio-chemical processing on a chip. Traditional electrokinetic processes governing free surface flows are mostly effective in manipulating fluids having characteristically low values of the electrical conductivity (lower than 0.085 S/m). Biological and biochemical processes, on the other hand, typically aim to manipulate fluids having higher electrical conductivities. To circumvent the problem of traditional electrokinetic processes in manipulating highly conductive fluids mediated by free surface flows, here we demonstrate a novel on-chip methodology for the same by exploiting the interaction between an alternating electric current and a thermal field. The consequent local gradients in electrical properties can be tuned to direct the flow towards a specific direction on the interface, opening up a new realm of on-chip process control without necessitating the creation of a geometric confinement. Similar concept of free-surface flow can be used to trigger the fluid along a open-channel flow

physics.flu-dyn↗

arXiv2018

Electrohydrodynamics of compound droplet in a microfluidic confinement

The present study deals with the dynamics of a double emulsion confined in a microchannel under the presence of a uniform electric field. Towards investigating the non-trivial electrohydrodynamics of a compound droplet, confined in between two parallel plate electrodes, a phase field approach has been adopted. Under the assumption of negligible fluid inertia and small shape deformation, an asymptotic model is also developed to predict the transient as well as the steady state droplet dynamics in the limiting case of an unbounded suspending medium. The phases involved are either assumed to be perfect dielectric or leaky dielectric. Subsequent investigation shows that shape deformation of either of the interfaces of the droplet increase with rise in the channel confinement for a perfect dielectric system. However, for a leaky dielectric system, the deformation of inner droplet and outer droplet can increase or decrease with the rise in channel confinement depending on the electric properties (conductivity and permittivity) of the system. The present study further takes into account the effect of eccentricity of the inner droplet. Depending on the relative magnitude of the permittivity and conductivity of the system, the inner droplet, if eccentrically located, may exhibit a to and fro or a simple translational motion. It is seen that increase in the channel confinement results in a translational motion of the inner droplet thus rendering its motion independent of any electrical properties. Since the compound droplet model approximately mimics the structure of a cell, the current investigation is expected to provide a significant impact towards predicting the effect of an externally imposed electrical field on the dynamics of a cell in any fluidic confinement and hence has the potential towards widespread applications in the field of medical diagnostics.

physics.flu-dyn↗

arXiv2018

Rotational Instabilities in Microchannel Flows

Mixing in numerous medical and chemical applications, involving overly long microchannels, can be enhanced by inducing flow instabilities. The channel length, is thus shortened in the inertial microfluidics regime due to the enhanced mixing, thereby rendering the device compact and portable. Motivated by the emerging applications of lab-on-a-CD based compact microfluidic devices, we analyze the linear stability of rotationally actuated microchannel flows commonly deployed for biochemical and biomedical applications. The solution of the coupled system of Orr-Sommerfeld (OS) and Squire (SQ) equations yields the growth rate and the neutral curve of the two types of instabilities: (i) the Tollmien-Schlichting (TS) wave and (ii) the Coriolis force-driven instability. We report the existence of four distinct unstable modes (Modes I-IV) at low Reynolds numbers of which only the existence of Mode I is previously known for the present flow configuration. Furthermore, Modes I and II exhibit competing characteristics. We infer that Modes III and IV might cause strong mixing locally by virtue of strong velocity perturbation in proximity to the interface. We quantify the potential of all the modes to induce such localized mixing near the interface using the notion of penetration depth. Further, insight into the mechanism of energy transfer, drawn from the evaluation of kinetic-energy budget, reveals that the Reynolds stress first transfers energy from the mean flow to the streamwise velocity fluctuations. The Coriolis force, thereafter, redistributes the axial momentum into spanwise and wall-normal directions, generating the frequently observed roll-cell structures. A qualitative comparison of our predictions with reported experiments on roll-cells indicates co-existence of Modes I and II.

physics.flu-dyn↗

arXiv2018

Auto-electrophoresis in non-Newtonian media: interaction of rheology and electrocatalytic parameters

Recent findings of possible applications of bio-friendly synthetic self-phoretic swimmers, have motivated the researchers in investigating the various motion-generating mechanisms to optimize the operating characteristics of the same. In this paper we model the auto-electrophoretic motion of a bimetallic (Au-Pt) spherical swimmer in a non-Newtonian medium. In view of the fact that many bio-fluids closely follow the generalized Newtonian rheology, the rheology of the surrounding swimming medium is considered to follow the Carreau-Yasuda viscosity model. Further to capture the experimentally observed effects of peroxide concentration and feedback of generated proton concentration on the surface cation flux, we incorporate a Michaelis-Menten like surface reaction kinetics. The electrocatalytic efficiency defined as a ratio between the cost of swimming of an equivalent passively dragged particle and the input chemical energy via surface reaction, is utilized to assess the influence of the rheological parameters on the swimmer performance. The results indicate that the shear thinning and shear thickening nature of the surrounding fluid causes an enhancement and reduction in swimmer velocity, respectively. However, an increase in the shear thinning effect does not always guarantee an efficient operation of the microswimmer. A competition between the particle velocity and reduction in the drag force on the particle, decides whether the propulsion efficiency will have a relative augmentation or attenuation with varying power law index. We further report the existence of maximum efficiency points for some specific range of Weissenberg number and optimum power law indices. Moreover the behaviour of such optimum operating conditions strongly depends on the non-trivial and highly coupled interplay among the rheology and electrocatalytic parameters.

physics.flu-dyn↗

arXiv2018

Weighted Exponential Random graph models: Scope and large network limits

We study models of weighted exponential random graphs in the large network limit. These models have recently been proposed to model weighted network data arising from a host of applications including socio-econometric data such as migration flows and neuroscience. Analogous to fundamental results derived for standard (unweighted) exponential random graph models in the work of Chatterjee and Diaconis, we derive limiting results for the structure of these models as the number of nodes goes to infinity. Our results are applicable for a wide variety of base measures including measures with unbounded support. We also derive sufficient conditions for continuity of functionals in the specification of the model including conditions on nodal covariates. Finally we include a number of open problems to spur further understanding of this model especially in the context of applications.

math.PR↗

arXiv2018

Electrohydrodynamic settling of drop in uniform electric field at low and moderate Reynolds numbers

Dynamics of a liquid drop falling through a quiescent medium of another liquid is investigated in external uniform electric field. The electrohydrodynamics of a drop is governed by inherent deformability of the drop (defined by capillary number), the electric field strength (defined by Masson number) and the surface charge convection (quantified by electric Reynolds number). Surface charge convection generates nonlinearilty in a electrohydrodynamics problem by coupling the electric field and flow field. In Stokes limit, most existing theoretical models either considered weak charge convection or weak electric field to solve the problem. In the present work, gravitational settling of the drop is investigated analytically and numerically in Stokes limit considering significant electric field strength and surface charge convection. Drop deformation accurate upto higher order is calculated analytically in small deformation regime. Our theoretical results show excellent agreement with the numerical and shows improvement over previous theoretical models. For drops falling with moderate Reynolds number, the effect of Masson number on transient drop dynamics is studied for (i) perfect dielectric drop in perfect dielectric medium (ii) leaky dielectric drop in the leaky dielectric medium. For the latter case transient deformation and velocity obtained for significant charge convection is compared with that of absence in charge convection which is the novelty of our study. The present study suggests that for both the regimes, surface charge convection tends to increase or decrease the settling speed depending upon the ratios of electrical properties. Notably, in the inertial regime, deformation and velocity are seen to be altered prominently in the existence of significant charge convection.

physics.flu-dyn↗

arXiv2018

Electrohydrodynamic migration of a surfactant-coated deformable drop in Poiseuielle flow

In this study we attempt to explore the consequences of surfactant coating on the electrohydrodynamic manipulation of a drop motion in a plane Poiseuielle flow. In addition we consider bulk insoluble surfactants and a linear dependency of the surface tension on the surfactant concentration. Subsequently a double asymptotic perturbation method is used in terms of small electric Reynolds number and capillary number in the limit of a diffusion-dominated surfactant transport mechanism. Also going beyond the widely employed axisymmetric framework, the coupled system of governing differential equations in three dimensions are then solved by adopting the `generalized Lamb solution technique'. The expressions of key variables suggest that the flow curvature of the external flow, the electric field effects and the surfactant effects are coupled in a non-trivial manner, well beyond a linear superposition. A careful investigation shows that surfactant-induced Marangoni stresses interacts with the electrohydrodynamic stresses in a highly coupled fashion. Owing to this, under different combinations of electrical conductivity and permittivity ratios, the Mason number and the applied electric field direction, the surfactants affect differently on the longitudinal as well as cross-stream migration velocity of the drop. The present results may be of utmost importance in providing a deep insight to the underlying complex physical mechanisms. Most importantly the ability of surfactants in selectively controlling the drop motion in different directions, makes them suitable for achieving a new degree of freedom in the electrical actuation of droplets in the microfluidic devices.

physics.flu-dyn↗

arXiv2018

Electrohydrodynamic interaction between droplet pairs in confined shear flow

The present study deals with the numerical as well as asymptotic analysis of the electrohydrodynamic interaction between two deformable droplets in a confined shear flow. Considering both the phases as leaky dielectric, we have performed numerical simulations to study the effect of channel confinement on the drop trajectories in the presence of a uniform electric field. Two important varieties of motion are identified in the present analysis, namely (i) the reversing motion and (ii) the passing over motion. The study suggests that conversion of the passing over motion to the reversing motion or vice versa is possible via modulating the strength of the imposed electric field. Such a conversion of the pattern of droplet migration is also possible in a confined domain due to change in different electrical properties of the system. The present numerical model is also able to predict the pattern of the trajectory of individual droplets depending on the initial distance separating the two. For example, a smaller initial distance results in a reversing motion whereas a passing over motion is predicted when the distance between the droplets is significantly large. However, the final positions of the droplets are found to be independent of their initial positions. Interestingly, presence of electric field is found to prevent droplet coalescence to a certain extent depending on its strength, thus rendering the emulsion stable. A small deformation asymptotic model is also developed under the assumption of negligible fluid inertia to support the numerical results for the limiting case of an unbounded flow. The current investigation successfully presents a novel technique to predict the precise positions of a system of droplets in a micro-channel and how electric field can be used as a tool to modulate droplet trajectories in an emulsion.

physics.flu-dyn↗

arXiv2018

Transient drift of Escherichia coli under diffusing Step nutrient profile

Bacteria such as Escherichia coli (E. coli) exhibit biased motion if kept in a spatially non-uniform chemical environment. Here, we bring out unique time-dependent characteristics of bacterial chemotaxis, in response to a diffusing spatial step ligand profile. The experimentally obtained temporal characteristics of the drift velocity are compared with the theoretical and Monte-Carlo simulation based estimates, and excellent agreements can be obtained. These results bring in new insights on the time-responsive facets of bacterial drift, bearing far reaching implications in understanding their migratory dynamics in the quest of finding foods by swimming toward the highest concentration of food molecules, or for fleeing from poisons, as well as towards the better understanding of therapeutic response characteristics for certain infectious diseases.

physics.bio-ph↗