arXiv subjects

Suman Chakraborty

Publications and source records attributed to Suman Chakraborty.

At least 91 records · Page 5Linked to original sources

arXiv2018

Electric field-induced droplet deflection in microconfined flow

The deflection of liquid droplet driven through a liquid medium under the combined action of transverse electric field and pressure driven flow has been studied in the present analysis. The present experimental and numerical analysis identifies the domain confinement as a key parameter for transverse migration of the droplets in the presence of a transverse electric field. Notably, the droplet migrates at a faster rate in highly confined domain. The present analysis also illustrates that the droplet can migrate toward the wall electrode or centerline depending on the physical and electrical properties of the system. The achieved steady state transverse position is found independent of its initial positions.

physics.flu-dyn↗

arXiv2018

Effect of uniform electric field on the deformation of a 2D liquid droplet in confined simple shear flow

In the present study, we have studied the electro-hydrodynamic of a physical system where a Newtonian dielectric liquid column or droplet suspended in another Newtonian dielectric liquid medium in presence of a simple shear flow. Taking both the phases as leaky dielectric and perfect dielectric in to consideration, we have performed 2D numerical solution for capturing the essential features of droplet deformation in between the parallel plate configuration. For a perfect dielectric system, this study shows that the deformation characteristic follows a monotonic as well as non-monotonic variation with domain confinement depending on the values of electrical permittivity ratio of the droplet and the surrounding fluid. For a leaky-dielectric system, presence of small conductivity further alters the deformation characteristic and it is happened that, at low electric field strength, the deformation increases with confinement monotonically. On contrary, deformation parameter shows non-monotonic variation with the domain confinement at higher electric field strength. Furthermore, in confined domain, the transient evolution of the deformation parameter is also markedly altered by the electric field strength in terms of steady state value of the deformation parameter and steady state time. Finally, the present analysis shows that the domain confinement significantly augment the deformation parameter in presence of electric field that leads to possible droplet break up phenomenon. From the present study, it is worthy to mention that domain confinement can be used to modulate the droplet morphology that has potential applications in modern-days droplet-based micro-fluidic devices.

physics.flu-dyn↗

arXiv2018

Electrothermally Modulated Dispersion in Viscoelastic Fluids on Patterned Surfaces

Augmenting the dispersion of a solute species remains a challenging topic in electroosmotically-actuated flows due to the inherent plug-like feature of its velocity profile. In this study, we improvise on electrothermal flow of viscoelastic fluids through a parallel-plate microchannel with patterned surface charge to depict augmented dispersion coefficient of a solute. Here we consider the physical properties of the fluid to be temperature-dependent, thus, bringing out the combinatorial effect of both the thermal perturbation as well as the surface modulation on the resulting dynamics. This analysis unveils that by making a judicious choice of pertinent parameters one can control the flow rate with the desired account of dispersion. We envisage that the intricate coupling between the electrothermal perturbation and surface charge modulation combined with the fluid rheology may have tremendous implications towards the efficient designing of microfluidic devices.

physics.flu-dyn↗

arXiv2018

Alteration in contact line dynamics through competition between thermocapillary and electrothermal effects

The contact line dynamics of a thermal field assisted flow configuration of two immiscible fluids is considered in a narrow thermofluidic pathway. The surfaces of the channel are wetted with predesigned wettabilities and interdigitated electrodes are mounted on the substrates to generate non-uniform electric field. In this study, the interplay of thermocapillary and electrothermal forces on interfacial dynamics are studied in detail. The former is caused by surface tension gradients while the latter results from gradients in permittivity and electrical conductivity. Our investigations reveal that the relative strength of interfacial forces and electrothermal forces and their interactions can be effectively used to control the capillary filling time as well as the flow dynamics. For same strength of thermocapillary and electrothermal forces (characterized by individual dimensionless numbers) electrothermal forces dominate over thermocapillary forces. However, interfacial forces at some wettabilities dominate over electrothermal forces and set the fluid motion toward the entry of the channel.

physics.flu-dyn↗

arXiv2018

Alternating current driven interfacial dynamics of a binary fluid in a patterned fluidic environment modulated by electrothermal effects

In this paper, we depict interfacial electro-thermo-chemical-hydrodynamics of two immiscible fluids in a microchannel with substrates patterned by ribs. The motion of the binary fluids is set by an alternating current electrothermal (ACET) mechanism. Our investigations, based on free-energy-based phase field formalism, reveal that the capillary filling dynamics and the contact line motion are strong functions of the wetting characteristics and geometric parameters of the patterned ribs. Modulation of these parameters alters the surface energy over the rib surface, which, in turn, facilitates the interaction between interfacial tension and driving electrothermal forces. The competition between two forces may speed up or slow down the fluid-fluid-solid contact line motion over the rib surface. At the edges of the ribs, the interface can halt for a sufficiently long time owing to contact line pinning. Alteration in positions of the rib between the electrode pairs changes the electric field strength and thereby bulk ACET forces across the contact line. Further, by suitable arrangement of the ribs, various intricate shapes of the liquid front can be achieved over a short distance, which can have significant implications in the morphological control of microscale flows.

physics.flu-dyn↗

arXiv2018

Electrothermally modulated stop-go microvalves for capillary transport systems

In this article, we show a novel approach to implementing valving action in a dynamically evolving capillary filling process, by exploiting alternating current electrothermal (ACET) mechanism. The surfaces of the top and bottom walls of the capillary are asymmetrically patched with two different wettabilities. Our observations reveal that by exploiting an intricate interplay of the electrothermal effects and interfacial tension modulation, the acceleration-deceleration motion of the interface and interface pinning at specific locations may be delicately tuned. This, in turn, modulates the flow speed and residence time of the interface at designated locations, with a simple tuning of the frequency of the electrical signal. The present study, thus, unveils a novel micro valving mechanism achieved via effective manoeuvring of the interfacial electrochemistry where active and passive control mechanisms can be utilized simultaneously to achieve control over the interfacial motion of the two-phase system. This may bear far-reaching consequence in optimizing several applications in the bio-microfluidic domain, including devising novel miniaturized platforms for locally investigating reaction chemistries.

physics.flu-dyn↗

arXiv2018

Surfactant-induced-retardation in lateral migration of droplets in a microfluidic confinement

In the present study, the role of surfactants on the cross-stream migration of droplets is investigated both experimentally and theoretically. For experimental analysis, sunflower oil is used as the carrier phase and DI water as the dispersed phase which is intermixed with Triton X-100 that acts as the surfactant. A T-junction is used in the microchannel for the purpose of droplet generation. Presence of an imposed pressure driven flow induces droplet deformation and disturbs the equilibrium that results in subsequent surfactant-redistribution along the interface. This further induces a gradient in the 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. Further, it is shown that the effect of surfactants in reducing the cross-stream migration is significantly enhanced for a larger droplet as compared to a smaller one within the same time span, provided the channel height is kept constant. In addition, a larger surfactant concentration is found to induce a greater retardation in cross-stream migration of the droplet, the effect of which is reduced when the initial transverse position of the droplet is shifted closer to the channel centerline. The present analysis can be applied to various droplet based microfluidic as well as medical diagnostic devices where manipulation of droplet trajectory is a major issue. For the theoretical counterpart, an asymptotic approach is adopted in the presence of bulk-insoluble surfactants and under the assumption of negligible fluid inertia. A good match between our theoretical prediction and the experimental results is obtained.

physics.flu-dyn↗

arXiv2018

Electrokinetic Energy Harvesting using Paper and Pencil

We exploit the combinatorial advantage of electrokinetics and tortutosity of cellulose-based paper network on a laboratory grade filter paper for the development of a simple, inexpensive, yet extremely robust (shows constant performance till 12 days) paper-and-pencil-based device for energy harvesting application. We successfully achieve to harvest maximum output power of 640 pW in single channel, while the same is significantly improved (by about 100 times) with the use of multichannel microfluidic array (maximum up to 20 channels). 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 for extreme point of care diagnostics applications.

physics.app-ph↗

arXiv2018

Modeling D-Region Ionospheric Response of the Great American TSE of August 21, 2017 from VLF signal perturbation

Solar eclipse is an unique opportunity to study the lower ionospheric variabilities under a controlled perturbation when the solar ultraviolet and X-ray are temporally occulted by the lunar disk. Sub-ionospheric Very Low Frequency (VLF) radio signal displays the ionospheric response of solar eclipse by modulating its amplitude and phase. During the Total Solar Eclipse (TSE) on August 21, 2017 in North America, data was recorded by a number of receivers as presented in public archive. Out of these, two receiving stations YADA in McBaine and K5TD in Tulsa could procure a reasonable quality of noise free data where the signal amplitude was clearly modulated due to the eclipse. During the lunar occultation, a C3.0 solar flare occurred and the signal received from Tulsa manifested the effect of sudden ionization due to the flare. The VLF amplitude in Tulsa shows the effect which is generally understood by superimposing effects of both the solar eclipse and flare. However, the signal by YADA did not perturb by the solar flare, as the flaring region was totally behind the lunar disk for the entire period. We numerically reproduced the observed signal amplitude variation at both the receiving locations by using Wait's two component D-region ionospheric model and the well-known Long Wavelength Propagation Capability (LWPC) code. The perturbed electron density for both the cases is computed which matches satisfactorily with the true ionospheric conditions.

astro-ph.EP↗

arXiv2018

Influence of interfacial slip on the suspension rheology of a dilute emulsion of surfactant-laden deformable drops in linear flows

The present study deals with the effect of interfacial slip on the deformation and emulsion rheology of a dilute suspension of droplets in a linear flow. The droplets are laden with surfactants that are bulk-insoluble and get transported only along the interface. An asymptotic approach is adopted for the present analysis in order to tackle the nonlinearity present due to deformation of droplets. The analysis is carried out under two different limiting scenarios namely: surface diffusion-dominated-surfactant transport and surface convection-dominated surfactant transport. For either of the limiting cases we look into the droplet dynamics for two commonly encountered bulk flows - uniaxial extensional and simple shear flow. Under the assumption of negligible fluid inertia in either phase, it is shown that slip at the droplet interface significantly affects the surfactant-induced Marangoni stress and hence droplet deformation and emulsion rheology. Presence of interfacial slip not only brings about a decrease in the droplet deformation but also reduces the effective viscosity of the emulsion. The fall in both droplet deformation and effective viscosity is found to be more severe for the limiting case of surface convection-dominated surfactant transport. For the case of an imposed simple shear flow, the normal stress differences generated due to droplet deformation are affected as well due to the presence of interfacial slip.

physics.flu-dyn↗

arXiv2017

Site percolation on non-regular pseudo-random graphs

We study site percolation on a sequence of graphs $\{G_n\}_{n\geq1}$ on $n$ vertices where degree of each vertex is in the interval $(np -a_n, np+a_n)$ and the co-degree of every pair of vertices is at most ${n}p^2+ b_n$, where $p \in (0,1)$ and $\{a_n\}_{n\geq1}$, $\{b_n\}_{n\geq1}$ are sequences of real numbers. Under suitable conditions on $p \in (0,1)$, $a_n$'s and $b_n$'s we show that site percolation on these sequences of graphs undergo a sharp phase transition at $\frac{1}{np}$. More precisely for $\varepsilon>0$, we form a random set $R(ρ_n)$ by including each vertex of $G_n$ independently with probability $ρ_n$. If $ρ_n = \frac{1-\varepsilon}{np}$, then for every small enough $\varepsilon>0$ and $n$ large enough, all connected components in the subgraph of $G_n$ induced by $R(ρ_n)$ are of size at most poly-logarithmic in $n$ with high probability. If $ρ_n = \frac{1+\varepsilon}{np}$, then for every small enough $\varepsilon>0$ and $n$ large enough, the subgraph of $G_n$ induced by $R(ρ_n)$ contains a 'giant' connected component of size at least $\frac{\varepsilon}{p}$ with high probability. Further, we show that under an additional assumption on $\{b_n\}_{n\geq 1}$ the giant component is unique. This partially resolves a question by Krivelevich \cite{krivelevich2016phase} regrading uniqueness of the giant component of site percolation in a general class of regular pseudo-random graphs. We hope that our method of proving uniqueness of the giant component will be applicable in other contexts as well.

math.PR↗

arXiv2017

Charged Obstacles Augment Electrokinetic Energy Conversion Efficiency

In the present study, we delineate the effect of introducing flow obstructions on streaming potential and energy conversion efficiency in a narrow fluidic confinement taking into consideration the wall hydrodynamic slip, finite ionic size, and local permittivity variation effects. We consider two types of the geometric pattern of flow obstacle between which regular pattern turns out to be more effective. It is observed that, implementing structured flow obstacles of radii greater than 10% of channel height and charge density ratio (charge density of flow obstacle surface to that of channel wall) greater than or equal to 9 renders significant enhancement of power generation efficiency. On the other hand, it is effective to operate at a charge density ratio~1 for obstacles radii less than or equal to 10% of channel height. An effective normalized pitch length of magnitude 0.6 or above has to be maintained in order to obtain optimum energy conversion efficiency. We found that implementation of charged flow obstacle causes a significant enhancement of energy conversion efficiency (~18% with regular pattern) comparing to planar slit channel (~4%) without considering wall hydrodynamic slip and steric effects. Introducing finite ionic size and wall hydrodynamic slip effect, pertinent to flows in narrow confinement, leads to further enhancement in electrokinetic energy conversion efficiency.

physics.flu-dyn↗

arXiv2017

Thermophoretically driven capillary transport of nanofluid in a microchannel

We investigate the interplay of thermophoretic force and interfacial tension on the capillary filling dynamics of a Newtonian nanofluid in a microchannel. In our model, we also consider an intricate thermofluidic coupling by taking the temperature dependence of viscosity aptly into account. This, in turn, determines the evolution of the viscous resistive force as the capillary front progresses, and presents an involved inter-connection between the driving thermophoretic force and the viscous resistive force. The two distinct regimes of particle transport in a fluid medium, delineated by particle size, are expounded to peruse the impact of imposed thermal gradients and particle size on particle retaining propensity of the nanofluid. Additionally, we witness a significant reduction in particle bearing proclivity of the nanofluid with enhancement in a thermal gradient. The results demonstrate the efficacy of the thermophoretic actuation towards the filling of narrow capillaries under the influence of a thermal gradient.

physics.flu-dyn↗

arXiv2017

Complex Fluid-Fluid Interface may Non Trivially Dictate Droplet Deformation in an Incipient Flow

The present study theoretically predicts the effect of interfacial viscosity on the deformation of a compound drop as well as on the bulk rheology. The system at hand comprises of a dilute emulsion of concentric compound drops, laden with surfactants and suspended in a linear flow. Two types of linear flows are considered in this study, namely, a uniaxial extensional flow and a simple shear flow. Presence of surfactants along the drop surface leads to the generation of an interfacial viscosity, which is different from the bulk. This interfacial viscosity generates a viscous drag that along with bulk flow-induced nonuniform surfactant distribution on the drop surface significantly alters drop dynamics. For the present study an asymptotic approach is used to solve the flow field under the limiting case of diffusion-dominated-surfactant transport. Assuming the surfactants to be bulk-insoluble and negligible inertia to be present in fluid flow, it is shown that presence of interfacial viscosity reduces the deformation of a compound drop and enhances the stability of a dilute double emulsion. At the same time the effective viscosity of the emulsion also increases with rise in interfacial viscosity. For large values of interfacial dilatational viscosity the drop deformation is seen to increase and hence the stability of the double emulsion is questionable.

physics.flu-dyn↗

arXiv2017

Thermally modulated cross-stream migration of a surfactant-laden deformable drop in a Poiseuille flow

In the present study, we investigate the cross-stream migration of a deformable droplet suspended in a non-isothermal Poiseuille flow in the presence of bulk-insoluble surfactants. Owing to the non-linearity present in the system of governing equations, an asymptotic approach is adopted, in an effort to capture the intricate and non-trivial coupling between the various influencing parameters. With the assumption of negligible inertia in fluid flow and convective transport of thermal energy, we obtain the droplet migration velocity through small-deformation perturbation analysis for two different limiting cases, namely, convection-driven-surfactant transport and surface-diffusion-dominated surfactant transport. Under each of these limiting cases, the cross-stream migration of droplet is studied for a constant temperature gradient applied in the same direction as well as in a direction opposite to the imposed flow. For the former limiting case, the droplet is always migrates towards the centerline of flow. For a highly viscous droplet, the direction of its cross-stream migration reverses. When the temperature decreases in the direction of the imposed flow, cross-stream migration velocity reduces with increase in the applied temperature gradient till a critical point is reached at which there is no cross-stream migration. Beyond the critical point, there is a gradual increase in the magnitude of the cross-stream velocity. The droplet, below the critical temperature gradient, migrates towards the flow centerline; however, above it the droplet moves away from the centerline. For the other limiting case of surfactant transport dominated by surface convection, the magnitude of the cross-stream velocity is found to be significantly larger and at the same time independent of the droplet-carrier phase viscosity ratio.

physics.flu-dyn↗

arXiv2017

Effect of Marangoni stress on the bulk rheology of a dilute emulsion of surfactant-laden deformable droplets in linear flows

In the present study we analytically investigate the deformation and bulk rheology of a dilute emulsion of surfactant-laden droplets suspended in a linear flow. We use an asymptotic approach to predict the effect of surfactant distribution on the deformation of a single droplet as well as the effective shear and extensional viscosity for the dilute emulsion. The non-uniform distribution of surfactants due to the bulk flow results in the generation of a Marangoni stress which affects both the deformation as well as the bulk rheology of the suspension. The present analysis is done for the limiting case when the surfactant transport is dominated by the surface diffusion relative to surface convection. As an example, we have used two commonly encountered bulk flows, namely, uniaxial extensional flow and simple shear flow. With the assumption of negligible inertial forces present in either of the phases, we are able to show that both the surfactant concentration on the droplet surface as well as the ratio of viscosity of the droplet phase with respect to the suspending fluid has a significant effect on the droplet deformation as well as the bulk rheology. It is seen that increase in the non-uniformity in surfactant distribution on the droplet surface results in a higher droplet deformation and a higher effective viscosity for either of linear flows considered. For the case of simple shear flow, surfactant distribution is found to have no effect on the inclination angle, however, a higher viscosity ratio predicts the droplet to be more aligned towards the direction of flow.

physics.flu-dyn↗

arXiv2017

Cross-stream migration characteristics of a deformable droplet in a non-isothermal Poiseuille Flow through Microfluidic Channel

The migration characteristics of a suspended deformable droplet in a parallel plate microchannel is studied, both analytically and numerically, under the combined influence of a constant temperature gradient in the transverse direction and an imposed pressure driven flow. Any predefined transverse position in the micro channel can be attained by the droplet depending on the applied temperature gradient in the cross-stream direction or how small the droplet is with respect to the channel width. For the analytical solution, an asymptotic approach is used, where we neglect any effect of inertia or thermal convection of the fluid in either of the phases. To obtain a numerical solution, we use the conservative level set method. Variation of temperature in the flow field causes a jump in the tangential component of stress at the droplet interface. This jump in stress component, which is the thermal Marangoni stress, is an important factor that controls the trajectory of the droplet. The direction of cross-stream migration of the droplet is decided by the magnitude of the critical Marangoni stress, corresponding to which the droplet remains stationary. In order to analyze practical microfluidic setup, we do numerical simulations where we consider wall effects as well as the effect of thermal convection and finite shape deformation on the cross-stream migration of the droplet.

physics.flu-dyn↗

arXiv2017

Cross-stream migration of a surfactant-laden deformable droplet in a Poiseuille flow

The motion of a viscous deformable droplet suspended in an unbounded Poiseuille flow in the presence of bulk-insoluble surfactants is studied analytically. Assuming the convective transport of fluid and heat to be negligible, we perform a small-deformation perturbation analysis to obtain the droplet migration velocity. The droplet dynamics strongly depends on the distribution of surfactants along the droplet interface, which is governed by the relative strength of convective transport of surfactants as compared with the diffusive transport of surfactants. The present study is focused on the following two limits: (i) when the surfactant transport is dominated by surface diffusion, and (ii) when the surfactant transport is dominated by surface convection. In the first limiting case, it is seen that the axial velocity of the droplet decreases with increase in the advection of the surfactants along the surface. The variation of cross-stream migration velocity, on the other hand, is analyzed over three different regimes based on the ratio of the viscosity of the droplet phase to that of the carrier phase. In the first regime the migration velocity decreases with increase in surface advection of the surfactants although there is no change in direction of droplet migration. For the second regime, the direction of the cross-stream migration of the droplet changes depending on different parameters. In the third regime, the migration velocity is merely affected by any change in the surfactant distribution. For the other limit of higher surface advection in comparison to surface diffusion of the surfactants, the axial velocity of the droplet is found to be independent of the surfactant distribution. However, the cross-stream velocity is found to decrease with increase in non-uniformity in surfactant distribution.

physics.flu-dyn↗