Controlling particle migration in confined flows of viscoelastic fluids using an electric field by Huges Bodiguel, Université de Grenoble Alpes
We aim at manipulating particles at small scales in the context of particle sorting applications, or clogging phenomenon in porous media. At low Reynolds numbers, in Newtonian fluids and in dilute conditions, particles mainly follow the streamlines and exhibit a uniform probability density function of their position in the whole cross-section of a channel flow. However, when the carrying fluid is non-Newtonian and for moderate to high confinement in pressure driven flows, a transverse force is acting on the particle, due to the gradient of shear rate. This viscoelastic migration directly involves normal stress anisotropy. In most cases, and in absence of electric field, particle migrates towards the center where the shear rate gradient is minimal. When a external force is applied on the (such as in the case of an electric field in the flow direction), the sense of the migration can be inverted and particles can migrate towards the wall. We have developed an accurate experimental method to determine efficiently the particle position density function in order to investigate these effects. A nice agreement with the theory is found in the case of dilute polymer solutions exhibiting a steady viscosity. In the case of shear thinning polymer solutions, some theoretical efforts seem to be needed to account for the experimental results. Outlooks of this work deal with electrophoresis in non-Newtonian fluids.
Acoustic streaming : two examples in droplets and channel microfluidics by Philippe Brunet, Université Paris Diderot
After a short introduction of acoustic streaming phenomena, I will present recent results on how to generate flows with acoustic waves in two situations of confined geometries, where mixing is often limited : (1) a micro channel with sharp-edged tips, in which streaming and mixing can be generated even for kHz-range frequencies, (2) a sessile droplet excited by surface acoustic waves, in which an analogue of whispering galleries can set a flow with one or two pairs of vortices. Both situations were investigated experimentally and numerically.
Levitation and breakup of charged droplets in quadrupole traps by Rochish Thaokar, I.I.T. Bombay
Levitation, surface oscillations and breakup of charged droplets in quadrupole traps have applications in electrospray and ion-mass spectrometry. I shall present experiments, theory and numerical simulations on the levitation and breakup of charged ethylene glycol droplets levitated in an in-house quadrupole trap. The levitated charged droplets show degenerate structures that depend upon the charge to mass ratio of the droplets. An important application of a quadrupole trap is in studying the dynamics and breakup of a charged droplet. The surface oscillations of a sub-Rayleigh charged droplet in a quadrupole field could be explained using a viscous-potential theory. Our numerical study on the Rayleigh breakup of a charged droplet, within the perfect conductor assumption yields a method to estimate the total charge loss in Rayleigh instability. We then demonstrate that accounting for fast, but finite charge dynamics is essential to describe formation of jets in breakup of charged drops. Our own experiments on Rayleigh instability in a “loose” trap demonstrates “field influenced asymmetric, subcritical Rayleigh instability” in contrast to “field induced Rayleigh breakup” studied in the literature.
Drop Sedimentation under influence of electric fields by Aditya Bandopadhyay, I.I.T. Kharagpur
We investigate the motion of a sedimenting drop in the presence of an electric field in an arbitrary direction, otherwise uniform, in the limit of small interface deformation and low-surface-charge convection. We show that tilt angle, which quantifies the angle of inclination of the applied electric field with respect to the direction of gravity, has a significant effect on the magnitude and direction of the drop velocity. Lateral migration of the drop is also discussed. Our experimental investigation further confirms the presence of lateral migration of the drop in the presence of a tilted electric field, which is in support of the essential findings from the analytical formalism.
Phase-field modelling and computational microfluidics by Suman Chakraborty, I.I.T. Kharagpur
Phase-field method is a diffuse interface approach which has far-reaching applications in modeling of fluid-fluid interfaces. Not merely restricted to the modelling of simple fluid-fluid interfaces present in multiphase flows, the phase-field method is an important tool for modelling of complex fluid-fluid interfaces such as vesicles and red blood cells. Modelling of fluid-fluid interfaces is challenging due to the moving and deforming nature of the interfaces. The fluid-fluid interfaces can be handled simultaneously with ease and thermodynamic rigour in the paradigm of phase-field method. Phase-field method replaces the sharp interfaces separating the fluids by a diffuse transition layer where the interfacial forces are smoothly distributed. The diffuse layer in phase-field field method is different from the diffuse layer in volume of fluid or level set method as in the later cases the diffuse layer is an artificial (numerical) transition layer, while the phase-field method introduces diffuse layer from a physical perspective. In sharp contrast to moving mesh methods, the phase-field method automatically captures the fluid-fluid interfaces and thus the explicit tracking of the interfaces and calculation of interface curvature are no longer required. However, one has to solve an extra equation for order parameter. The order parameter is a measure of phase variation which gives the distribution of the participating phases. It is important to note that the order parameter is not arbitrary like the level-set function but can be linked to the physical situation encountered in different phenomena. In sharp contrast to other multiphase methods which uses surface tension forces, the phase-field method is based on free energy of fluids. Phase-field method represents the fluid-fluid interface in terms of a diffuse interface in which the two phases are mixed and store a mixing energy. Simplest form of free energy density for isothermal two-phase system can be constructed by a combination of bulk free energy and gradient free energy. The dynamic evolution of the system (equation for order parameter) can be described by Allen-Cahn or Cahn-Hilliard formulations. Depending on the physical situation, Allen-Cahn or Cahn-Hilliard dynamics can be used to study the following two kinds of problems: (a) Problems related to moving boundary: Phase-field acts as a numerical tool to simulate multiphase flows e.g. Droplet or bubble dynamics. (b) Problems in which the interface profile is of great importance: Phase-field captures microscopic physics on the interface e.g. solidification of alloys, near-critical systems, contact line dynamics. In the later category, the phase-field method gives physically realistic description when the interface width is comparable to the length scale of the phenomenon. As the phase-field method is based on free energy based formulation, by suitably incorporating different free energies, this method can be used to address biological systems which are inherently driven by gradients of free energies. So, the phase-field method can be effectively used to couple biological systems with transport phenomena. The lecture will first introduce the speakers to the broad motivation and the fundamental theoretical developments associated with phase field modelling in perspective of computational microfluidics. Finally, it aims to outline some typical examples encountered in microfluidics, including electro-hydrodynamic flows, in order to demonstrate the efficacy of the method.
Blood : Passive and Active Motion by Misbah Chaouqi, Université de Grenoble Alpes
Electrokinetics for earth sciences by Laurence Jouniaux, Université de Strasbourg
Seismo-electromagnetics methods used in geophysics originate from the electrokinetic phenomenon. Seismoelectrics deals with the electromagnetic waves generated by electrokinetic conversion of seismic waves. The observed seismoelectric conversions at geological interfaces are due to electric and mechanical contrasts between different media and depend on numerous parameters such as the water content, the permeability, the porosity, the fluid viscosity. Unfortunately the interfacial response is usually difficult to detect because of its small amplitude compared to the coseismic signal. Recent studies showed that depending on the type of source and the geometrical acquisition this signal can be measured more easily. Moreover theoretical developments showed that we can take into account the effect of water content including the effect of interfaces such as rock/water and water/air and different types of soils. This prospecting method aims at combining the resolution of the seismic method with the sensitivity of the electric methods to fluid content in porous rocks. Observations, experimental measurements, and numerical modelling of both seismoelectrics and electroseismics have been developed to gain understanding of the nature, characteristics and behaviour of near surface Earth systems, such as aquifers, gas/oil/gas-hydrate reservoirs, CO2 geological storage sites, glaciers.
Microfluidic devices and systems for stem cell engineering and cancer diagnosis by Yong Chen, E.N.S. Paris
In this talk, I will begin with a critical assessment on the current approaches of microfluidics and organ-on-a chip systems. Then, I will describe our strategy and methods toward a higher degree of biomimicry and a higher degree of microengineering, including artificial basement membrane in the form of culture patch, microfluidic devices for patch and tissue integration, and automated culture platforms for long-term human pluripotent stem cell differentiation. Afterward, I will give a few examples for tissue modeling of such as cardiomyocytes, alveolus, neurons, etc. to illustrate the high potential of our approach for both fundamental research and advanced applications. Finally, I will present our efforts on capture and modeling of circulation tumor cells for a large scale application.
Electro-encapsulation using AC actuation and hydrodynamics inside Taylor cone by Pradipta Kumar Panigrahi, I.I.T. Kanpur
The potential of low frequency alternating current (AC) electric field actuation for micro-encapsulation using coaxial electrospray has been investigated. Ethanol, olive oil and glycerol fluid combinations have been used as working fluid. Dye visualization of the Taylor cone and high speed visualization of electrospray have been carried out. PIV measurements inside the Taylor cone has been carried out to demonstrate the flow instability of Taylor cone. Confocal microscopy has been used to characterize the capsules structure. Current measurement has been used to quantify the net charge content of the capsules. DC actuation shows straight generatrix of Taylor cone while AC actuation shows cusp shape. Difference in charge accumulation on the interface of the core and shell liquid for the DC and AC actuation and the resulting Maxwell stress influence the curvature of Taylor cone. Biorthogonal decomposition is used to characterize the stability of the electrospray process. The minimum potential required for stable cone jet formation is lower for AC actuation compared to that of DC actuation. The present study demonstrates that square wave AC actuation can successfully generate stable coaxial cone jet and capsules.
Action at a distance - leveraging magnetic particles in microfluidic platforms by Ranjan Ganguly, Jadavpur University
Iron oxide magnetic nanoparticles (MNPs), in the forms of ferrofluids and magnetic microspheres can be advantageously used in microfluidic applications, since several unique features of these particles offer promising solutions for major microfluidic challenges. These particles are commercially available, not only in a wide range of sizes (from a few nanometers to a few tens of microns), physical states (ferrofluids, SPIONs or magnetic microspheres), but also with variety of surface functionalizations and properties that can be tailored to suit different microfluidic and bioanalytical tasks. The useful combination of selective biochemical functionalization and “action-at-a-distance” that a magnetic field provides has considerable potential for use of superparamagnetic nanoparticles in micro-total analysis systems (μ-TAS). Ferrofluids can be transported by applying external magnetic fields to realize controlled mass transfer or field-assisted self-organization and assembly in a microfluidic environment. Tunable three dimensional microscale structures can be built inside microchannels using a ferrofluid in the presence of a localized magnetic field gradient. These have interesting applications, such as for microfluidic valves, plungers or droplet manipulators. Magnetic microspheres can also be advantageously used as “mobile substrates” for microscale bio-assays for a wide variety of bioanalytical applications. They can also be used as tags for the magnetic detection of biological entities in MEMS biosensors. The targeted assembly of magnetic microspheres can be used for the soft-fabrication of flexible magnetic templates or for binding sites in microfluidic devices. Particle transport in microfluidic media and particle-fluid momentum interactions can be utilized to achieve enhanced mixing in a microchannel or the manipulation of droplets on a digital microfluidic platform. This talk provides an insight into the behavior of magnetic particles and their transport in a microfluidic environment, and discusses the salient microfluidic applications of superparamagnetic nanoparticles in the form of ferrofluids and magnetic microspheres.
Flow Actuated by Thermoviscous Expansion: Fundamentals and Applications by Debashis Pal, I.I.E.S.T. Shibpur
Thermoviscous expansion of a fluid along a periodically heated solid wall constitutes a novel thermo-mechanical pumping method that is capable of inducing net fluidic transport along the wall. Unsteady heat diffusion, temperature dependent density and viscosity variation and interplay among multiple length scales play incredibly critical roles in such a transport process. In this presentation, we have provided analytical foundation to thermoviscous transport phenomena through meticulous scaling analysis of basic conservation equations and derivation of mean flow governing equations in open configurations as well as in narrow confinements. Identification of all the pertinent length and velocity scales (including a universal scaling law for characteristic net velocity in microchannels) reveals appropriate handles for controlling the flow rate and velocity profiles induced by thermoviscous actuation. Subsequent numerical (CFD) analysis has been instrumental in corroborating the theoretical findings as well as exploring some nontrivial issues (for example, reduction of net throughput for low Prandtl number fluids). We further investigated the problem of solutal transport in presence of thermoviscous expansion triggered by a travelling temperature wave. Based on the temporal evolution of the axial dispersion coefficient, new regimes of dispersion – such as a short-time “oscillating regime” and a large-time “stable regime” – have been identified, which are absent in traditionally addressed flows through miniaturized fluidic devices. The new physical paradigm for dispersion may have significant contextual relevance to widely diverse fields of applied sciences ranging from micro-reactors & DNA amplifiers to separation and detection of species.
Opto-electrothermally induced vortex flows in microfluidic systems by Aloke Kumar, Indian Institute of Science
Hybrid opto-electric manipulation in microfluidics/nanofluidics refers to a set of methodologies employing optical modulation of electrokinetic schemes to achieve particle or fluid manipulation at the micro- and nano-scale. Over the last decade, a set of methodologies, which differ in their modulation strategy and/or the length scale of operation, have emerged. These techniques offer new opportunities with their dynamic nature, and their ability for parallel operation has created novel applications and devices. Hybrid opto-electric techniques have been utilized to manipulate objects ranging in diversity from millimetre-sized droplets to nano-particles. We will discuss the development of a specific hybrid technique called rapid electrokinetic patterning (REP), which utilizes optical landscapes to create thermal 'hotspots' creating an opto-electrothermal tweezer.
High performance EHD pump for macro and micro applications by Christophe Louste, Université de Poitiers, Institut P’
Electrohydrodynamics is the study of the complex relationships between electricity and liquid flows. Electrohydrodynamics is not a new research area. The first experiment was realized by Michael Faraday in 1838. In a popular article published in the Philosophical Transactions of the Royal Society, he reported that when “two wires be dipped into a pint of turpentine oil in different places one leading to an electrical machine and the other to the discharging train on working machine, the fluid be thrown into violent motion throughout is whole mass”. This experiment seems to be simple but it took over a century to fully explain the phenomenon. The work developed at the University of Poitiers focuses both fundamentally and experimentally on the development EHD actuators. In an EHD actuating systems, a liquid flow is created by applying an intense electric field on a dielectric liquid. Two main forces can be used for that purpose. The first one is the Coulomb Force and the second one, is the dielectrophorectic force. It has been demonstrated that Electrohydrodynamic actuators are particularly well adapted for space application. They are low energy consuming and low weight, generate no noise and no vibrations, have short response time and are not affected by takeoff vibrations. Moreover, only electrodes are needed to create an electrodynamic actuator then the EHD technology is also cheap. In satellites, most of electronic equipment and components must be maintained at constant temperature to work in a normal way. Conventional electronic cooling systems usually consist in a pump system connected to complex pipes. A liquid refrigerant is used to transport the heat from higher temperature region to the lower temperature region and to attain uniform heat distribution. However, a number of limitations exist. In space applications, mechanical pumps can be damaged by takeoff vibrations and capillary pump have limited pumping capacity, start-up difficulties, and dry-out problems. EHD could appear to be only fundamental but since few years a lot of applications start to be developed: microfluidic systems, zero gravity cooling systems, robot muscle … and that is very promising.
Current trends in numerical computation of EHD flows by Philippe Traore, Université de Poitiers, Institut P’
My talk will address the specificities of numerical simulation in Electro-Hydro-Dynamic problems. One characteristic in EHD problems is that the transport equation of the charge density or the ones for the ionic species density are hyperbolic. Therefore, specific numerical schemes are needed to solve them accurately. I will present the features of the numerical code we have developed at the P’ institute for EHD problems purposes and the type of problem we can solve with it.
A brief overview of ElectroFluidoDynamics at P’ Institute in Poitiers by Hubert Romat, Université de Poitiers, Institut P’
The objective of this presentation is to better make known the four following research topics proposed by the ElectroFluidoDynamics Team of P’ Institute for a future collaboration with India. 1. “Electroaerodynamic phenomena in electric discharges - Application to airflow control by plasma actuators and ionic wind propulsion”; 2.“Electrofluidic applied to electrolyte and electrochemical systems”; 3.“New advances in Electrical Double Layer : impact on EHD applications”; 4.“Particles/Electric field Interactions - Applications to electrostatic precipitation, cleaning and separation”
Electrohydrodynamics for Electronic Thermal Management by Anandaroop Bhattacharya , I.I.T. Kharagpur
The challenge of electronics cooling has existed since the creation of the first transistor. During the decades since the advent of the integrated circuit (IC), electronics cooling has grown in complexity to cover devices from the smallest cellular phone to large server banks containing literally thousands of processors. Increasing microprocessor performance has historically been accompanied by increasing power and increase in chip power density both of which present a thermal challenge. This has called for the development of new and innovative cooling solutions at the device, package and system levels. Electrohydrodynamic based solutions have come to the fore in the last few years as alternatives to conventional techniques. This presentation talks about some of these cooling technologies with focus on electrowetting and ionic winds.
Electrohydrodynamic instabilities in microchip electrophoresis by Supreet Singh Bahga, I.I.T. Delhi
Microchip electrophoresis techniques rely on differential migration velocities of ionic species in an electrolyte solution, under applied electric field, to separate ionic species from complex mixtures. Certain electrophoresis techniques, such as field amplified sample stacking and isotachophoresis, involve gradients in electrolyte conductivity for improving detection sensitivity. However, presence of conductivity gradients along with high electric field often lead to undesirable phenomena of electrohydrodynamic (EHD) instabilities. EHD instabilities in microchip electrophoresis occur in two flow configurations, wherein applied electric field and conductivity gradient are either orthogonal or colinear. During the sample injection step of microchip electrophoresis, the electric field and conductivity gradient are orthogonal to each other. On the other hand, during the separation step the electric field and conductivity gradient are colinear. We have performed a detailed experimental investigation of both of these EHD instabilities. Based on these experiments, we have identified the spatio-temporal coherent structures that represent the dynamics of instability. These coherent structures provide a valuable insight into the dynamics and the spatio-temporal scales of the EHD instabilities.
Origins and consequences of surface charge and its modulation in microfluidics and nanofluidics by Subhra Datta, I.I.T. Delhi
The origin of surface charges in microfluidics and consequences of surface charge patterning will be the key themes of the proposed talk. Concerning origin of surface charges, the intricate way in diffusion, electromigration and crucially confinement to the nanoscale interplays with the electrochemistry of the solid-liquid interface will be discussed in the first part of the talk. Applications explored will be nanofluidic diodes the emerging green technology of electrokinetic power generation. When suitably designed using micro-patterning, microfluidic and nanofluidic conduits can provide significantly faster transport of fluids under electric fields and pressure drives than possible at the macroscale. The second part of the talk will deal with the electrokinetic and hydrodynamic influence of wall-micropatterns in surface-charge, interfacial-chemistry and topography. Applications to drag reduction, separation and microfluidic mixing will be discussed.
Electrokinetic flows through narrow deformable confinements by Jeevanjyoti Chakraborty , I.I.T. Kharagpur
Electrokinetic flows particularly within the microlufluidics context have traditionally been studied through rigid confinements. Yet, there are many physiological settings where the confining walls are soft and where the surfaces naturally articulate past each other. Within such narrow deformable confinements, electrokinetics can play a major role in not only affecting the lubrication flow itself but also in augmenting the lift force on the two surfaces. In this talk, by considering a canonical lubrication geometry of two non-parallel surfaces sliding past each other, these roles of electrokinetic flows will be highlighted with particular focus on streaming potential flows. A major finding that will be discussed is the influence of finite size of the ions and the importance of accounting for such finite size through a consistent mathematical model in correcting older anomalous predictions.
Electrophoresis of an ion-selective micro granule for weak and moderate external electric fields by Sakir Amiroudine, Université de Bordeaux
Electrophoresis of a spherical ion-selective microparticle in a binary diluted electrolyte solution is considered for weak and strong electric fields. For the weak electric field the asymptotic solution shows a linear response of electrophoretic velocity as a function of the electric field. In the case of strong electric field, when ions pass through the ion-selective spherical particle, three boundary layers that are nested inside each other, are formed at the front of the particle in the region of incoming ion flux: the classical electric double layer (EDL), a space charge region (SCR) and a thin diffusion layer. Near the surface of the outgoing ion flux (formed only by the EDL), a region of the enriched electrolyte with a strong concentration gradient emerges. At some critical value of the electric field, the steady-state solution loses stability and the system becomes unstable. The instability manifests itself in the region of the incoming ion flux. For both cases, the electrophoretic velocity of the particle in the weak and strong electrical fields was compared with experimental data. This comparison shows good agreement of analytical and numerical approaches with experimental observations.
Bottom-up construction of cell aggregates in microfludic devices using negative dielectrophoresis by M. Frénéa-Robin, Ecole Centrale Lyon
Cell aggregates are an intermediate model between single cells and cell tissues and are used in many fields such as tissue engineering and in vitro drug screening. Such aggregates can be used for instance as tumor models for the study of electrochemotherapy or electro-gene-therapy (Chopinet et al, Int. J. Pharm., 2012). These therapies are based on reversible electroporation, the application of one or more electric field pulses inducing a transient permeabilization of the membrane and thus allowing the introduction of anti-cancer agents or genes into cells. There is therefore a great interest in developing tools enabling to relate the structure and size of the aggregate to electroporation conditions (pulse intensity, number, duration). In this context, we need reliable approaches for the fabrication of well controlled cell assemblies and their electrical characterization. Several approaches are investigated in parallel for the fabrication of 3D cell constructs prior to possible use for electroporation or/and electrical characterization. Here we focus on electric-field driven assembly of cells following a bottom-up approach. This approach exploits negative dielectrophoresis, in which cells are trapped at regions of lowest electric field intensity to form aggregates (Cottet et al, Biophys. J., 2019; Menad et al, Acta Biomater., 2019; Cottet et al, Electrophoresis, 2019; Cottet et al, Microfluidics Nanofluidics, 2019). After a few minutes of field application, it can be observed that cells remain confined together despite field removal. Dielectrophoresis is a promising approach for the collective fabrication of cell aggregates presenting well controlled features. By providing cell-cell contact, the use of electric field could contribute to accelerate the establishment of cell-cell interactions, but this hypothesis needs to be confirmed by further investigations.
Convective flows driven by the dielectrophoretic force in nonisothermal liquid dielectrics by Harunori Yoshikawa, Université Nice Sophia Antipolis
Application of alternating electric fields of high frequency to non-isothermal dielectric liquid can induce thermal convection by an electrohydrodynamic (EHD) force arising from differential polarization of the liquid. This thermo-electrohydrodynamic (TEHD) convection allows active control of heat transfer, in particular, when the Earth's gravity has no significant effects, e.g., in microgravity environments, in microfluidic systems. The driving polarization force can be regarded as a thermal buoyancy force in an electric effective gravity. This analogy with the ordinary thermal Archimedean force also motivates the research of the convection, as it would enable us to simulate certain geophysical convections by EHD flow systems. My talk is devoted to the TEHD convection. Its generation mechanism and resulting heat transfer will be discussed. In particular, it will be shown that disturbances in electric fields prevent convection generation and impedes heat transfer. The effects of dielectric loss that has been ignored in many studies will also be examined.
Electrokinetics in complex fluids in the thin double layer regime by Uddipta Ghosh, I.I.T. Gandhinagar
Complex fluids are fascinating in nature owing to their starkly contrasting behavior as compared to their Newtonian counterparts. In particular, dynamics of viscoelastic fluids under various actuating conditions has long been a vital area of research among the fluid dynamists around the globe. These fluids also play a central role in many microfluidic applications for their ability to closely mimic the dynamical responses of a number of biological fluids, such as human blood, plasma etc. At the same time, various kinds of interfacial phenomena also play important roles in dictating the dynamical responses of such small-scale systems. Electrokinetics is a prime example of one such paradigm, which hinges upon the formation of an electrical double layer (EDL), essentially a charged layer of fluid adhearing to a solid substrate. Although electrokinetic phenomena has long been identi ed to be a key part of life at small scales across various kinds of systems, its manifestation in complex mediums, i.e., mediums which do not necessarily obey Newtonian constitutive models, has not been adequately addressed. In particular, electrokinetic flows of non-Newtonian viscoelastic fluids remain under-explored, especially for systems with non-uniform surface properties. Motivated by the above shortcoming in the literature, we here, set out to explore electrokinetics in viscoelastic fluids, obeying linear and quasi-linear viscoelastic fluids. We particularly focus on the unique interactions between the complex rheology of the fluid and non-uniform surface properties and pay close attention to systems with thin EDLs. It is well known that the theoretical description of electrokinetic flows in thin EDL regime can be greatly simpli ed, by calling upon the idea of Smoluchowski Slip velocity, which replaces the no-slip boundary condition at the solid surface with an equivalent slip velocity condition at the edge of the EDL. Here, we demonstrate that this same idea may also be extended to quasi-linear viscoelastic fluids, in a mathematically rigorous manner. The resulting modified slip velocity exhibits many non-trivial features, otherwise absent from Newtonian fluids. One of the central outcomes of the analysis is that the slip velocity becomes explicitly dependent on the curvature of the bounding solid surface, a facet not observed in Newtonian fluids. We subsequently explore one of the consequences of this feature and show how it might alter electrophoretic mobility of particles depending on their size. Our results might have potentially important applications in devices dealing with particle separation and transport at small scales.
Droplet dynamics with and without electric field by Kirti Chandra Sahu, I.I.T. Hyderabad
The beauty and complexity of the dynamics of bubbles and drops have fascinated scientists for centuries. The three-dimensional behavior of drops was first documented by Leonardo Da Vinci in the 1500s. In this talk, the dynamic of deformation and topological change of droplets with and without the action of the electric field will be discussed. The talk is divided into two parts; the first part deals with the droplet dynamics without the electric field and the second part addresses the influence of the electric field of the droplet behavior. Firstly, we will discuss different regimes in terms of path instability and the shape of a bubble rising in a viscous medium. Our phase plot in the Galilei–Eotvos plane shows five distinct regimes with sharply defined boundaries. Two symmetry-loss regimes are found: one with minor asymmetry restricted to a flapping skirt; and another with marked shape evolution. A perfect correlation between large shape asymmetry and path instability is established. In regimes corresponding to peripheral breakup and toroid formation, the dynamics is unsteady. Then, the complex deformation dynamics of a nonspherical droplet falling in air which is mainly due to the competition between the surface tension force and inertial force will be discussed. It is found that, in the inertia-dominated region and for high initial aspect ratio, a droplet undergoes predominant nonlinear oscillations and subsequently breaks up. In the second part, we will discuss the electrohydrodynamics of an initially spherical droplet in a leaky dielectric medium under the influence of an external AC electric field. In sharp contrast to reported theory on electro-mechanics of droplets that trivially predicts shape oscillations of a droplet subjected to alternating current electric field about the steady-state deformation under an equivalent root mean square DC electric field under all possible electrical conductivity and permittivity ratios of the droplet and the carrier phase, here we bring out a novel dimensionality driven physical paradigm under which the same does not necessarily hold true. We found that the droplet becomes prolate (elongates in the direction of electric field) and oscillates about a mean value of degree of deformation in the case of an AC electric field. In contrast, for high permittivity and low electrical conductivity ratios, the droplet becomes oblate in the case of the equivalent DC electric field. This behavior is similar to that observed by Torza et al. (1971) experimentally.