Golak Kunti

Rapid mixing with high-throughput in a semi-active semi-passive micromixer

In this paper, we investigate a novel alternating current electrothermal (ACET) micromixer driven by a high efficiency ACET micropump. The micromixer consists of thin film asymmetric pairs of electrodes on the microgrooved channel floor and array of electrode pairs fabricated on the top wall. By connecting electrodes with AC voltage, ACET forces are induced. Asymmetric microgrooved electrodes force the fluids along the channel, while lateral vortex pairs are generated by symmetric electrode pairs located on the top wall. Waviness of the floor increases contact area between two confluent streams within a narrow confinement. An active mixer operates as a semi active semi passive mixer. Effects of various parameters are investigated in details in order to arrive at an optimal configuration that provides for efficient mixing as well as appreciable transport. It is found that using a specific design, uniform and homogeneous mixing quality with mixing efficiency of 97.25% and flow rate of 1.794μm2/ min per unit width of the channel can be achieved.

Numerical investigations of electrothermally actuated moving contact line dynamics: Effect of property contrasts

This article reports interfacial electro-thermo-chemical-hydrodynamics of binary fluids with contrasting viscosity, thermal conductivity, and electrical conductivity of fluids under AC electrokinetics, called alternating current electrothermal (ACET) mechanism, over wetted substrates. The interfacial kinetics of the two phases are modulated by the coupled influence of electrothermal, viscous, and capillary forces. Numerical investigations of contact line dynamics reveal that at low viscosity of displaced fluid, viscous drag force significantly reduces leading to faster progression of the contact line. Larger viscous drag force at higher viscosity of the displaced fluid resists the interface motion to travel along the capillary. ACET forces are the direct consequences of the thermal and electric fields. For low thermal conductivity of the displaced fluid, the temperature gradient becomes much stronger leading to higher ACET forces and contact line velocity. Below a threshold limit of thermal conductivity, ...

Electrothermally actuated moving contact line dynamics over chemically patterned surfaces with resistive heaters

In this paper, we explore the moving contact line dynamics of two Newtonian immiscible fluids over substrates patterned with two different alternative chemical patches. The bulk fluid motion is actuated using electrothermal kinetics where the thermal field is generated by incorporating resistive heaters on the substrate. The electrothermal forces, which arise from the local gradient in electrical conductivity and permittivity, strongly depend on the local temperature and potential distributions. The thermal field and the potential distribution can be modulated by altering the heater characteristics and electrode patterning. The contact line motion and its intricate physics can be effectively tuned by altering the geometrical parameters of the heaters and electrode arrangement. Further, a comparison is executed between conventional electrothermal and heater-assisted electrothermal processes. The interfacial dynamics of the immiscible binary fluids is greatly affected by the present electrothermal mechanism and shows advantages over the conventional electrothermal process. The results presented here are effective for developing various smart devices involving multiphase flow dynamics within an electrokinetic paradigm.

Electro-thermally driven transport of a non-conducting fluid in a two-layer system for MEMS and biomedical applications

Biomedical and biochemical applications pertaining to ion exchange or solvent extraction from one phase to another phase often deal with two-fluid flows, where one layer is non-conducting and the other layer is a biofluid. In the present study, we investigate the transport of two-layer immiscible fluids consisting of one non-conducting fluid and another conducting fluid layer in a micro-grooved channel, employing an alternating current electrothermal (ACET) mechanism. The conducting fluid, driven by the influence of ACET forces, transfers its induced momentum across the fluid-fluid interface allowing the movement of the non-conducting fluid layer. We use an order parameter based approach to track the interface of the two-layer fluid transport via the coupled Cahn-Hilliard-Navier-Stokes equation, while the potential and temperature distribution are solved using the Laplace equation and the thermal energy balance equation, respectively. The efficiency with which the non-conducting layer gets transported is ...

Alteration in contact line dynamics of fluid-fluid interfaces in narrow confinements through competition between thermocapillary and electrothermal effects

The paper reports the results of our numerical investigation on contact line dynamics of a thermal field assisted flow configuration of two immiscible fluids 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 a non-uniform electric field. In this study, the interplay of thermocapillary and electrothermal forces on interfacial dynamics are considered. The former is caused by temperature-induced surface tension gradients while the latter is originated from the temperature-induced 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 flow dynamics. For the same strength of thermocapillary and electrothermal forces (characterized by individual dimensionless numbers), electrothermal effects dominate over thermocapillary effects. However, interfacial forces dominate over electrothermal forces at certain wettabilities (characterized by the imposed contact angle on the surfaces), and depending on the direction of the interfacial forces, the contact line travels toward the entry or exit of the channel.The paper reports the results of our numerical investigation on contact line dynamics of a thermal field assisted flow configuration of two immiscible fluids 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 a non-uniform electric field. In this study, the interplay of thermocapillary and electrothermal forces on interfacial dynamics are considered. The former is caused by temperature-induced surface tension gradients while the latter is originated from the temperature-induced 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 flow dynamics. For the same strength of thermocapillary and...

Electrothermally modulated contact line dynamics of a binary fluid in a patterned fluidic environment

In this paper, we depict the 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 investigation, based on the free-energy-based phase field formalism, reveals 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 the interfacial tension and the driving electrothermal force. An interplay of these 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 the contact line pinning. Alteration in the position of the ribs between the electrode pairs changes the electric field strength and thereby the bulk ACET forces across the contact line. Furthermore, by suitable arrangement of these ribs, various intricate shapes of the liquid front can be achieved over a short distance, which can have significant implications on the morphological control of microscale flow.In this paper, we depict the 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 investigation, based on the free-energy-based phase field formalism, reveals 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 the interfacial tension and the driving electrothermal force. An interplay of these 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 the contact line pinning. Alteration in the position of the ribs between the electrode pairs changes the electric fie...

Energy-efficient generation of controlled vortices on low-voltage digital microfluidic platform

Generating controlled vortices in a sessile surface droplet configuration in an energy efficient manner is an outstanding research problem of interdisciplinary relevance, having implications in widely varying areas ranging from biomedical diagnostics, thermal management to digital microfluidic technology. Here, we experimentally and theoretically demonstrate a simple yet energy efficient strategy for generating controlled vortices inside a surface droplet, by deploying interacting electrical and thermal fields over inter-digitated electrodes on an electrically wetted platform. Unlike the traditional electrically driven mechanisms, this strategy involves significantly low voltage ( ≤ 10 V) to induce rotational structures inside the droplet, by exploiting the strong spatial gradient of electrical properties on account of the prevailing thermal field as attributable to intrinsically induced Joule heating effects. Our experiments demonstrate that fluid velocities typically of the order of mm/s can be generated inside the droplet within the standard regimes of operating parameters, bearing far-reaching consequences towards enhancing internal mixing in multifarious droplet based microfluidic applications. An inherent integrability with the existing electrowetting on dielectric platforms renders the process ideal to be used in conjunction with digital microfluidic technology.Generating controlled vortices in a sessile surface droplet configuration in an energy efficient manner is an outstanding research problem of interdisciplinary relevance, having implications in widely varying areas ranging from biomedical diagnostics, thermal management to digital microfluidic technology. Here, we experimentally and theoretically demonstrate a simple yet energy efficient strategy for generating controlled vortices inside a surface droplet, by deploying interacting electrical and thermal fields over inter-digitated electrodes on an electrically wetted platform. Unlike the traditional electrically driven mechanisms, this strategy involves significantly low voltage ( ≤ 10 V) to induce rotational structures inside the droplet, by exploiting the strong spatial gradient of electrical properties on account of the prevailing thermal field as attributable to intrinsically induced Joule heating effects. Our experiments demonstrate that fluid velocities typically of the order of mm/s can be genera...