Principles of microfluidic actuation by modulation of surface stresses

Development and optimization of multifunctional devices for fluidic manipulation of films, drops, and bubbles require detailed understanding of interfacial phenomena and microhydrodynamic flows. Systems are distinguished by a large surface to volume ratio and flow at small Reynolds, capillary, and Bond numbers are strongly influenced by boundary effects and therefore amenable to control by a variety of surface treatments and surface forces. We review the principles underlying common techniques for actuation of droplets and films on homogeneous, chemically patterned, and topologically textured surfaces by modulation of normal or shear stresses

Characterization of the Heat Transfer Accompanying Electrowetting-Induced Droplet Motion

Electrowetting (EW) involves the actuation of liquid droplets using electric fields and has been demon- strated as a powerful tool for initiating and controlling droplet-based microfluidic operations such as droplet transport, generation, splitting, merging and mixing. The heat transfer resulting from EW- induced droplet actuation has, however, remained largely unexplored owing to several challenges under- lying even simple thermal analyses and experiments. In the present work, the heat dissipation capacity of actuated droplets is quantified through detailed modeling and experimental efforts. The modeling involves three-dimensional transient numerical simulations of a droplet moving under the action of grav- ity or EW on a single heated plate and between two parallel plates. Temperature profiles and heat trans- fer coefficients associated with the droplet motion are determined. The influence of droplet velocity and geometry on the heat transfer coefficients is parametrically analyzed. Convection patterns in the fluid are found to strongly influence thermal transport and the heat dissipation capacity of droplet-based systems. The numerical model is validated against experimental measurements of the heat dissipation capacity of a droplet sliding on an inclined hot surface. Infrared thermography is employed to measure the transient temperature distribution on the surface during droplet motion. The results provide the first in-depth analysis of the heat dissipation capacity of electrowetting-based cooling systems and form the basis for the design of novel microelectronics cooling and other heat transfer applications

Electrical Actuation of Dielectric Droplets

Electrical actuation of liquid droplets at the microscale offers promising applications in the fields of microfluidics and lab-on-a-chip devices. Much prior research has targeted the electrical actuation of electrically conducting liquid droplets; however, the actuation of dielectric droplets has remained relatively unexplored, despite the advantages associated with the use of a dielectric droplet. This paper presents modeling and experimental results on the electrical actuation of dielectric droplets between two flat plates. A first-order analytical model, based on the energy-minimization principle, is developed to estimate the electrical actuation force on a dielectric droplet as it moves between two flat plates. Two versions of this analytical model are benchmarked for their suitability and accuracy against a detailed numerical model. The actuation force prediction is then combined with available semi-analytical expressions for predicting the forces opposing droplet motion to develop a model that predicts transient droplet motion under electrical actuation. Electrical actuation of dielectric droplets is experimentally demonstrated by moving transformer oil droplets between two flat plates under the influence of an actuation voltage. Droplet velocities and their dependence on the plate spacing and the applied voltage are experimentally measured and showed reasonable agreement with predictions from the models developed

Numerical Modeling of Deformation, Oscillation, Spreading and Collision Characteristics of Droplets in an Electric Field

Electric field induced flows, or electrohydrodynamics (EHD), have been promising in many fast-growing technologies, where droplet movement and deformation can be controlled to enhance heat transfer and mass transport. Several complex EHD problems existing in many applications were investigated in this thesis. Firstly, this thesis presents the results of numerical simulations of the deformation, oscillation and breakup of a weakly conducting droplet suspended in an ambient medium with higher conductivity. It is the first time that the deformation of such a droplet was investigated numerically in a 3D configuration. We have determined three types of behavior for the droplets, which are less conducting than ambient fluid: 1) oblate deformation (which can be predicted from the small perturbation theory), 2) oscillatory oblate-prolate deformation and 3) breakup of the droplet. Secondly, a numerical study of droplet oscillation placed on different hydrophobic surfaces under the effect of applied AC voltage including the effect of ambient gas was investigated. The presented algorithm could reproduce droplet oscillations on a surface considering different contact angles. It has been found that the resonance frequency of the water droplet depends on the surface property of the hydrophobic materials and the electrostatic force. Thirdly, a new design of an electrowetting mixer using the rotating electric field was proposed which offers a new method to effectively mix two droplets over a different range of AC frequencies. Two regimes were observed for droplet coalescence: 1) coalescence due to the high droplet deformation, 2) coalescence due to the interaction of electrically induced dipoles. Fourthly, the spreading and retraction control of millimetric water droplets impacting on dry surfaces have been investigated to examine the effect of the surface charge density and electric field intensity. The effect of the surface charge on the spreading of droplets placed gently on surfaces was investigated in the first part. It was found that the maximum spreading diameter increases with an increasing charge. In the second part, the impact of a droplet on a ground electrode was considered. It was also found that in order to keep the maximum diameter after the impact, less charge is needed for surfaces with lower contact angle. Finally, the interaction between two identical charged droplets was investigated numerically. The effects of the impact velocity, drop size ratio and electric charge on the behavior of the combined droplet were investigated. It was shown that two conducting droplets carrying charges of the same polarity under some conditions may be electrically attracted. The formation of charged daughter droplets has been investigated and it was found that the number of the satellite droplets after collision appears to increase with an increase in the droplet charge