Modeling of electrowetted surface tension for addressable microfluidic systems: dominant physical effects, material dependences, and limiting phenomena (original) (raw)
Related papers
Journal of Applied Physics, 2003
This article describes the equilibrium shape of a liquid drop under applied fields such as gravity and electrical fields, taking into account material properties such as dielectric constants, resistivities, and surface tension coefficients. The analysis is based on an energy minimization framework. A rigorous and exact link is provided between the energy function corresponding to any given physical phenomena, and the resulting shape and size dependent force term in Young's equation.
Use of electrowetting to measure dynamic interfacial tensions of a microdrop
Lab on a Chip, 2012
The adsorption of surface active species to liquid-liquid and to solid-liquid interfaces can have dramatic effects in microfluidics. In this paper we show how electrowetting on dielectric can be used to monitor a dynamic liquid-liquid interfacial tension (IFT) with a time resolution of O(1 s) using amplitude modulation of the AC voltage. This straightforward method, which requires less than a microlitre of sample, is demonstrated for aqueous drops containing Triton X-100 surfactant on a Teflon AF-coated substrate and with heptane as the immiscible oil ambient. Under these conditions, next to extracting the oil-water IFT (c ow), also the effective water-substrate IFT difference (Dc ws) can be obtained from the oil-water IFT and the Young's angle. Both c ow and c ws decrease over time due to adsorption. The measured dynamic oil-water IFT compares well to results of pendant drop experiments.
Experimental characterization of droplet dispensing in electrowetting-based microfluidics
Applied Physics Letters, 2017
In this study, the effect of various parameters on the dispensed droplet size in microchannels based on the electrowetting on dielectric technique is experimentally investigated. A printed circuit board (PCB)-based microfluidic chip is used as a platform for the experiments. A crescent configuration for the channel electrodes is fabricated, which leads to a higher electrowetting force which improves the motion of the droplet. In addition, two electrode designs are proposed, which provide a nearly constant overlapping length on the reservoir electrode. The focus of this paper is on the geometry of the reservoir and the channel electrode; therefore, the channel dimensions, surface conditions, and applied voltage are kept constant. The experiments are performed for various reservoir liquid volumes and different electrode shapes of the reservoir and the microchannel. The results show that decreasing the length of the small reservoir electrode reduces the size of the dispensed droplet. It is also observed that using a channel electrode curved in the opposite direction of the droplet motion leads to a smaller dispensed droplet. Published by AIP Publishing.
Electrowetting-on-dielectric (EWOD) of sessile liquid drops on rheologically tuned soft surfaces
The inter-connection between the elasticity of a dielectric film and the wetting of a sessile drop on the same, under an applied electrical voltage, remains unaddressed. Here, we report the electrowetting-on-dielectric (EWOD) behaviour of sessile drops on dielectric elastomer films of varying elasticities-from an apparently rigid dielectric film to a soft, deformable dielectric film. Our results reveal that the elasticity of the underlying dielectric film provides an additional control over the droplet electrowetting behaviour, which may be best addressed from free energy based consideration, leading to a modification of the classical Lippman-Young paradigm. We also provide an explanation on the displacement profiles for the deformation of the soft dielectric surface, due to the interfacial electro-elastocapillary interaction triggered by the electrowetted sessile droplet. These results can be of profound importance in various emerging applications, ranging from the development of soft liquid lenses to drug delivery.
Electrowetting in a water droplet with a movable floating substrate
Physical Review E, 2016
Electrowetting (EW) enables facile manipulation of a liquid droplet on a hydrophobic surface. In this study, manipulation of an electrolyte droplet having a small floating object on it was investigated on a solid hydrophobic substrate under the EW process. Herein, the floating object exhibited a vertical motion under an applied electric field owing to the spreading and contraction of the droplet on its connecting substrates. The field-induced height variation of the floating object was significantly influenced by the thicknesses of the dielectric and hydrophobic materials. A small mass was also placed on the top floating object and its effect on the spreading of the droplet was observed. In this system, the height of the top floating object is precisely controllable under the application of an electric voltage. The proposed system is expected to be highly useful in the design of nano-and micro-oscillatory systems for microengineering.
Droplet transport by electrowetting: lets get rough!
Microfluidics and Nanofluidics, 2013
Since the pioneering works of Wenzel and Cassie Baxter in the 1930s, and now with the trivialization of the micro-and nanotechnology facilities, superhydrophobic surfaces have been announced as potentially amazing components for applications such as fluidic, optical, electronic, or thermal devices. In this paper, we show that using superhydrophobic surfaces in digital microfluidic devices could solve some usual limitations or enhance their performances. Thus, we investigate a specific monophasic (air environment) microfluidic device based on electrowetting integrating either a hydrophobic or a superhydrophobic surface as a counter-electrode. The droplet transport using a superhydrophobic surface compared with a classical hydrophobic system led to some original results. Characterization of the dynamic contact angle and the droplet shape allows us to get new insight of the fluid dynamics. Among the remarkable properties reported, a 30 % lower applied voltage, a 30 % higher average speed with a maximum instantaneous speed of 460 mm/s have been measured. Furthermore, we have noticed a huge droplet deformation leading to an increase by a factor 5 of the Weber number (from 1.4 to 7.0) on SH compared to hydrophobic surfaces. Finally, we discuss some of the repercussions of this behaviour especially for microfluidic device.
We study, experimentally and numerically, the effect of liquid electrical properties (conductivity and permittivity) and frequency of the applied potential on the electrodynamic forces generated on droplets in digital microfluidic (DMF) devices. Electrodynamic forces were found to increase with increasing the electrical conductivity in the range between 10-5 S/m to 10-3 S/m. Moreover, forces decreased significantly with increasing frequency of the applied voltage beyond a certain threshold, which increases with the increase of droplet conductivity. These results enable one to choose optimum voltage and frequency required to generate highest actuation forces and, consequently, highest droplet speed and actuation reliability.
Simulations of Interfacial Electrokinetics with Applications to Microfluidic Systems
2014
Electrokinetics plays an important role in facilitating fluid transport and particle manipulation in microfluidic systems. This dissertation studies the mechanics of electrokinetic phenomena for microscale particles and drops. The work aims to increase the understanding of complex electrokinetic phenomena for applications in Lab-on-Chip technology, assembly of colloidal particles and two-phase flow sensing. The standard model consisting of the Poisson-Nernst-Planck equations is used to study the electric double layer polarization of charged dielectric particles and channel wall which plays a major role in control and manipulation of colloidal particles and understanding of electrohydrodynamic flow field. The cases of polarization of "soft" particle under the influence of alternating current field, influence of residual charges and particle size on electrostatic interaction between charged particles at oil-water interface, and characterization of streaming potential due to ...