Athanasios G. Papathanasiou

On the connection between dielectric breakdown strength, trapping of charge, and contact angle saturation in electrowetting.

Electrowetting on dielectric (EWOD) is simulated by solving the equations of capillary electrohydrostatics, by the Galerkin/finite element method. Aiming to provide reliable predictions of the voltage dependence of the apparent contact angle, close to or beyond the saturation limit, special attention is given in the treatment of the dielectric properties of the solid dielectric where the liquid sits. It is proposed that in regions where the electric field strength locally exceeds the material breakdown strength, the dielectric locally switches to a conductor. Without using any fitting parameter, the implementation of the proposed phenomenological idea realized a surprising matching of published experimental data concerning materials ranging from SiO(2) to Parylene N and Teflon. Charge trapping is naturally connected to the field-induced transition, and its distribution as well as its dependence on the applied voltage is calculated.

Manifestation of the connection between dielectric breakdown strength and contact angle saturation in electrowetting

Limiting phenomena on the electrostatically assisted wetting of dielectric solids by conducting liquids are illuminated by means of computer-aided analysis. The importance of the electrostatic edge effects and their influence on the dielectric properties of the solid is raised to demonstrate that contact angle saturation sets in when the electric field strength locally exceeds the breakdown strength of the dielectric solid where the liquid sits. The proposed argument along with the computed predictions is tested against published experimental measurements showing remarkable agreement.

Mechanisms of wetting transitions on patterned surfaces: continuum and mesoscopic analysis

Micro-or nano-structurally roughened solid surfaces exhibit a rich variety of wetting behavior types, ranging from superhydro- or superoleophobicity to superhydro- or superoleophilicity. Depending on their material chemistry, the scale and morphology of their roughness or even the application of external electric fields, their apparent wettability can be significantly modified giving rise to challenging technological applications by exploiting the associated capillary phenomena at the micrometer scale. Certain applications, however, are limited by hysteretic wetting transitions, which inhibit spontaneous switching between wetting states, requiring external stimuli or actuation like thermal heating. The presence of surface roughness, necessary for the manifestation of the superhydrophobicity, induces multiplicity of wetting states and the inevitable hysteresis appears due to considerable energy barriers separating the equilibrium states. Here, by using continuum as well as mesoscopic computational analysis we perform a systems level study of the mechanisms of wetting transitions on model structured solid surfaces. By tracing entire equilibrium solution families and determining their relative stability we are able to illuminate mechanisms of wetting transitions and compute the corresponding energy barriers. The implementation of our analysis to ‘real world’ structured or unstructured surfaces is straightforward, rendering our computational tools valuable not only for the realization of surfaces with addressable wettability through roughness design, but also for the design of suitable actuation for optimal switching between wetting states.

Superior performance of multilayered fluoropolymer films in low voltage electrowetting

The requirement for low operational voltage in electrowetting devices, met using thin dielectrics, is usually connected with serious material failure issues. Dielectric breakdown (visible as electrolysis) is frequently evident slightly beyond the onset of the contact angle saturation. Here, plasma-enhanced chemical vapor deposition (PECVD) is used to deposit thin fluorocarbon films prior to the spin-coating of Teflon® amorphous fluoropolymer. The resulting multilayered hydrophobic top coating improves the electrowetting performance of the stack, by showing high resistance to dielectric breakdown at high applied voltages and for continuous long term application of DC and AC voltage. Leakage current measurements during electrowetting experiments with the proposed composite coating showed that current remains fairly constant at consecutive electrowetting tests in contrast to plain Teflon® coating in which material degradation is evident by a progressive increase in the leakage current after multiple electrowetting tests. Since the proposed composite coating demonstrates increased resistance to material failure and to dielectric breakdown even at thin configurations, its integration in electrowetting devices may impact their reliability, robustness, and lifetime.

Enabling efficient energy barrier computations of wetting transitions on geometrically patterned surfaces

Proper roughness design is important in realizing surfaces with fully tunable wetting properties. Engineering surface roughness boils down to an energy barrier optimization problem, in which the geometric features of roughness serve as the optimization parameters. Computations of energy barriers, separating admissible equilibrium wetting states on patterned surfaces, have been demonstrated utilizing fine-scale simulators (e.g., lattice-Boltzmann for mesoscale and molecular dynamics for microscale simulations), however with substantial computational requirements. Here, by solving an augmented Young–Laplace equation with a disjoining pressure term, we demonstrate accurate and efficient computations of equilibrium shapes of entire millimeter sized droplets on patterned surfaces. In particular, by adopting a natural parameterization of the Young–Laplace equation along the liquid/air and liquid/solid interfaces, the tedious implementation of the Youngs contact angle boundary condition at multiple three phase contact lines is bypassed. We, thus, enable the computation of wetting transition energy barriers, separating the well-known Cassie–Baxter and Wenzel states, as well as intermediate states, but with negligible computational cost. We demonstrate the methods efficiency by computing the equilibrium of droplets on stripe-patterned surfaces, and compare the results with mesoscopic lattice Boltzmann simulations. Our computationally efficient continuum-level analysis can be readily applied to patterned surfaces with increased and unstructured geometric complexity, and straightforwardly coupled with shape optimizers towards the design of surfaces with desirable wetting behavior.

Some twists and turns in the path of improving surface activity

Abstract The average reactivity of a catalytic surface was appreciably enhanced through spatio-temporally variable operation. Computer steering of a focused laser beam allowed the realization of controlled temperature profiles and their interaction with intrinsic system (reaction/transport) time and space scales. Real-time monitoring of the product concentration then enabled the exploration/implementation of strategies towards optimizing the overall reaction rate. The ability to dictate reaction conditions in space and time, whether in open or in closed loop [1–3] [Science 292 (2001) 1357; Nature 361 (1993) 240; Science 294 (2001) 134], opens new directions for reaction control (e.g., of activity and selectivity in more complex reaction networks) through the combination of chemistry and systems theory.

Front initiation on microdesigned composite catalysts

We first briefly review the subject of spatiotemporal pattern formation on microdesigned composite catalysts. One of the most significant interaction mechanisms between different reacting domains (consisting of different metal catalysts such as Pt and Rh, coupled through surface diffusion) is the initiation of reaction fronts at the interface between them. We then explore in some detail the effect of two-dimensional composite geometry on this basic building block of composite catalyst dynamics. (c) 2002 American Institute of Physics.

Mesoscopic model for microscale hydrodynamics and interfacial phenomena: Slip, films, and contact-angle hysteresis

We present a model based on the lattice Boltzmann equation that is suitable for the simulation of dynamic wetting. The model is capable of exhibiting fundamental interfacial phenomena such as weak adsorption of fluid on the solid substrate and the presence of a thin surface film within which a disjoining pressure acts. Dynamics in this surface film, tightly coupled with hydrodynamics in the fluid bulk, determine macroscopic properties of primary interest: the hydrodynamic slip; the equilibrium contact angle; and the static and dynamic hysteresis of the contact angles. The pseudo-potentials employed for fluid-solid interactions are composed of a repulsive core and an attractive tail that can be independently adjusted. This enables effective modification of the functional form of the disjoining pressure so that one can vary the static and dynamic hysteresis on surfaces that exhibit the same equilibrium contact angle. The modeled fluid-solid interface is diffuse, represented by a wall probability function that ultimately controls the momentum exchange between solid and fluid phases. This approach allows us to effectively vary the slip length for a given wettability (i.e., a given static contact angle) of the solid substrate.

Evaluating the Robustness of Top Coatings Comprising Plasma-Deposited Fluorocarbons in Electrowetting Systems

Abstract Thin dielectric stacks comprising a main insulating layer and a hydrophobic top coating are commonly used in low voltage electrowetting systems. However, in most cases, thin dielectrics fail to endure persistent electrowetting testing at high voltages, namely beyond the saturation onset, as electrolysis indicates dielectric failure. Careful sample inspection via optical microscopy revealed possible local delamination of the top coating under high electric fields. Thus, improvement in the adhesion strength of the hydrophobic top coating to the main dielectric is attempted through a plasma-deposited fluorocarbon interlayer. Interestingly enough the proposed dielectric stack exhibited (a) resistance to dielectric breakdown, (b) higher contact angle modulation range and (c) electrowetting cycle reversibility. Appearance of electrolysis in the saturation regime is inhibited, suggesting the use of this hydrophobic dielectric stack for the design of more efficient electrowetting systems. The possible causes of the improved performance are investigated by nanoscratch characterization.

Droplet spreading on rough surfaces: Tackling the contact line boundary condition

The complicated dynamics of the contact line of a moving droplet on a solid substrate often hamper the efficient modeling of microfluidic systems. In particular, the selection of the effective boundary conditions, specifying the contact line motion, is a controversial issue since the microscopic physics that gives rise to this displacement is still unknown. Here, a sharp interface, continuum-level, novel modeling approach, accounting for liquid/solid micro-scale interactions assembled in a disjoining pressure term, is presented. By following a unified conception (the model applies both to the liquid/solid and the liquid/ambient interfaces), the friction forces at the contact line, as well as the dynamic contact angle are derived implicitly as a result of the disjoining pressure and viscous effects interplay in the vicinity of the substrate’s intrinsic roughness. Previous hydrodynamic model limitations, of imposing the contact line boundary condition to an unknown number and reconfigurable contact lines, when modeling the spreading dynamics on textured substrates, are now overcome. The validity of our approach is tested against experimental data of a droplet impacting on a horizontal solid surface. The study of the early spreading stage on hierarchically structured and chemically patterned solid substrates reveal an inertial regime where the contact radius grows according to a universal power law, perfectly agreeing with recently published experimental findings.