Search

A chemically driven dewetting effect is demonstrated using sessile droplets of dilute hydrofluoric acid on chemically oxidized silicon wafers. The dewetting occurs as the thin oxide is slowly etched by the droplet and replaced by a hydrogen-terminated surface; the result of this is a gradual increase in the contact angle of the droplet with time. The time-varying work of adhesion is calculated from the time-varying contact angle; this corresponds to the changing chemical nature of the surface during dewetting and can be modeled by the well-known logistic (sigmoid) function often used for the modeling of restricted growth, in this case, the transition from an oxidized surface to a hydrogen-terminated silicon surface. The observation of the time-varying contact angle allows one to both measure the etch rate of the silicon oxide and estimate the hydrogenation rate as a function of HF concentration and wafer type. In addition to this, at a certain HF concentration, a self-running droplet effect is observed. In contrast, on hydrogen-terminated silicon wafers, a chemically induced spreading effect is observed using sessile droplets of nitric acid. The droplet spreading can also be modeled using a logistical function, where the restricted growth is the transition from hydrogen-terminated to a chemically induced oxidized silicon surface. The chemically driven dewetting and spreading observed here add to the methods available to study dynamic wetting (e.g., the moving three-phase contact line) of sessile droplets on surfaces. By slowing down chemical kinetics of the wetting, one is able to record the changing profile of the sessile droplet with time and gather information concerning the time-varying surface chemistry. The data also indicates a chemical interface hysteresis (CIH) that is compared to contact angle hysteresis (CAH). The approach can also be used to study the chemical etching and deposition behavior of thin films using liquids by monitoring the macroscopic droplet profile and relating this to the time-varying physical and chemical interface phenomena. [ABSTRACT FROM AUTHOR]

A voltage polarity dependent, voltage-induced wetting transition is demonstrated using a liquid metal-oxide-semiconductor (MOS) junction. A droplet of mercury can be made to spread out on an oxidized silicon wafer upon the application of a voltage. The wetting is seen to depend on the doping type and doping density of the silicon-a voltage polarity related asymmetrical electrowetting is observed. Impedance measurements on Al-SiO2-Si MOS capacitors link the space-charge effects with the electrowetting on the Hg-SiO2-Si MOS junction. A modified Young-Lippmann equation for electrowetting at a MOS junction is derived-the model agrees with the experiments. [ABSTRACT FROM AUTHOR]

We demonstrate a proof-of-concept that microelectromechanical systems (MEMS) can be optically actuated using photoelectrowetting. A 30 µm thick aluminium cantilever is actuated using an ordinary white light source via the modulation of capillary forces in a liquid bridge on a Teflon® coated commercial silicon wafer. A deflection of 58 µm is observed using a light power of 100 mW at a bias of 7 V. The deflection of the cantilever relies on the photoelectrowetting effect [S. Arscott, Sci. Rep. 1, 184 (2011)]. Such wireless actuation could be useful for optical addressing and control of autonomous wireless sensors, MEMS, and microsystems. [ABSTRACT FROM AUTHOR]

We report electrowetting at a liquid metal-semiconductor (Schottky) junction using of a mercury droplet resting on silicon. This is demonstrated using n-type and p-type single-crystal silicon wafers of different doping levels. The voltage-dependent wetting contact angle variation of the mercury droplet is observed to depend on both the underlying semiconductor doping density and type. The electrowetting behavior can be explained by the voltage-dependent modulation of the capacitance of a Schottky junction; current-voltage and capacitance-voltage measurements indicate this to be the case. A modified Young-Lippmann electrowetting equation-formulated using a well-established metal-semiconductor junction model-agrees well with the observations. [ABSTRACT FROM AUTHOR]

A proof-of-concept demonstration of the electrowetting-on-dielectric of a sessile soap bubble is reported here. The bubbles are generated using a commercial soap bubble mixture-the surfaces are composed of highly doped, commercial silicon wafers covered with nanometer thick films of Teflon®. Voltages less than 40 V are sufficient to observe the modification of the bubble shape and the apparent bubble contact angle. Such observations open the way to inter alia the possibility of bubble-transport, as opposed to droplet-transport, in fluidic microsystems (e.g., laboratory-on-a-chip)-the potential gains in terms of volume, speed, and surface/volume ratio are non-negligible. [ABSTRACT FROM AUTHOR]