Laser machined micropatterns as corrosion protection of both hydrophobic and hydrophilic magnesium.

Magnesium and its alloys are promising candidate materials for medical implants because they possess excellent biocompatibility and mechanical properties comparable to bone. Furthermore, secondary surgical operations for removal could be eliminated due to magnesium's biodegradability. However, magnesium's degradation rate in aqueous environments is too high for most applications. It has been reported that hydrophobic textured surfaces can trap a surface gas layer which acts as a protective barrier against corrosion. However, prior studies have not investigated separately the role of the texture and hydrophobic treatments on magnesium corrosion rates. In this study, pillar-shaped microstructure patterns were fabricated on polished high purity magnesium surfaces by ablation with a picosecond laser. Some micropatterned samples were further processed by stearic acid modification (SAM). Micropatterned surfaces with SAM had hydrophobic properties with water droplet contact angles greater than 130°, while the micropatterned surfaces without SAM remained hydrophilic. The corrosion properties of textured and smooth magnesium surfaces in saline solution were investigated using electrochemical impedance spectroscopy (EIS) and optical microscopy. Corrosion rates on both hydrophobic and hydrophilic laser machined surfaces were reduced ∼90% relative to polished surfaces. Surprisingly, corrosion rates were similar for both hydrophobic and hydrophilic surfaces. Indirect evidence of local alkalization near microstructures was found and was hypothesized to stabilize the Mg(OH)2 layer, thereby inhibiting corrosion on hydrophilic surfaces. This is different than the corrosion resistance mechanism for superhydrophobic surfaces which makes use of gas adhesion at the liquid solid interface. These results suggest additional processing to render the magnesium hydrophobic is not necessary since it does not significantly enhance the corrosion resistance beyond what is conferred by micropatterned textures.

Continuous electrowetting via electrochemical diodes.

We describe a novel method for droplet transport combining electrowetting on dielectric (EWOD) and the diode-like behavior of valve metals to achieve unique actuation performance. While traditional EWOD droplet transport requires switching of voltage between multiple electrodes, our method, which we term continuous rectified electrowetting, utilizes a simple single electrode and a DC voltage to move a 50 µl droplet 28 mm with velocities up to 32 mm s(-1).

Characterization of electrowetting processes through force measurements.

A new method of characterizing electrowetting is presented. In this method, the electrowetting actuation forces are measured rather than the contact angle. The forces on the liquid are measured by trapping a droplet between a flat nanoindenter tip and the test substrate. When voltage is applied to electrodes in the substrate, lateral and normal forces are exerted on the tip and measured by the nanoindenter transducer. Proper selection of the tip geometry permits direct prediction of the resulting in-plane lateral forces using analytical formulas derived from the Young-Lippmann equation. Experimental results show good agreement with both analytical and numerical predictions. Numerical modeling using SURFACE EVOLVER shows that the lateral forces are relatively insensitive to most alignment errors and that the analytical model is most accurate when the flat tip is close to the substrate. Evaporation of the test liquid can introduce modest errors in long measurements, but compensation methods are presented. As the droplet undergoes almost no movement, the fluid dynamics have minimal impact on the measured forces and transient electrowetting events are readily detected. Experimental results show significant response at frequencies up to 40 Hz. This setup is useful in measuring electrowetting responses at high speeds and in measuring system degradation processes.