Controlled Wetting Using Ultrasonic Vibration

Many industrial processes such as printing and cleaning, as well as products like adhesives, coatings, and biological testing devices, rely on the wetting of liquids on a surfaces. Wetting is commonly controlled through material selection, coatings, and/or surface texture, but these means are sensitive to environmental conditions. Wetting is influenced by variables like surface tension, density, the surface chemistry, local energy barriers like surface roughness, and how the droplet is placed on the surface. Wetting of droplets can also be influenced externally in many ways such as introducing surfactants, applying electrical fields, or by dynamically excitation. Low-frequency, high amplitude vibration can initiate wetting changes prompted by droplet contact line oscillations that exceed the range of stable contact angles inherent of a droplet on a solid surface. The study of ultrasonic vibration wetting and spreading effects is sparse [1, 2], and is usually only qualitatively analyzed. Therefore, the specific goal of this thesis is somewhat unique, but also has potential as a means to controllably reverse surface adhesion. High frequency vibration effects and the governing mechanisms are relatively uncharacterized due to difficulties posed by the spatial and temporal scales. To investigate, droplets of 10, 20, and 30 µL are imaged as they vibrate on a hydrophobic surface forced via a piezoelectric transducer over different high frequencies (\u3e10 kHz). Wetting transitions occur abruptly over a range of parameters, but coincide with transducer resonance modes. The magnitude of contact angle change is dependent on droplet volume and surface acceleration, and remains after cessation of vibration, however new droplets wet with the original contact angle. A more detailed investigation of this phenomenon was necessary to obtain a better understanding. This required repeatable testing conditions, which relies heavily on surface integrity. However, some “hydrophobic” coatings are sensitive to extended water exposure. To determine which hydrophobic coatings may be appropriate for investigating dynamic wetting phenomena, samples of glass slides coated with a series of fluoropolymer coatings were tested by measuring water contact angle before, during, and after extended submersion in deionized water and compared to the same coatings subjected to ultrasonic vibration while covered in deionized water. Both methods caused changes in advancing and receding contact angle, but degradation rates of vibrated coatings, when apparent, were significantly increased. Prolonged soaking caused significant decreases in the contact angle of most coatings, but experienced significant recovery of hydrophobicity when later heat-treated at 160 C. Dissimilar trends apparent in receding contact angles suggests a unique degradation cause in each case. Roughening and smoothing of coatings was noted for coatings that were submerged and heat-treated respectively, but this did not correlate well with the changing water contact angle. Degradation did not correspond to surface acceleration levels, but may be related to how well coatings adhere to the substrate, indicative of a dissolved coating. Most coatings suffered from contact angle degradation between 20-70% when exposed to water over a long period of time, however the hydrophobic fluoropolymer coating FluoroSyl was found to remain unchanged. For this reason it was found to be the most robust coating for providing long term wetting repeatability of vibrated droplets. Droplets (10 to 70 µL) were imaged on hydrophobic surfaces as they were vibrated with ultrasonic piezoelectric transducers. Droplets were vibrated at a constant frequency with ramped amplitude. Spreading of droplets occurs abruptly when a threshold surface acceleration is exceeded of approximately 20,000 m/s2. Droplet contact area (diameter) can be controlled by varying acceleration levels above the threshold. The threshold acceleration was relatively independent of droplet volume, while initial contact angle impacts the extent of spreading. Wetting changes remain after cessation of vibration as long as the vibrated droplet remained within the equilibrium contact angle range for the surface (\u3e the receding contact angle), however new droplets wet with the original contact angle except for some cases where vibration of liquid can affect the integrity of the coating. Reversible wettability of textured surfaces is a desired effect that has various industry applications where droplet manipulation is used, like biomedical devices, coating technologies, and agriculture [3-5]