Driving Wetting Transitions on Textured Surfaces Using Ultrasonic Vibration

Abstract

Adhesives, medical devices, and many cleaning products depend on the wetting of liquids on solid surfaces. The liquid/solid interaction depends on chemistry, surface topology, and external energy input. For instance, surfactants are commonly used in cleaning solutions to improve their effectiveness, and electrical fields are frequently used to control the contact angle of liquid droplets. Low frequency vibration has been used to spread, move, and manipulate droplets using the mode shape oscillations of the droplet to displace the contact line. Ultrasonic vibration (above 20 kHz) can also cause a liquid droplet to wet or spread out on a solid surface under the right circumstances. We have previously demonstrated that ultrasonic vibration can be used to control the wetting/spreading of liquid droplets on smooth hydrophobic surfaces and that the response is relatively insensitive to excitation frequency or fluid properties [1]. This paper reports on the use of ultrasonic vibration to initiate spreading on surfaces with etched pillars. Ultrasonic vibration successfully initiated a transition from Cassie to Wenzel states in all geometries with no apparent need to tune excitation frequencies to the geometry. However, the magnitude of the acceleration required to initiate the transition decreased with increased pillar spacing. For small pillar spacing, some smooth spreading in the Cassie wetting mode was observed before transition.