Driving Wetting Transitions on Textured Surfaces Using Ultrasonic Vibration

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.

Ultrasonic vibration may also be used to alter the typical wetting behaviors of liquids on textured or rough surfaces. However, most real surfaces are not ideally smooth but inherently rough and may thereby act to impede contact line motion or trap gas beneath a droplet, thereby reducing adhesion (wetting). To test this hypothesis, textured surfaces were patterned using photolithography and DRIE to create 90 µm square pillars spaced at varying distances (from 30 to 90 µm) on a silicon substrate. The texture was subsequently coated with a hydrophobic fluoropolymer coating, FluoroSyl 3750 (Cytonix). A piezoelectric transducer was used to vibrate 10, 20, and 30 µL droplet of deionized water on the textured surfaces. The surface acceleration magnitude was measured with a surface mount accelerometer. Droplets were imaged with a high speed camera as they were vibrated. The surface was vibrated at a frequency of 41.4 kHz (near the transducer resonance), and the vibration amplitude was increased from zero to 350 V with four separate voltage ramp rates in each instance.

Droplets began in the Cassie state with gas visibly trapped under the droplet. As vibration amplitude increases, droplets begin to partially wet into the trenches between the pillars, while the contact line may spread/jump along the tops of the pillars. When acceleration levels are sufficiently high, the droplet abruptly wets the substrate – transitioning from a Cassie to Wenzel state – while simultaneously spreading significantly. The surface acceleration level at which droplets partially wet the substrate was relatively constant for all parameters tested. The surface acceleration required to spread the contact line of droplets along the top of the pillars increases with increasing pillar spacing, but if the pillar spacing is greater than 60 µm, the droplet usually transitions to Wenzel state before spreading. The surface acceleration required for a droplet to completely wet the substrate decreases with increasing pillar spacing. After cessation of surface vibration, wetting changes that occurred during the transitions remain, and the liquid has a new state of equilibrium. However, when subsequent droplets are placed on the surface, they wet the surface similarly to the previous droplet before vibration. No dependency on droplet volume was observed, but higher acceleration ramp rates require high surface acceleration levels for each wetting transition to occur.

The use of ultrasonic vibration and textured surfaces could be used to improve the bond strength of adhesives. Ultrasonic vibration substantially lowers the advancing and receding angle of liquid droplets on textured surfaces. In one instance using a 45 µm spacing texture, the advancing, and receding angle were lowered on average from 165 and 120 degrees to 120 and 70 degrees when the droplet was vibrated at 41.4 kHz with 100 V amplitude driving signal. The average roll off angle was also increased from 10 to 45 degrees. Varying the depth and spacing of the texture, as well as the vibration characteristics, could maximize adhesion in critical applications where only small bonding surface areas are available.