Acoustic droplet vaporization on hydrophobic and hydrophilic solid surfaces

Acoustic-driven vaporization of volatile droplets, such as dodecafluoropentane (DDFP) droplets with a lower boiling temperature of 29°C, receives attention as a promising means for medical diagnostic and therapeutic applications. The acoustic droplet vaporization (ADV) process on hydrophobic and hydrophilic surfaces is experimentally investigated using a microscope and high-speed camera system with temporal and spatial resolutions of 150,000 frames/s and 5 pixels/um, respectively. A water-repellent agent is coated on a slide glass for a hydrophobic surface whereas an anti-fogging agent for a hydrophilic surface. The DDFP droplet has a contact angle of 105 degree on the hydrophobic surface and 45 degree on the hydrophilic surface. Ultrasound with a center frequency of 5 MHz was applied to DDFP droplets immersed in liquid water. The present ADV experiments show that the droplet vaporization rate is higher on the hydrophilic surface than on the hydrophobic surface. The droplet-bubble compound is always attached on the hydrophobic surface during the ADV process, but the compound grown on the hydrophilic surface slides off while leaving a small droplet portion. The effect of ambient water temperature on the droplet and bubble motion was quantified.