Ultrasonic estimation of the contact angle of a sessile droplet

• Published in: AIP conference proceedings Vol. 1581; no. 1
• Main Authors: Quintero, R, Simonetti, F
• Format: Conference Proceeding Journal Article
• Language: English
• Published: Melville American Institute of Physics 18.02.2014
• Subjects: Actuation; AMPLITUDES; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Contact angle; DEPOSITS; DROPLETS; EVAPORATION; FINITE ELEMENT METHOD; INTERACTIONS; MONITORING; RAYLEIGH WAVES; SCATTERING; SIMULATION; SOUND WAVES; SUBSTRATES; Surface acoustic waves; SURFACE TENSION; Wave propagation; Wave reflection
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/1.4865027

Radiation of energy by large amplitude leaky Rayleigh waves is regarded as one of the key physical mechanisms regulating the actuation and manipulation of droplets in surface acoustic wave (SAW) microfluidic devices. The interaction between a SAW and a droplet is highly complex and is presently the subject of extensive research. This paper investigates the existence of an additional interaction mechanism based on the propagation of quasi-Stoneley waves inside sessile droplets deposited on a solid substrate. In contrast with the leaky Rayleigh wave, the energy of the Stoneley wave is confined within a thin fluid layer in contact with the substrate. The hypothesis is confirmed by three-dimensional finite element simulations and ultrasonic scattering experiments measuring the reflection of Rayleigh waves from droplets of different diameters. Moreover, real-time monitoring of the droplet evaporation process reveals a clear correlation between the droplet contact angle and the spectral information of the reflected Rayleigh signal, thus paving the way for ultrasonic measurements of surface tension.