Substrate vibrations at frequencies comparable to HF and VHF radio frequencies and in contact with a liquid generate flow at micron and submicron scales that may lead to spreading of liquid films – acoustic spreading. This spreading mechanism is thought as a way of manipulating liquids and especially water, a natural carrier of biological and biochemical agents, on microfluidic platforms. Acoustic spreading was recently observed under the influence of piston-like substrate motion [1] and surface acoustic waves [2], where vibrational convective forces, invoked near the liquid/solid/air contact line, are found to drive steady contact line motion.
Previous studies on acoustic spreading concentrated on low surface tension liquids, i.e., silicon oil, where spreading is continuous as long as liquid and substrate vibrations are in contact. Water films under acoustic excitation, however, were observed to spread to a minute extent and only under high power levels that further invoke capillary instabilities [3]; physical mechanisms associated with acoustic spreading of water and other high/intermediate surface tension liquids are not well understood and are the topic of this presentation.
We will discuss acoustic spreading of liquid films with arbitrary surface tension. We will bridge between the mechanisms associated with acoustic spreading of low and high surface tension liquids and show that film spreading is governed by a balance between capillary and convective forces. This analysis elucidates the discrepancy, observed in earlier studies, between the response of oil and deionized water to acoustic excitation and further highlights an intermediate region, where precise manipulation of the rate and direction of liquid spreading is achieved by carefully balancing governing mechanisms.
[1] O. Manor, et al; Soft Matter 2011, 7, 7976
[2] A. Rezk, , et al; Nature Commun. 2012, 3, 1167
[3] D. Collins, et al; Phys. Rev. E 2012, 86, 056312
manoro@technion.ac.il
