Unique fingering instabilities and soliton-like wave propagation in thin acoustowetting films

Abstract

Acoustic-fluid interactions not only has had a long history but has recently experienced renewed scrutiny because of their vast potential for microscale fluid and particle manipulation. Here we unravel a fascinating and anomalous ensemble of dynamic 'acoustowetting' phenomena in which a thin film drawn from a sessile drop first spreads in opposition to the acoustic wave propagation direction. The advancing film front then exhibits fingering instabilities akin to classical viscous fingering, but arising through a different and novel mechanism: transverse Fresnel diffraction of the underlying acoustic wave. Peculiar 'solitonlike' wave pulses are observed to grow above these fingers, which, on reaching a critical size, translate away along the wave propagation direction. By elucidating the complex hydrodynamics underpinning the spreading, and associated flow reversal and instability phenomena, we offer insight into the possibility of acoustically controlling fast and uniform film spreading, constituting a flexible and powerful alternative for microfluidic transport.

Double flow reversal in thin liquid films driven by megahertz-order surface vibration

“…While the thicker pair of films and a single associated flow reversal was observed in an earlier study [7], none of the remaining phenomena has been reported. The ability to form a submicrometre film, and-between the thinner pair of these three film thickness ranges-a second flow reversal and an associated unstable thickness region have now been discovered and found to lead to the formation of sharply defined film protrusions and depressions that bizarrely move in different directions depending on their particular thicknesses.…”

“…While such Eckart flows usually dominate large sessile drop translation experiments, in which the drops are observed to move along the SAW propagation direction [15,16], we have shown that these flows are also dominant in the film in figure 2d(iii) and previously in the solitary wave-like ridges in [7], as long as the thickness of these features exceed the sound wavelength (h λ ). …”

“…The steppe motion corresponds to the motion of thicker, stable films with thicknesses λ /4 < h λ in figure 2c. Such films can nevertheless increase further in thickness [7] such that h λ SAW , as shown by the small steppe region in figure 2d(iii). Films possessing this greater thickness will once again move along the direction of the SAW-the second reversal.…”

“…The thicknesses are quantized to specific values [17] as dictated by the acoustic radiation pressure upon the film interface [18], discussed in §6. In our previous work, we have shown that such counterpropagating films arise owing to a drift flow induced at the outer edge of the viscous boundary layer-Rayleigh streaming-which has a corresponding volume flux per unit width of the film [7]:…”

“…Useful for many microfluidic applications [1,2], highfrequency sound waves (O(10-10 3 MHz)) strongly interact with fluids at the micro-to nanoscale and underpin a diverse range of complex and nonlinear phenomena, from colloidal patterning [3], chaotic advection [4] and interfacial jetting [5], to self-similar droplet generation [6] and fingering instabilities [7]. An oil strip is deposited on the piezoelectric substrate along a line perpendicular to the SAW propagation path, as shown from (a) the top and (b) the side.…”