SAW propagation along a piezoelectric substrate. (a) Configuration of the SAW device and the IDT electrode deposited on the piezoelectric substrate to generate the SAW. (b) Interaction of the SAW with a fluid drop causes the drop to deform into an asymmetric conical shape leaning roughly at an angle corresponding to the Rayleigh angle ; the free surface of the drop undulates with a wideband frequency response roughly indicated here with a single wavelength capillary wave. The SAW itself is a retrograde traveling wave: a point on the surface travels in an ellipse in a counterclockwise fashion if the wave moves from left to right. The SAW is largest in amplitude on the surface, decaying exponentially to almost negligible levels within four to five wavelengths below the surface. The acoustic irradiation causes drop deformation through first-order effects on the time scale of the acoustic wave and bulk fluid recirculation on a hydrodynamic time scale, known as acoustic streaming, through second-order time-averaged effects. The SAW amplitude is reduced by the interaction with the drop due to the effects of viscous dissipation. (c) Schematic representation of the elliptical trajectory of the particle elements on the surface as the SAW Rayleigh wave traverses the surface. Atomization occurs from the free surface of the irradiated drop.
Two sets of image sequences showing drop deformation into an asymmetric conical shape and the rapid destabilization of its free surface leading to atomization of small droplets from the parent drop due to SAW irradiation, acquired at and magnification. Both occur above a hydrophobic Teflon coated surface, spin coated above the lithium niobate substrate. In the second case, note the formation of an elongated axisymmetric cylindrical jet at and , which eventually pinches off to form a droplet at and . In both cases, the SAW propagation direction is from the right to the left of the image.
Drop deformation and atomization above a hydrophilic (bare lithium niobate) surface. The figure shows a sequence of images acquired at and magnification showing atomization of a drop which has spread into a thin film.
Results from the FFT frequency sweep using the scanning laser Doppler vibrometer showing the frequency at which the capillary wave on the free surface of the drop is excited. (a) Drop with finite thickness of approximately : the resonant frequency of the capillary wave is approximately . (b) Thin film of thickness : the resonant frequency of the capillary wave is approximately , the exciting frequency.
Sequence of images taken at showing the formation of a crest at the tip of an elongated drop on a hydrophobic substrate (Fig. 2) due to SAW irradiation. The crest is whipped outwards and upwards on the rise half-cycle of a traveling capillary wave on the free surface. On the fall half-cycle, the forward propagation of this wave essentially leaves the tail of the elongated crest behind, thinning the thread and resulting in pinch-off to form a large droplet. Due to its mass and low ejection velocity, this droplet falls back and recombines with the parent drop (not shown). The time between successive images in each row is ; however, the first image in each row is taken at the times shown.
Three separate instances of jetting phenomena from the free surface of a thin film. The liquid thread that forms the axisymmetric jet arising due to the destabilization of a capillary wave on the free surface elongates and pinches-off near the base to form an ejected droplet. The velocity of the jet is roughly of the order but the droplet velocity, upon pinch-off, is much slower, approximately . The images were acquired at and, hence, the time between successive images is for all cases.
Variation in the amplitude of the capillary wave on the free surface of a liquid film with the frequency, which is a function of the film aspect ratio . The onset of atomization occurs around .
Schematic representation of the destabilization of an initial slender sessile drop with height and length . The initial drop profile is given by the solid line whereas the destabilized drop is given by the dashed line. The spatiotemporal film height distribution is given by , whereas the wavelength of the capillary waves is .
Drop interfacial profiles at various dimensionless times for (a) , (b) , and . The dimensionless wave number and frequency are held at and , respectively. The initial drop profile (first frame) relaxes to its equilibrium position and translates a short distance to the right in the direction of propagation of the imposed traveling acoustic pressure wave (second frame). During this initial transient, interfacial waves are observed to appear, induced by the pressure wave. In the final frame, these interfacial undulations remain stable due to insufficient acoustic forcing to overcome the stabilizing capillary forces, as shown in the inset for small values in case (a), or are rapidly destabilized for values beyond a critical threshold leading towards atomization in cases (b) and (c). The critical threshold for interfacial destabilization therefore lies somewhere around .
Drop interfacial energy defined by Eq. (34) as a function of time for various acoustic capillary numbers . The other parameters are and .
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