Substrate dependent drop deformation and wetting under high frequency vibration Ofer Manor, Michael Dentry, James R. Friend and Leslie Y. Yeo * Received 6th June 2011, Accepted 8th July 2011 DOI: 10.1039/c1sm06054f We explore the peculiar steady component of a sessile drop response to MHz order vibration, found to be dependent on its initial wettability. Placed on a vibrating hydrophobic substrate, the drop elongates vertically in the direction of the incident sound wave while remaining hydrophobic. In contrast, the drop is seen to spread on a slightly hydrophilic substrate. We elucidate this discrepancy by revealing the competing effects between the radiation pressure exerted at the bulk air/water interface and the acoustic streaming force on the contact line, revealing the critical role of the flow in the viscous boundary layer. While a number of interesting phenomena pertaining to vibration- induced liquid spreading, such as climbing drops, 1 transition to superhydrophobicity, 2 liquid contour oscillation, 3 etc., have been reported, there have been few fundamental studies explaining wetting effects under high frequency (MHz order) vibration despite renewed interest in acoustically-driven microfluidic drop actuation. 4,5 Here, we investigate MHz frequency pistonlike vibration of a sessile drop, a rather simple experiment but one that reveals an interesting char- acteristic—the drop response has a steady component that behaves very differently depending on its initial wettability. There have only been attempts to explore the behavior of vibrated liquid drops and films excited up to tens of kHz to date; 6,7 the observations we report here, and elucidate with a fundamental theoretical model, were seemingly absent from these earlier experiments due to a key factor— at 10 kHz order, acoustic streaming within the viscous boundary layer is negligible. 7 For simplicity, we will limit our discussion to the steady deviation of the drop shape from its equilibrium state at rest and ignore fast fluctuations—a reasonable assumption given that the drop shape transitions and contact line dynamics occur on much longer time scales than that of the excitation or the drop oscillation. The qualitative agreement between the experiments and theoretical prediction, which allow for boundary layer streaming effects, suggest they play a fundamental role in the peculiar response observed. Two ml deionized water drops (Millipore, Billerica, MA) were dispensed onto a 20 mm diameter and 0.99 mm thick lead zirconate titanate (PZT) thickness polarized disk (C-203; Fuji Ceramics Corp., Tokyo, Japan), on which a thin film of polytetrafluoroethylene (PTFE) (TeflonÒ 60151-100-6; Dupont, Wilmington, DE) or polydimethylsiloxane (PDMS) (Sylgard 184; Dow Corning, Midland, MI) was coated (Fig. 1 inset). We spin coated PTFE on the disk at 50 rpm for 10 s and then at 500 rpm for 60 s, followed by baking at 250  C for 2 h. Another disk was dip-coated in a PDMS solution consisting of 0.4 g silicone elastomer base and 0.1 g curing agent dissolved in 10 ml toluene (601-021-00-3; Merck KGaA, Darmstadt, Germany). The coated disk was then kept at room temperature for 3 days over which the solvent slowly evaporated and subsequently baked at 70  C for 2 h to allow final curing of the polymer. To drive disk vibration, we applied an electrical signal at resonance (2.23 and 2.11 MHz for the PTFE and PDMS coated disks, respectively) using a signal generator (SML01; Rhode & Schwarz, North Ryde, NSW, Australia) and amplifier (10W1000C; Fig. 1 Response of a 2 ml drop atop a PZT disk coated with (a) PTFE or (b-i) PDMS, under varying vibration amplitudes x 0 . The initially pinned contact line becomes unpinned for x 0 > 5.8 nm (PTFE) and x 0 > 3.7 nm (PDMS). (b-ii) Time series plot of the drop atop PDMS illustrated in (b-i) showing the initially spreading drop (0 < t < 0.4 s) subsequently receding to its initial position as the power is relaxed (t > 1.2 s); the arrows show the contact line displacement direction. After spreading to its equilibrium position, the contact line continues to vibrate but without any net displacement at long times. Inset: Schematic of the experimental setup; the dashed line denotes the symmetry axis. Micro/Nanophysics Research Laboratory, Monash University, Clayton, VIC, 3800, Australia. E-mail: leslie.yeo@monash.edu 7976 | Soft Matter , 2011, 7, 7976–7979 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2011, 7, 7976 www.rsc.org/softmatter COMMUNICATION Downloaded by Monash University on 04 September 2011 Published on 04 August 2011 on http://pubs.rsc.org | doi:10.1039/C1SM06054F View Online