Article pubs.acs.org/Langmuir
Drop Motion Induced by Repeated Stretching and Relaxation on a Gradient Surface with Hysteresis Jonathan E. Longley,† Erin Dooley,† Douglas M. Givler,† William J. Napier, III,† Manoj K. Chaudhury,† and Susan Daniel*,‡ †
Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14850, United States
‡
S Supporting Information *
ABSTRACT: The motion of a droplet can be induced by periodically compressing and extending it between two similar gradient surfaces possessing significant wetting hysteresis. The shape fluctuation of the drop during repeated compression−extension cycles leads to its ratchetlike motion toward the region of higher wettability. A simple model requiring the volume preservation of the drop during the compression−extension cycles is sufficient to account for the effect and predict drop velocity across the surface when drop size and cycle frequency are specified. In connection with this study, we also report a variation of the standard vapor phase adsorption method of preparing a chemically graded surface that allows for good control over the steepness and the length of the active zone. The method can be used to produce a linear or a radial gradient, both of which are employed here to drive droplet motion along these patterns. This type of discrete droplet motion can be used to move drops on surfaces to transport materials within miniaturized digital fluidic devices.
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INTRODUCTION A chemical and/or a morphological gradient designed on a surface can be used to propel liquid drops in preferential directions.1,2 For this motion to occur, the gradient surfaces should be of negligible hysteresis. In the presence of the ubiquitous defects on a surface, a liquid droplet may not move unless the gradient force is strong enough to overcome the resulting hysteresis.3−6 The effect of hysteresis can, however, be mitigated if additional energy is gained from, for example, the coalescence of multiple drops on a surface.7,8 There have also been some recent developments in drop fluidics in which an oscillatory electric field,9−11 vibration,4,12−19 and electrowetting20−22 have been used to overcome hysteresis. In a previous paper,12 we demonstrated that the periodic squeezing and extension of a pinned drop lends itself to an inchworm type motion on a gradient surface. Similar observations have also been made when a drop is squeezed and extended between two nonparallel surfaces in a scissoring mode.23,24 In both cases, the hysteresis rectifies the shape fluctuation of the drop and the resulting capillary force generates the motion. Since we reported the phenomenon a few years back,12 no systematic study has been performed to describe the velocity of the drop as a function of either the drop volume or the driving frequency. In this paper, we extend our previous study, in which a drop of known volume is placed between two gradient surfaces, with one of the surfaces undergoing low frequency (1−10 Hz) vibration. The sequential pinning and depinning of the front and rear edges of the drop causes it to translate unidirectionally. Velocity of drop motion increases with the © 2012 American Chemical Society
drop volume as well as the frequency of oscillation, the magnitude of which can be predicted with a simple volume conservation model as we show here.
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EXPERIMENTAL SECTION
Preparation and Characterization of Surface Energy Gradients. Linear surface energy gradients were prepared on glass substrates by diffusion controlled silanization with dodecyltrichlorosilane, which is a variation of a method published previously.7 The method is somewhat similar to those of recent publications,25,26 in which a gradient was prepared by imagewise modulation of the diffusion path of an alkanethiol on gold. Glass slides (75 × 25 × 1 mm; Fisher Scientific) were first cleaned in a piranha solution (30% H2O2 (50% w/w in H20) and 70% H2SO4 by volume) for 30 min, which were then rinsed with copious amounts of deionized water (DI water; Barnstead) and blow dried with ultrahigh purity nitrogen (N2, Praxair Inc.). Next, the glass slides were treated with oxygen plasma at 0.2 Torr for 45 s (Harrick Plasma Cleaner, model PDC-32G). Chemical gradients were established on all the glass slides within ∼10 min after removing them from the plasma cleaner. This procedure was performed in a glovebox purged with dry N2 (
