Article pubs.acs.org/Langmuir
Stick−Jump (SJ) Evaporation of Strongly Pinned Nanoliter Volume Sessile Water Droplets on Quick Drying, Micropatterned Surfaces Damien Debuisson, Alain Merlen, Vincent Senez, and Steve Arscott* Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), CNRS UMR 8520, The University of Lille, Cité Scientifique, Avenue Poincaré, 59652 Villeneuve d’Ascq, France S Supporting Information *
ABSTRACT: We present an experimental study of stick− jump (SJ) evaporation of strongly pinned nanoliter volume sessile water droplets drying on micropatterned surfaces. The evaporation is studied on surfaces composed of photolithographically micropatterned negative photoresist (SU-8). The micropatterning of the SU-8 enables circular, smooth, troughlike features to be formed which causes a very strong pinning of the three phase (liquid−vapor−solid) contact line of an evaporating droplet. This is ideal for studying SJ evaporation as it contains sequential constant contact radius (CCR) evaporation phases during droplet evaporation. The evaporation was studied in nonconfined conditions, and forced convection was not used. Micropatterned concentric circles were defined having an initial radius of 1000 μm decreasing by a spacing ranging from 500 to 50 μm. The droplet evaporates, successively pinning and depinning from circle to circle. For each pinning radius, the droplet contact angle and volume are observed to decrease quasi-linearly with time. The experimental average evaporation rates were found to decrease with decreasing pining radii. In contrast, the experimental average evaporation flux is found to increase with decreasing droplet radii. The data also demonstrate the influence of the initial contact angle on evaporation rate and flux. The data indicate that the total evaporation time of a droplet depends on the specific micropattern spacing and that the total evaporation time on micropatterned surfaces is always less than on flat, homogeneous surfaces. Although the surface patterning is observed to have little effect on the average droplet fluxindicating that the underlying evaporation physics is not significantly changed by the patterningthe total evaporation time is considerably modified by patterning, up to a factor or almost 2 compared to evaporation on a flat, homogeneous surface. The closely spaced concentric circle pinning maintains a large droplet radius and small contact angle from jump to jump; the result is a large evaporation rate leading to faster evaporation.
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INTRODUCTION The evaporation of small liquid droplets has been studied for some time now.1−3 The reason for this is that there are many practical applications whose optimal functioning relies heavily on knowledge generated from fundamental studies: inter alia cooling,4 smart clothing,5 adhesives,6 printing7 and deposition,8 DNA chips,9 medical,10 micro-11 and nanofabrication,12 microsystems,13 mass spectrometry14 fire safety,15 agriculture,16 and combustion.17 Many of these applications are beginning to exploit the complex patterning of surfaces which can be obtained using micro- and nanotechnologies in order to control the wetting 18,19 and thus the evaporation of sessile droplets3,20−23where the latter behavior is complicated by the presence of the underlying surface24 and the three-phase (vapor−solid−liquid) interface.8,25−28 There are several classic sequential modes of evaporation of a sessile droplet (see comprehensive reviews3,20−23). In recent years, both the “stick−slip” and “stick−jump” (SJ) modes have been studied in terms of droplet evaporation and dissolution.29−37 The SJ evaporation mode proceeds as a droplet evaporates and © XXXX American Chemical Society
successively pins and depins from well-defined features, e.g., particles in the droplet34,38 or surface patterns created using microtechnology,39−41 where controlled evaporation is achieved via a designed surface. One particular characteristic of SJ evaporation mode is that the abrupt jump times are short (
