Wetting morphologies at microstructured surfaces

Abstract

The wetting of microstructured surfaces is studied both experimentally and theoretically. Even relatively simple surface topographies such as grooves with rectangular cross section exhibit a large variety of different wetting morphologies as observed by atomic force microscopy. This polymorphism arises from liquid wedge formation along the groove corners and from contact line pinning along the groove edges. A global morphology diagram is derived that depends only on two system parameters: (i) the aspect ratio of the groove geometry and (ii) The contact angle of the underlying substrate material. For microfluidics, the most interesting shape regimes involve extended liquid filaments, which can grow and shrink in length while their cross section stays essentially constant. Thus, any method by which one can vary the contact angle can be used to switch the length of the filament, as is demonstrated in the context of electrowetting.surface topography ͉ wetting phenomena ͉ microfluidics R apid and efficient handling of relatively small amounts of liquids is a crucial requirement in molecular biology or biomedicine, e.g., for decoding the human genome or for analysis of small blood samples. To do this, one would like to construct labs-on-a-chip on the micrometer scale (see, e.g., ref. 1). An obvious prerequisite for such a lab is appropriate compartments for the confinement of very small amounts of liquids. These microcompartments should have some basic properties: they should have a well defined geometry by which one can measure the precise amount of liquid contained in them; they should be able to confine variable amounts of liquid; and they should be accessible in such a way that one can add and extract liquid in a convenient manner.A variety of concepts has been developed for the construction of such microfluidic systems. In most cases, a solid matrix is used that surrounds micropipes, reservoirs, etc. Here, we explore an alternative system design, namely open microfluidic systems, which contain free liquid͞vapor (or liquid͞liquid) interfaces. One advantage of these open structures is that they are directly accessible and easy to clean.There are two general strategies to construct open microfluidic systems. The first one is to chemically pattern planar substrates and to prepare distinct surface domains that differ in their wettability (2-4). The second strategy, explored here, is to use nonplanar surface topographies that can be fabricated by available photolithographic methods. We find that even relatively simple topographies such as grooves with rectangular cross sections already exhibit a large variety of different liquid morphologies such as droplets, filaments, and wedges. A systematic comparison of experimental observations and theoretical calculations reveals, however, that this polymorphism is primarily determined by only two parameters: (i) the aspect ratio X of the groove geometry, i.e., the ratio of the groove depth to the groove width; and (ii) the contact angle of the underlying substrate material. For...

Leaf surface structures enable the endemic Namib desert grass Stipagrostis sabulicola to irrigate itself with fog water

“…Fog droplets landing within the grooves will tend to remain preferentially at the groove corners where adjacent droplets will start to coalesce [32]. For an aspect ratio that is typical for the surface grooves of S. sabulicola leaves and contact angles that can be expected for leaf waxes, the liquid is confined to the grooves.…”

“…The behaviour of a groove that becomes filled by vapour condensation was studied by Seemann et al [32] and shown to be dependent on the aspect ratio (ratio between depth and width of the groove) and intrinsic contact angle. Fog droplets landing within the grooves will tend to remain preferentially at the groove corners where adjacent droplets will start to coalesce [32].…”

Multiscale dynamic wetting of a droplet on a lyophilic pillar-arrayed surface

“…So unlike figure 3(b), the liquid encountered the pillars in sequence. The fringe would also be accelerated (Seemann et al 2005) and pinned around the pillar one after another, making the MCL a complex 'zipping' line (Courbin et al 2007). The progress was also periodically repeated.…”

Classical density functional study of wetting transitions on nanopatterned surfaces

“…Wetting and interfacial phenomena are dominated by the rich interplay between intermolecular forces and the fluctuation effects associated with thermal noise and interfacial capillary waves [8][9][10][11][12]. These lead to a very rich picture of possible surface phase behaviour occurring over different length-scales, that call for cross-disciplinary fundamental investigations.…”