Anisotropic wetting and de-wetting of drops on substrates patterned with polygonal posts

Abstract

Table of contentWe record contact line motion of ink-jet printed drops spreading and evaporating on surfaces patterned with polygonal microposts and, aided by lattice Boltzmann simulations, explain the drop shapes in terms of interface pinning at posts. AbstractWe present results showing how water drops, produced by ink-jet printing, spread on surfaces patterned with lattices of diamond or triangular posts. Considering post widths typically ~7 m and lattice spacings between 15-40 m, we observe drop shapes with 3,4 and 6-fold symmetry, depending on both the symmetry of the lattice and the shape of the posts. This is a result of the different mechanisms of interface pinning and depinning which depend on the direction of the contact line motion with respect to the post shape. Lattice Boltzmann simulations are used to describe these mechanisms in detail for triangular posts. We also follow the motion of the contact line as the drops evaporate showi ng that they tend to return to their original shape. To explain this we show that the easy direction for movement is the same for sprea ding and drying drops. We compare the behaviour of small drops with that of larger drops created by jetting several drops at the same position. We find that the contact line motion is unexpectedly insensitive to drop volume, even when a spherical cap of fluid forms above the posts. The findings are relevant to m icro-fluidic applications and to the control of drop shapes in i nk-jet printing.

Wetting of anisotropic sinusoidal surfaces—experimental and numerical study of directional spreading

“…patterns composed of several primary directions, as it has been done by Vrancken et al [18]. Hence, the authors managed to obtain droplets adopting polygonal-like footprints.…”

Modelling unidirectional liquid spreading on slanted microposts

“…The contact angle of the liquid with both the posts and the substrate is θ which we vary in the range 30 o to 70 o . This range is representative of different hydrophilic material, for example the polymers considered in[1] or [16].…”

Dynamic polygonal spreading of a droplet on a lyophilic pillar-arrayed surface

“…Jokinen et al [24] developed a method of fabricating irregular pillars on a square array surface, and found directional wetting property of droplets on this surface, where droplets spread to irregular square-like shapes. and Vrancken et al [7] reported the droplet shapes depend on the array geometry, pillar shape and array space. In previous studies, the researchers focused on the final shapes of droplets on lyophilic pillar-arrayed surfaces, while the evolution of the projected shape from an initial circle to a final polygon is essentially a dynamic process.…”

“…Spreading of a droplet on a micro-structured surface is prevalent in nature, [1,2] and is of key importance in a wide range of applications, such as DNA technologies, [3,4] fog-harvesting, [5,6] inkjet printing, [7][8][9] biomedicine [10,11] and microfluidics. [12,13] To develop these practical applications, dynamic wetting behaviours of a droplet on a textured surface have been investigated extensively in recent years.…”