Control of drop positioning using chemical patterning

Abstract

We explore how chemical patterning on surfaces can be used to control drop wetting. Both numerical and experimental results are presented to show how the dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic grid. The grid proves a successful way of confining drops and we show that it can be used to alleviate mottle, a degradation in image quality which results from uneven drop coalescence due to randomness in the positions of the drops within the jetted array.From microfluidic technology to detergent design and ink-jet printing it is important to investigate the way in which drops move across surfaces. The dynamics of the drops will be affected by any chemical heterogeneities on the surface [1,2,3]. Until recently such disorder was usually regarded as undesirable. However with the advent of microfabrication techniques it has become possible to control the chemical patterning of a substrate down to nanoscale, leading to the possibility of exploring how such patterning can control, rather than disturb, drop motion.A practical example where such an approach might be of use is in ink-jet printing. Although ink-jet printers are widely available for domestic use the quality of the images is still not sufficiently robust to allow widespread industrial applications. The possibility of replacing the traditional contact techniques with electronically controlled template design, particularly for small print runs, is highly desirable for both efficiency and cost.In the printed image a patch of colour is produced by jetting drops in a regular, square array. The closer the drops the more intense the colour of the patch appears to the eye. To achieve a solid colour the aim is that drops jetted at a distance apart comparable to their diameter should coalesce and form a uniform covering of ink. However, in practice, randomness in the positions at which the drops land, combined with surface imperfections, often lead to local coalescence and the formation of large, irregular drops with areas of bare substrate between them as shown in fig. 3(a) and the upper part of fig. 4. Such configurations are likely to lead to poor image quality, called mottle [4].In an attempt to overcome this problem we demonstrate how using a two-dimensional array of hydrophobic chemical stripes can be used to control the equilibrium shape, the position and the dynamic pathway of spreading drops. The hydrophobic stripes form barriers controlling the drops and allowing their relative positions to be tuned. The behaviour of single drops, and then an array of drops on the patterned surfaces, is explored both by solving the hydrodynamics equations of motion by means of a lattice Boltzmann algorithm and by performing suitable experiments.In the numerical modelling we consider a liquid-gas system of density n(r) and volume V . The surface of the substrate is denoted by S. The equilibrium properties of the drop are described by the free energywhere Einstein notation is understood for the Cartesian label α and where ψ b (n) is the free en...

Controlling Drop Size and Polydispersity Using Chemically Patterned Surfaces

“…This numerical approach has been shown to agree well with experiments in several cases [1,2,4], giving us confidence in using it for the more complicated situations considered here.…”

“…The behaviour of fluids spreading and moving across such surfaces is extremely rich, and is only just beginning to be explored [1,2,3,4,5,6,7,8,9,10,11]. Biosystems have evolved to use hydrophobic and hydrophilic patches to direct the motion of fluids at surfaces [12,13].…”

Anisotropic Drop Morphologies on Corrugated Surfaces

“…Regular arrays of chemical patches [4,5,6] and posts [7,8,9,10] of different shapes and sizes are regularly fabricated and several authors [11,12,13] have even shown the possibilities of manufacturing multi-scale surface patterns. Recently such patterning has been used to control the movement of drops [16,17] and to attempt to enhance flow in microchannels [18,19].…”

Lattice Boltzmann Simulations of Wetting and Drop Dynamics

“…In the first [55], we show how a (relatively) hydrophobic grid can be used to alleviate mottle [56] in ink-jet printing. In the second example, we demonstrate that chemical patterning can be used to control drop size and polydispersity [57].…”