Wed August 30th 2017
15:00 – 16:00
ZH286
Seminar Spreading of Micro-Droplets on Patterned Surfaces
Pallav Kant

Details:

The manufacture of high-resolution POLED displays via inkjet printing involves the simultaneous printing of large arrays of pixels in close proximity. It requires the ability to control the flow and spread of deposited liquid at small scales. This is usually achieved by micropatterning surfaces. These patterns are either in the form of micron-sized topographical features or variations in the wettability of substrates or a combination of both. Here we use a combination of experiments and numerical modelling to study the spreading of micro-droplets on such patterned surfaces.
During the course of this study, we developed a novel experimental system that can be used to study the dynamics of a single/multiple micro droplets deposited on patterned substrates. On substrates with only topographical features, we find that depending on whether the topography gradient ahead of the contact line is positive or negative, the spreading can either be locally enhanced or hindered. Locally enhanced spreading occurs via the formation of thin-pointed rivulets through a mechanism similar to the capillary rise in sharp corners. However, the efficacy of topographical features in restraining the spreading of deposited liquid in the desired region is critically dependent on the accurate positioning of droplets in a pixel. We demonstrate that this crucial dependence on the positioning of droplets can be evaded on the substrates with both topographical and wettability patterns.
Building on the experimental observations we demonstrate that a thin-film model combined with an experimentally measured spreading law, which relates the speed of the contact line to the contact angle, provides excellent predictions of the evolving liquid morphologies. We also show that the spreading can be adequately described by a Cox–Voinov law for the majority of the evolution. The model does not include viscous effects and hence, the timescales for the spreading of deposited liquid are not captured. Nonetheless, this simple model can be used very effectively as a useful design tool for systems that require precise control of liquid on substrates.
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The 10th Complex Motion in Fluids 2020
Max Planck Gesellschaft
MCEC
Twente
Centre for Scientific Computing
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