Evaporative deposition of polystyrene microparticles on PDMS surface

Ying-Song Yu,Xianfu Huang,Ming-Chao Wang

doi:10.1038/s41598-017-14593-5

Abstract

Evaporation of water and ethanol/water droplets containing large polystyrene (PS) microparticles on polydimethylsiloxane (PDMS) surface was experimentally investigated. It is found that no matter with or without small addition of ethanol, a compact monolayer deposition is formed for lower microparticle concentration while mountain-like deposition for higher concentration. Since the more volatile compound (ethanol) evaporates more quickly than the less volatile compound (water), evaporation of ethanol/water mixture droplet exhibits different characteristics from pure water. When the concentration of microparticle is low, the contact radius of ethanol/water mixture droplet decreases throughout the whole process, while the contact angle increases at first to a maximum, then keeps almost constant, and finally decreases sharply. However, the evaporation of ethanol/water mixture droplet with higher concentration of microparticle behaviors more complex. The settling time of microparticles was estimated and its theoretical value agrees well with the experimental one. Moreover, a mechanism of self-pinning of microparticles was used to elucidate the deposition behavior of microparticles, indicating that as the contact line is depinning, the liquid film covering the outmost microparticle becomes thicker and thicker, and the microparticles have to move spontaneously with the depinning contact line under the action of capillary force.

Highlights

Evaporation of water and ethanol/water droplets containing large polystyrene (PS) microparticles on polydimethylsiloxane (PDMS) surface was experimentally investigated
When the substrate is hydrophilic and the contact line is pinned, there will be a singular evaporation flux near the contact line, resulting in a strong outward capillary compensation flow, which will carry the particles towards the edge
Jung et al.[19] estimated the role of various forces such as drag, electrostatic, van der Waals, and capillary on the particle motion and found that i) the motion of a single particle suspended in liquid is mainly affected by drag force in the pinned contact line stage, and ii) capillary force controls its motion in the depinning contact line stage

Summary

Introduction

Evaporation of water and ethanol/water droplets containing large polystyrene (PS) microparticles on polydimethylsiloxane (PDMS) surface was experimentally investigated. The evaporation of ethanol/water mixture droplet with higher concentration of microparticle behaviors more complex. Shen et al.[4] pointed out that there will be no coffee ring effect when the liquid evaporates much faster than the movement of particle, and found that a coffee ring structure will be still formed until the droplet size decreases to a critical value. To better understand the coffee-ring phenomenon and apply evaporation-induced deposition, researchers have set up theoretical models[19,20,21,22,23,24] for self-pinning of microparticles at the contact line. Wong et al.[20] elucidated the physics of particle separation during coffee-ring formation based on a particle-size selection mechanism near the contact line of an evaporating droplet. Evaporation of mixture droplet might be used to eliminate the coffee ring effect

Methods

Results

Conclusion