Abstract
The coffee-ring effect, ubiquitously present in the drying process of aqueous droplets, impedes the performance of a myriad of applications involving precipitation of particle suspensions in evaporating liquids on solid surfaces, such as liquid biopsy combinational analysis, microarray fabrication, and ink-jet printing, to name a few. We invented the methodology of laser-induced differential evaporation to remove the coffee-ring effect. Without any additives to the liquid or any morphology modifications of the solid surface the liquid rests on, we have eliminated the coffee-ring effect by engineering the liquid evaporation profile with a CO2 laser irradiating the apex of the droplets. The method of laser-induced differential evaporation transitions particle deposition patterns from coffee-ring patterns to central-peak patterns, bringing all particles (e.g. fluorescent double strand DNAs) in the droplet to a designated area of 100 μm diameter without leaving any stains outside. The technique also moves the drying process from the constant contact radius (CCR) mode to the constant contact angle (CCA) mode. Physical mechanisms of this method were experimentally studied by internal flow tracking and surface evaporation flux mapping, and theoretically investigated by development of an analytical model.
Highlights
The drying of a droplet of water carrying colloidal particles naturally gives rise to non-homogenous deposition pattern, with most of particles migrating to the edge of the droplet, forming the well-known coffee-ring deposition pattern[1]
Given the small size of the droplets in most applications, we propose the idea of using a CO2 laser to create the effect of laser-induced differential evaporation
With the experimental results, the analytical solutions derived in this paper confirm the importance of differential evaporation in reversing the coffee-ring effect
Summary
The drying of a droplet of water carrying colloidal particles naturally gives rise to non-homogenous deposition pattern, with most of particles migrating to the edge of the droplet, forming the well-known coffee-ring deposition pattern[1]. Thermally-induced Marangoni effect has been proven effective to create a re-circulation flow to reverse the coffee-ring effect, and to reduce the deposition pattern size of solution droplets with volatile solvents[9,10]. Water molecules have strong absorption (greater than 3000 cm−1) at 10.6 μm wavelength of CO2 laser, is highly effective in generating a differential evaporative flux profile on water surface This laser-induced differential evaporation method has successfully reduced the deposition pattern size of aqueous solution droplets from 1.5 mm to the designated 100 μm spot without addition of any ionic salts or surfactants. With the experimental results, the analytical solutions derived in this paper confirm the importance of differential evaporation in reversing the coffee-ring effect
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