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Pattern Formation during Drying of a Nanoliter Droplet on a Flat Substrate

Rajneesh Bhardwaj and Daniel Attinger. Mechanical Engineering, Columbia University, 500 W 120th street, mudd 220, New York, NY 10027

An efficient way to pattern biomarkers on glass surfaces for bioassays is to deposit colloidal drops and evaporate them. The remaining deposits, however are not always homogeneous and sometimes exhibit ring-like patterns. In this work, the process of ring-like patterns investigated numerically and experimentally. The numerical modeling accounts for the evaporation of a colloidal nanoliter drop on a solid, non-isothermal substrate. The equations governing fluid, heat and mass transport are expressed in a Lagrangian framework. The diffusion of vapor in the gas surrounding the drop is solved numerically and depends on the drop-substrate geometry and thermodynamic conditions. Numerical results predict a radial flow pattern, as well as the formation of multiple rings. Experimental results are presented for the evaporation of nanoliter water drops with a low concentration of polymer microspheres on glass and PDMS substrates. Different deposit patterns are observed, such as a peripheral ring on glass and a uniform deposit on PDMS, which is modeled and explained by the influence of wetting angle on evaporation and by the attraction forces [1] between the particles and PDMS substrate.

[1] L. V. Andreeva, A. V. Koshkin, P. V. Lebedev-Stepanov, A N Petrov, and M. V. Alfimov, "Driving forces of the solute self-organization in an evaporating liquid droplet," Colloids and Surfaces A: Physicochem. Eng. Aspects, vol. 300, pp. 300-306, 2007.