Nano-particle separation using marangoni flow in evaporating droplets

Abstract

The “coffee ring effect” is known to result from the concentration of particles at the perimeter of a dried droplet. Raising the temperature of the substrate provides a temperature gradient induces “strong” Marangoni circulatory flow inside the droplet. This “strong” Marangoni can reverse the “coffee ring” effect. Here, we describe a method of particle separation based on the size of particles using Marangoni flow and the related “coffee ring” effect. Suspended particles of different sizes ranging from 100nm to 15µm are separated by using a combination of the “strong” Marangoni flow and coffee ring effects. Additionally, the effects of droplet contact angle with the substrates and temperature gradient in droplets are evaluated. Experiments show that, a temperature gradient of 1°C/mm, and contact angle (15°) can separate 100nm-particles from 1μm-particles successfully. The distance between the dried ring patterns is ~ 30 μm. In contrast, the separation between rings of particles without “strong” Marangoni is only 5.4μm. The “strong” Marangoni flow induced by temperature gradient enhances the separation of nano-particles at the contact line of a droplet.