PHYSICAL PRINCIPLES OF MARANGONI DRYING

An account is given about the relevant physical phenomena that govern Marangoni drying of hydrophilic surfaces. Marangoni drying is possible when vapor of a water-soluble organic liquid absorbs into the water meniscus against a partially immersed solid substrate. Because of the meniscus curvature, vapor absorption induces a surface tension gradient along the meniscus. The gradient causes the meniscus to partially contract via a Marangoni flow and assume an apparent finite contact angle. Experimental evidence is presented which shows that the magnitude of the observed contact angle reflects the magnitude of the induced surface tension gradient. The surface tension gradient enables a hydrophilic surface to be withdrawn from water with a visibly dry surface. After Marangoni drying, only a very thin water film remains on the surface. Its thickness depends on the nature of the organic vapor, the surface roughness, and the substrate withdrawal speed. Under suitably chosen conditions, this thickness is more than 1 order of magnitude smaller than the remaining film thickness after conventional spin drying. This ensures Marangoni drying to be an intrinsically much cleaner drying process with important technological applications.