Wetting on a-C:H coatings decorated with sub-micron structures

Abstract

A novel technique that enables the production of a-C:H coatings decorated either by sub-micron concave or convex structures is presented. The technique is based on several steps that involve i) pre-seeding of substrates with C:H sub-micron particles produced by a gas aggregation cluster source, ii) overcoating of the C:H particles by thin a-C:H coatings produced by plasma enhanced chemical vapor deposition and iii) optional ultrasonic lift-off of the C:H particles. The main focus is set on the characterization of the morphology of the resultant structures as well as on the comparison of their wettability. It is shown that the properties of the structured a-C:H coatings may be precisely controlled by the number of the C:H particles. Different dynamic wetting of convex and concave structures is reported, although static wettability is the same. For concave wells (maximum depth 90 nm) the dynamics of water droplet drying is similar to the one observed on smooth a-C:H films and is characterized by three well-distinguishable phases – the constant contact radius phase, the constant contact angle phase and the mixed phase – independently of the number of wells. By contrast, the increasing number of convex bumps (300 nm in height) gradually impedes the movement of a triple line which progressively suppresses the constant contact angle phase. Subsequently, the differences in the droplet drying have a strong impact on the bovine serum albumin patterns formed after the complete evaporation of droplets. Spatially irregular protein pattern is observed on smooth and well-decorated concave a-C:H coatings while a well-defined “coffee-ring” structure is formed on the a-C:H coatings decorated with convex structures.