Size dependence of bubble wetting on surfaces: breakdown of contact angle match between small sized bubbles and droplets.

For a bubble on a smooth and rigid substrate, its contact angle is always assumed to be supplementary to the droplet contact angle under the same wetting conditions. Here we revisit bubble wetting on smooth solid surfaces via both free energetic analysis and molecular dynamics (MD) simulations. Our study shows a fundamental difference between bubble wetting and droplet wetting: the size dependence of the isothermal compressibility of the gas bubble leads to a size-dependent bubble contact angle. Based on theoretical analysis we develop a new relationship between the bubble contact angle and droplet contact angle, which is verified with MD simulations for nano-sized bubbles and droplets. In general, our studies show that for bubbles having a size greater than 10 micrometers, the traditional relation of the bubble contact angle being supplementary to the droplet contact angle holds. But when the bubble size decreases to several micrometers or even to the nanoscale size, the relation of contact angle match breaks down, and the deviation from contact angle match increases with decreasing bubble size. This study reveals a unique wetting behavior of bubbles, implying that the wetting of micro- and nano-bubbles deserves serious consideration.