Micro and nano bubbles on polystyrene film/water interface

Surface bubbles at polystyrene (PS) film/water interface were imaged using the atomic force microscope (AFM), the surface roughness ranged from 0.58nm to 3.36nm in scan area of 5μm². Big microbubble with a lateral size up to 13μm and a height up to 400nm was reported. The possible reasons for nucleation of big microbubbles were investigated and found that surface roughness and surface properties play a significant role. Further, we focused on the problem "how does the contact angle (measured through air) rely on the bubble size" in a lateral size of 200nm to 13μm, which is the largest size scale for surface bubbles found so far. It was found that the dependence of contact angle on lateral size θ(2r) and height θ(h) is linear for bubbles on smooth substrates, but nonlinear and even keep constant with the increase of bubble size for bubbles on rough substrates. While studying the dependence of contact angle on curvature radius θ(R_c), an inversion in direction between the bubbles in different size scale was found. The results obtained were in close resemblance with the results of other studies. The line tension of surface bubbles on the seven PS substrates in our experiments was calculated and all of the seven line tension values are negative (the average line tension in this study was τ ≈ - 1.07 nN), which should be responsible for the anomalous low contact angle and the size-dependence of the surface bubbles.