Bubble Growth on Hydrophobic Rough Surfaces in the Shear Flow

It is well known that bubbles will form on a hydrophobic rough surface immersed in water, which can create a surface covered with bubbles and leads to drag reduction. However, it is still not clear how bubbles grow on the surface under flow conditions. In this work, a rotating flow field is created using a parallel-plate setup of a rotational rheometer, and sample surfaces with different roughnesses and wettabilities are examined with different shear rates. The growth of bubbles is exclusively observed on the hydrophobic rough surface, and subsequent drag reduction is also detected simultaneously. The growth of bubbles is attributed to heterogeneous nucleation in the crevices under a local pressure reduction generated by the shear flow. A geometric model is established to describe the profile evolution of the trapped bubble in the crevice based on the contact angle and the pressure balance across the gas-liquid interface, which involves the variations of the Laplace pressure resulting from changes in the local liquid pressure. The growth of bubbles on the hydrophobic rough surface does not need a large decrease of the surrounding pressure or a high moving speed, which will have potential applications in drag reduction under the condition of a moderate shear rate.