Bubbles climbing on two vertical superhydrophobic sticks

Abstract

Bubble manipulation using superhydrophobic surfaces (SBS) is currently a subject of extensive research. Although flat surfaces have been the primary focus of investigation, with dimensions significantly larger than those of the bubbles, there has been limited attention given to the study of long slender SBSs, such as filaments and thin rods, which closely approximate the size of the bubbles. However, these elongated SBSs hold promise for enabling a wider range of bubble motion modes and an increased surface area for the bubbles. This study introduces the concept of double superhydrophobic sticks (DSBS) as a novel method to enhance the rising velocity (U) of buoyancy-driven bubbles. The examination identifies three distinct bubble configurations on the DSBS: nut-shaped, bell-shaped, and apple-shaped. Remarkably, the DSBS exhibits exceptional superaerophilicity, achieving a threefold increase in U for larger bubbles (with a diameter of approximately 4 mm). Prior research has already established the substantial impact of the length of the moving three-phase contact line (MCL) on the migration velocity of bubbles sliding on diverse planar SBSs. In contrast, our findings indicate that even the MCL length remains constant, the rising velocity of bubbles on DSBSs can be modulated by varying both the bubble size, the spacing between double sticks (S), and the diameter of the stick (D_s). Specifically, U is dependent on a revised Ohnesorge number ($O{h}^{*}=L^{*}\frac{\mu }{\sqrt{\rho \sigma S}}$), following a log-linear-scaling relationship with U. Considering the altered impact of the S on the bubble shape, we propose the S as the characteristic length and the shape correction factor L* ($L^{*}=1/{\epsilon }^{*}=d_{\text{eq}}/S$) for the revised Ohnesorge number. Here, d_eq is the equivalent diameter of the bubble, ε* is the normalized stick gap ratio. The scaling law identified in this investigation effectively enables the simultaneous prediction of both the morphology and rising velocity of bubbles on the double sticks. Hence, our bubble manipulation method and its associated predictive model exhibits promising prospects for implementation across a range of gas-liquid systems, encompassing improved electrolytic hydrogen production, gas-liquid separation, adjustable flotation, and advanced surface chemistry.