Effect of a surface tension gradient on the slip flow along a superhydrophobic air-water interface

Abstract

Superhydrophobic surfaces have been shown to produce significant drag reduction in both laminar and turbulent flows by introducing an apparent slip velocity along an air-water interface trapped within the surface roughness. In the experiments presented within this study, we demonstrate the existence of a surface tension gradient associated with the resultant Marangoni flow along an air-water interface that causes the slip velocity and slip length to be significantly reduced. In this study, the slip velocity along a millimeter-sized air-water interface was investigated experimentally. This large-scale air-water interface facilitated a detailed investigation of the interfacial velocity profiles as the flow rate, interfacial curvature and interface geometry were varied. For the air-water interfaces supported above continuous grooves (concentric rings within a torsional shear flow) where no surface tension gradient exists, a slip velocity as high as 30% of the bulk velocity was observed. However, for the air-water interfaces supported above discontinuous grooves (rectangular channels in a Poiseuille flow), the presence of a surface tension gradient reduced the slip velocity and in some cases resulted in an interfacial velocity that was opposite to the main flow direction. The curvature of the air-water interface in the spanwise direction was found to dictate the details of the interfacial flow profile with reverse flow in the center of the interface for concave surfaces and along the outside of the interface for convex surfaces. The deflection of the air-water interface was also found to greatly affect the magnitude of the slip. Numerical simulations imposed with a relatively small surface tension gradient along the air-water interface were able to predict both the reduced slip velocity and back flow along the air-water interface.

References in corpus (4)

• Quantifying effective slip length over micropatterned hydrophobic surfaces
• Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces
• Drag and diffusion coefficients of a spherical particle attached to a fluid interface
• Visco-elastic drag forces and crossover from no-slip to slip boundary conditions for flow near air-water interfaces

Cited by in corpus (10)

• A theory for the slip and drag of superhydrophobic surfaces with surfactant
• A single parameter can predict surfactant impairment of superhydrophobic drag reduction
• Slip of submerged two-dimensional liquid-infused surfaces in the presence of surfactants
• Laminar drag reduction in surfactant-contaminated superhydrophobic channels
• On the self-similarity of unbounded viscous Marangoni flows
• Unsteady evolution of slip and drag in surfactant-contaminated superhydrophobic channels
• The influence of incompressible surfactant on drag in flow along an array of gas-filled grooves
• Local slip length and surfactant effects on liquid-infused surfaces
• Drag reduction in surfactant-contaminated superhydrophobic channels at high Péclet numbers
• Shear flow over a surface containing a groove covered by an incompressible surfactant phase