The Force Required to Detach a Rotating Particle from a Liquid–Fluid Interface

Abstract

The force required to detach a particle from a liquid–fluid interface is a direct measure of the capillary adhesion between the particle and the interface. Analytical expressions for the detachment force are available but are limited to nonrotating particles. In this work, we derive analytical expressions for the force required to detach a rotating spherical particle from a liquid–fluid interface. Our theory predicts that the rotation reduces the detachment force when there is a finite contact angle hysteresis between the particle and the liquid. For example, the force required to detach a particle with an advancing contact angle of 120° and a receding contact angle of 80° (e.g., polydimethylsiloxane particle at a water–air interface) is expected to be 25% lower when the particle rotates while it is detached.

Paddlebots: Translation of Rotating Colloidal Assemblies near an Air/Water Interface

“…The film thickness between the beads and the interface can be calculated by balancing the weight of the beads with the electrostatic repulsive forces between the beads and interface. , For the air/water interface, this film thickness should vary with μbot size and is estimated to be ∼50–60 nm. Supporting this, we observe no evidence of capillary interactions under normal experimental conditions. , Without added surfactant, however, beads do become pinned at the interface. − Once bound, surface tension forces dominate any magnetic forces preventing paddlebots from reorienting out of the interfacial plane and translating. The system can be further characterized by the Bond number Bo = Δ ρgL 2 /γ where Δρ is the density difference between the water and the beads, g is the acceleration due to gravity, L is the diameter of an individual bead, and γ is the surface tension of water at experimental surfactant concentrations .…”

The behavior of capillary suspensions at diverse length scales: From single capillary bridges to bulk

“…This also had an impact on the force required to detach rotating particles from a liquid interface, with a decrease of around 25% compared with nonrotating particles. [61] Similarly, a threshold torque for the yielding of silica particle rods in saline water was discovered using optical tweezer experiments [62]. As the capillary force should create even stronger particle contacts, this hindrance is likely the cause of rigid body movement that occurs when deforming capillary networks, as shown in Figure 2.…”

Effects of particle roughness on the rheology and structure of capillary suspensions

“…[6] This pinning effect due to contact angle hysteresis was also recently shown to enhance the capillary torque of a capillary bridge between a particle and a liquid-liquid interface. [35,36] Therefore, Wenzel wetting might be preferred over Cassie-Baxter wetting to create the most stable capillary suspensions.…”