Session Q27: Surface Tension Effects: Interfacial Phenomena II

Stick-Slip Dynamics in Non-Isothermal Flow

We have investigated the effect of a solidifying contact line on the dynamics of flowing fluid down an inclined plate. A complete set of experimental studies were performed for continuous flow at constant flow rate on an aluminum substrate for advancing contact angle values less than 90 degrees. As the fluid propagates downhill, the contact line gets pinned to the surface due to solidification. The liquid builds up behind the pinned contact line till it reaches a critical angle. The fluid then over flows the solid barrier, and the process repeats over the length of the inclined plate at a specific frequency. This work focuses on understanding the physics behind this stick-slip behavior and determining the governing characteristics such as critical pinning contact angle, critical spreading length, and pinning frequency as well as the important physical parameters involved in non-isothermal spreading fluids . The results of experimental and analytical studies are presented. 

  

Abstract Withdrawn

We provide a laminar, weakly compressible, numerical investigation of surface effects on the problem of a water droplet, initial radius R=1mm, de-wetting a horizontal plate that is oscillating in the vertical direction with velocity -Vcosωt, V=2800m/s and ω=280s^-1. We distinguish surface effects into cohesive and adhesive forces. We model cohesion by imposing the interface tension force γ along the interface. The approach is validated for two static cohesion problems: pressure inside a bubble and contact angle. We model adhesion following peeling theory, where the adhesive force σ decays with distance from the triple line. Our results, in agreement with the available experimental data, show that during upward acceleration the water droplet wets the plate. As the plate decelerates, the rim of the droplet continues an upward motion while the bottom gradually de-wets the plate, generating a conical cavity. We show that our results depend primarily on the integrated adhesive energy, not the exact form of the decay function. Our key new finding for this problem, where the Weber number is We=ρ2RV²/γ ≈O(10⁸), is that the de-wetting speed depends on adhesion, but not surface tension. Surface tension is important only after the droplet has separated from the plate.   

Capillary Adhesion on Rough Surfaces: Is Splitting Droplets Beneficial?

A wetting droplet confined between two surfaces will pull them together due to capillary forces. This mechanism is believed to (at least partially) explain how many species of insects adhere to surfaces so well — they secrete a thin film of fluid beneath their feet to attach themselves.  Close observations of the fluid in certain species have revealed it to be a water-in-oil emulsion. One suggestion for why this is preferred is that the emulsion droplets themselves form liquid bridges, and that splitting into a large number of small bridges strengthens adhesion. However,  there has been debate as to whether this is actually the case in reality: does droplet splitting really increase the adhesive force for physiologically relevant wetting properties? We suggest that droplet splitting may be particularly beneficial in ensuring good adhesion to a rough surface, and discuss some fluid mechanical features of drops on rough surfaces that give rise to this improvement.    

Universal solution for the retracting edge of a stretched viscous sheet

Surface tension acting alone causes the edge of a fluid sheet to retract and thicken. However, whether at the edge of the hole in a bursting bubble or at the edge of a falling ribbon of molten glass, such retracting edges are often simultaneously being stretched parallel to the edge, thus modifying the flow, the rate of retraction and even thinning the edge. Remarkably, a universal similarity solution for Stokes flow in a stretched edge shows that the scaled shape is actually independent of the stretching rate, and decays rapidly to the far-field thickness. This solution justifies the use of an integrated normal-stress boundary condition in long-wavelength models of viscous sheets, and gives the detailed shape of the edge, resolving its position to the order of the sheet thickness.   

Discussion of the impact of surfactant on the drag-reduction potential of superhydrophobic surfaces

Recent studies, Peaudecerf et al. (PNAS 2017) and Song et al. (PRF 2018), have investigated the negative effect of surfactant on the drag-reduction performance of superhydrophobic surfaces (SHS). As SHS could have a large impact in reducing energy utilisation for many internal and external flow applications, it is important to understand and predict how surfactant-Marangoni stresses affect the flow over SHS. We review experimental and numerical work which show how surfactant can deteriorate the effective slip of SHS and lead to the undesired no slip boundary condition. By studying the governing equations of surfactant transport over SHS, we show the complexity of modelling this coupled nonlinear problems where up to eleven dimensionless parameters can appear. We reveal different regimes in surfactant dynamics and highlight the important dimensionless parameters. We show the impact on the effective slip and the drag of the SHS. Finally, we discuss how surfactant effect could be mitigated through changes in the geometry and flow conditions.   

Capillary Induced Particle Motion in Thin Liquid Films

We demonstrate a new form of capillary force experienced by neutrally buoyant spherical particles adsorbed simultaneously at both interfaces of a thin liquid film of spatially varying thickness. The force is proportional to the slope of the interface and the difference between the local contact angle and the equilibrium value, and exists even when the two bounding interfaces have zero curvature. We derive the expression for the force, which when balanced against the hydrodynamic drag gives the trajectory of the particle. The measured trajectories for spherical particles of varying diameters in thin films compare well with predictions.   

Dynamic super menisci: spontaneous film climbing of colloidal particle solutions

We report experiments showing that suspensions can form a “super meniscus” above a critical interfacial concentration of particles. As a pre-wetted film is inserted into a liquid interface covered with particles, we observe a climbing liquid front as well as a lagging particle front. Previously, the Marangoni effect has been used to produce the growth of climbing films, where a surface tension gradient drives flow with the aid of temperature, surfactant or solute gradients. Particles at an interface may also change the effective interfacial properties. A high-speed interferometric technique is used to track in-situ the film thickness of a climbing colloidal solution. We find that the particle size and particle concentration control the rising and falling rates of the climbing film as well as the final deposition patterns of the particles. We also present a mathematical model to rationalize our results.