Marangoni convection in droplets on superhydrophobic surfaces

Abstract

We consider a small droplet of water sitting on top of a heated superhydrophobic surface. A toroidal convection pattern develops in which fluid is observed to rise along the surface of the spherical droplet and to accelerate downwards in the interior towards the liquid/solid contact point. The internal dynamics arise due to the presence of a vertical temperature gradient; this leads to a gradient in surface tension which in turn drives fluid away from the contact point along the interface. We develop a solution to this thermocapillary-driven Marangoni flow analytically in terms of streamfunctions. Quantitative comparisons between analytical and experimental results are presented as well as effective heat transfer coefficients.

Ice-dependent liquid-phase convective cells during the melting of frozen sessile droplets containing water and multiwall carbon nanotubes

“…Literature values for maximum velocities attained within evaporating sessile water droplet systems on hydrophobic surfaces with the same temperature difference are on the order of 1-20 mm/s [44,45], which are comparable to the speeds obtained in the current system. Once again it is difficult to quantify flow in the pure water system.…”

Mechanism of supercooled droplet freezing on surfaces

“…The velocity of a single particle was evaluated from a PIV (particle image velocity) analysis of two consecutive frames. Unlike the usual bidirectional symmetric circulation observed inside a falling water droplet in air or the Marangoni flow inside a sessile water droplet evaporating on a surface 20,27 , only one direction of circulation is visible in Figure 4b because of the asymmetric shear flow around the sessile droplet 28 . High-speed recordings were made through the observation window using a high-speed camera system (Photron Fastcam PCI1024) with a maximum image resolution of 1,024×1,024 pixels, mounted on a binocular microscope (Olympus SZX10).…”

Interactions and Stress Relaxation in Monolayers of Soft Nanoparticles at Fluid-Fluid Interfaces

“…The nonuniform rate of change of area over the surface of a pendant drop results in gradients in surface coverage, which could produce unaccounted for Marangoni stresses. These stresses do not affect the measurement if the time scale of the convective flow that relaxes the gradient in surface coverage, τ M ¼ ηR=Δγ, is sufficiently fast [24,25]. The maximum difference in surface tension over the surface of the drop can be estimated as the change in surface tension upon compression, Δγ ≈ 1 mN=m.…”