Related Experiment Video
Updated: Jun 8, 2026

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure
Published on: April 17, 2018
Enhanced mixing of droplets during coalescence on a surface with a wettability gradient
Yu-Hsuan Lai1, Miao-Hsing Hsu, Jing-Tang Yang
1Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan.
Abstract:
We investigated the dynamics of head-on collisions between a moving droplet and a stationary droplet on a surface with a wettability gradient. The mixing of fluids is achieved passively through convective mass transfer caused by the release of surface energy during coalescence, and also through diffusive mass transfer. The coalescence dynamics were visualized with a high-speed camera; the internal flow patterns were resolved with measurement of micro-PIV (particle image velocimetry). The results show that the released surface energy creates a pair recirculation flow inside the merged droplet when the stationary droplet is placed near the gradient, whereas most released surface energy is converted into oscillation when the stationary droplet is far from the gradient. This distinction is attributed to the motion of the contact line during coalescence. The mixing of fluorescently labeled oligonucleotides in these two modes is revealed with confocal micro-laser induced fluorescence technique. The results of 3D scans demonstrate that the motion of the contact line during coalescence distributes the fluids in a complicated manner, thus beneficial for mixing. This mechanism of enhanced mixing is applicable also for platforms other than a surface with a wettability gradient; prospective applications include improving the mixing of biochemical fluids.
Related Concept Videos
Cohesion
On a surface,...
Surface Tension, Capillary Action, and Viscosity
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid. However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. Because of the unbalanced molecular attractions on the surface molecules, liquids contract to form a shape that minimizes the number...
Colloidal precipitates
Surface Tension of Fluid
Surface tension varies with...
Surface Tension
Coagulation

