Synopsis

Gravity-Driven Flows in Two-Fluid Drops

Physics 11, s127
The direction in which fluid circulates in binary drops doesn't change when the drops are tilted, implicating gravity—not surface tension—as the driver of flow in these systems.
A. M. J. Edwards et al., Phys. Rev. Lett. (2018)

Drops of spilled coffee dry and leave behind ring-shaped patterns. The shape and uniformity of these patterns are determined by fluid flows that drive particle deposition within the drops. Now, Fouzia Ouali of Nottingham Trent University in the UK and colleagues have shown that in the case of drops composed of two fluids, gravity, not surface tension, as researchers previously thought, dominates convective flows. The team says that this result could allow for improved control of material deposition in applications like printing that involve drying drops.

In a drop containing two fluids, unequal evaporation of the fluids leads to changes in the drop’s local composition. This composition change can give rise to gradients in both the surface tension and the density of the drop, two parameters that can drive fluid flow.

To see which effect dominates, Ouali and colleagues studied water-ethanol drops and water–n-butanol drops in which they suspended micrometer-sized polystyrene particles. They placed a 1-mm-diameter drop on a flat surface and then monitored flows within the evaporating drop by tracking the motion of the particles. They repeated the experiment for drops on surfaces tilted at different angles to determine the relative importance of gravity versus surface tension in controlling the flows.

The team found that the direction of fluid flow within the drop was unaffected by the angle of the surface—the fluid circulated in the same direction for drops that were both right-side up and upside-down on horizontal surfaces. The team says that this finding suggests that fluid buoyancy and hence gravity—not surface tension—drives fluid flow.

This research is published in Physical Review Letters.

–Christopher Crockett

Christopher Crockett is a freelance writer based in Arlington, Virginia.


Subject Areas

Fluid Dynamics

Related Articles

Liquid Metal Experiment Mimics Accretion Disks
Astrophysics

Liquid Metal Experiment Mimics Accretion Disks

Using a magnetically stirred liquid metal, researchers have reproduced a key feature of astrophysical accretion disks: a turbulence-based transfer of angular momentum. Read More »

Turbulence-Surfing Plankton Can Double Their Speed
Fluid Dynamics

Turbulence-Surfing Plankton Can Double Their Speed

Simulations indicate that plankton can gain quicker access to food by riding ascending turbulent ocean currents. Read More »

Effectively Mixing Two Fluids
Fluid Dynamics

Effectively Mixing Two Fluids

Want to mix two fluids? Researchers have developed a path to optimize the stirrer shape and velocity to give the best outcome.   Read More »

More Articles