Synopsis: Viscous pull

Scaling laws are a useful way to characterize fluid flow over a wide range of flow rates and experimental conditions. Theorists now explain several earlier experiments by finding a scaling law that describes how a liquid-liquid interface changes shape when driven by viscous forces.
Synopsis figure

Shape transitions are ubiquitous in fluids. Consider for example what happens when a pipe is suspended just above the interface between two liquids. As the pipe begins to suction liquid from the top layer, the liquid flow will pull the interface between the two fluids upwards. As the flow rate through the pipe increases, this interface undergoes a shape transition from a gentle hump to a sharp neck at the point when the pipe starts to draw fluid from the lower liquid.

This type of necking effect can occur, for example, in microfluidic devices where fluids are channeled down extremely narrow passages or in the encapsulation of biological cells. But, researchers have not been able to capture the experimental details of the process with a simple scaling law that holds for different types of liquids and length scales. In a paper appearing in Physical Review Letters, François Blanchette and Wendy Zhang at the University of Chicago in the US treat this problem analytically and resolve the discrepancy between different experiments.

Blanchette and Zhang argue that the part that makes this problem hard to solve—the discontinuous neck that forms at the transition—is in fact, not important. Instead, they show that it is the gentle deflection of the interface far away from the opening of the tube that matters. With this simplifying assumption, they find a simple scaling relation between the critical flow rate at which the shape transition occurs and the height of the pipe above the interface and outline the experimental regimes where this law applies. – Jessica Thomas


More Features »


More Announcements »

Subject Areas

Fluid DynamicsSoft Matter

Previous Synopsis

Semiconductor Physics

Dirt and geometry insulate electrons

Read More »

Next Synopsis

Nuclear Physics

Nuclear star gazing

Read More »

Related Articles

Focus: <i>Video</i>—Fluid Video Contest Winners
Fluid Dynamics

Focus: Video—Fluid Video Contest Winners

Swimming starfish larvae, dripping paint, and swirling gas jets are featured in the APS Division of Fluid Dynamics’ winning videos. Read More »

Synopsis: How to Make Superhydrophobicity Last
Fluid Dynamics

Synopsis: How to Make Superhydrophobicity Last

Researchers find tricks to prolong the typically short-lived water repellency of a superhydrophobic surface. Read More »

Focus: Drops Falling in Clouds Make More Drops
Fluid Dynamics

Focus: Drops Falling in Clouds Make More Drops

Experiments with a simplified version of the atmosphere show that falling drops seed many smaller droplets in their wake. Read More »

More Articles