Synopsis: Hold the ketchup

A different take on shearing forces in gooey fluids provides a better description of viscoelasticity.
Synopsis figure
Credit: Sami Mitra

If we fill the space between two horizontal plates with a fluid and then move the top plate sideways, we say that the fluid is subject to a shearing force. Viscoelasticity determines how a fluid flows or deforms when a shearing force is applied. “Complex” (non-Newtonian) fluids—pudding, blood, ketchup, toothpaste—are often described as “squishy” or “gooey.” For such fluids, viscoelasticity depends, in a mathematically complex way, on the magnitude of the applied force.

Aditya Khair and Todd Squires, writing in Physical Review Letters, propose a new technique in active microrheology, in which two micron-sized particles are pulled through a complex fluid. The pulling force between the particles is used to determine the nonlinear properties of the fluid viscoelasticity. The authors show that, in principle, varying the pulling speed and direction, as well as the particle spacing, allows for direct measurement of the nonlinear viscoelasticity. They expect the proposed technique to work on a variety of complex fluids, thus opening a new direction in microrheology. – Jane Throwe


More Announcements »

Subject Areas

Soft Matter

Previous Synopsis

Next Synopsis

Interdisciplinary Physics

Popularity contest

Read More »

Related Articles

Synopsis: Hidden Forces in a Pile of Sand
Soft Matter

Synopsis: Hidden Forces in a Pile of Sand

A map of the contact forces between grains in a sand pile provides a first glimpse of how force is distributed in a 3D granular system. Read More »

Viewpoint: How to Fracture a Fluid
Fluid Dynamics

Viewpoint: How to Fracture a Fluid

High-speed imaging shows that fluids can break like brittle glass under the right conditions. Read More »

Synopsis: In, Yet Out of Equilibrium
Statistical Physics

Synopsis: In, Yet Out of Equilibrium

An analysis of a popular model for active matter, like bacteria and buffalo herds, defines the conditions under which such systems can be described with the tools of equilibrium statistics. Read More »

More Articles