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 Features »


More Announcements »

Subject Areas

Soft Matter

Previous Synopsis

Next Synopsis

Interdisciplinary Physics

Popularity contest

Read More »

Related Articles

Synopsis: How Ice Bridges Form

Synopsis: How Ice Bridges Form

New theoretical work predicts the conditions under which sea ice will clog a narrow channel to create a natural bridge across it. Read More »

Synopsis: Little Spheres Are Pushy
Soft Matter

Synopsis: Little Spheres Are Pushy

A simple diffusion model explains why small particles tend to push big ones to the bottom of a drying colloid film. Read More »

Viewpoint: A Crumpled Sheet’s Remembrance of Things Past
Soft Matter

Viewpoint: A Crumpled Sheet’s Remembrance of Things Past

Crumpled sheets “remember” the application and removal of a force for days, a newly discovered memory effect that suggests crumpled sheets are a lot like glasses. Read More »

More Articles