Team models sudden thickening of complex fluids

January 16, 2014
Sheared suspension of spheres occupying 52 percent of a volume, alongside images of "force chains;" i.e., the paths connecting spheres as they come into contact with their neighbors. The top image shows the spheres sheared before the transition to discontinuous sheared thickened. Credit: Benjamin Levich Institute, The City College of New York

(Phys.org)—A new model by a team of researchers with The City College of New York's Benjamin Levich Institute may shed new understanding on the phenomenon known as discontinuous shear thickening (DST), in which the resistance to stirring takes a sudden jump. Easily observed in a 'kitchen experiment' by mixing together equal amounts of cornstarch and water, DST occurs because concentrated suspensions of hard particles in a liquid respond differently than normal fluids to shear forces.

Because many materials are handled in industrial applications as suspensions, engineers need to understand how to predict their flow properties. However, they have been hard-pressed to understand DST and calculate when it will occur during processing. Since concentrated suspensions are found in everything from consumer products to cement, a range of industries stand to find better ways to manage suspensions.

For simple liquids, such as water or glycerin, resistance increases in proportion to the stirring speed. However, in complex fluids such as suspensions, emulsions and polymers the relationship is not proportional. Resistance increases either faster than the stirring speed – shear thickening – or slower – shear thinning. DST occurs when resistance increases suddenly.

Research associates Ryohei Seto and Romain Mari and chemical engineering Professors Jeffrey Morris and Morton Denn set out to develop a model that would predict how resistance changes in relation to stirring speed. Their findings were reported in Physical Review Letters in November.

The model modifies the classical fluid mechanics approach to suspension behavior, in which liquid lubrication keeps from contact, to permit contact and to include forces resulting from friction as particles move past one another.

As friction increases two or more particles can act like a single entity with regard to the surrounding fluid. When concentrations of particles and the shear rate reach sufficient levels, the particles form temporary chains that can fill space. This causes jamming, which can make flow difficult or even impossible.

An increase in the rate of shearing of just 50 percent puts the suspension above the transition to DST, as depicted by the dense network of connections. Credit: Benjamin Levich Institute, The City College of New York

Using the model, the team was "able to accurately reproduce experimental observations of the steep viscosity versus shear rate curves and their evolution with packing friction, both of which are first for numerical simulations," Dr. Eric Brown, Yale University assistant professor of mechanical engineering and materials science, wrote in a commentary published in Physics.

Future investigations will address moving beyond a simulation "that follows the mechanics of a finite number of particles in a cube to predicting the flow of concentrated suspensions in arbitrary geometries," Professor Denn noted. "A long-term goal is to be able to develop a 'continuum' description of concentrated in which the behavior is averaged over large numbers of particles."

Explore further: Physicists capture microscopic origins of thinning and thickening fluids

More information: Paper: prl.aps.org/abstract/PRL/v111/i21/e218301

Related Stories

Images capture split personality of dense suspensions

March 30, 2012

Stir lots of small particles into water, and the resulting thick mixture appears highly viscous. When this dense suspension slips through a nozzle and forms a droplet, however, its behavior momentarily reveals a decidedly ...

Stirred, not shaken: Physicists gain more particle control

March 22, 2013

Cornell physicists can now precisely control how particles in viscous liquids swirl, twirl and whirl. Think of coffee and adding cream—and gaining control of the particles in the cream. Understanding this concept could ...

Why cement tends to thicken

October 1, 2013

Researchers at the ETH Zurich have unraveled the microscopic mechanism behind shear thickening: the increase in viscosity with speed observed for dense particle suspensions under flow. The study has a direct impact on the ...

Recommended for you

Magnetism at nanoscale

August 3, 2015

As the demand grows for ever smaller, smarter electronics, so does the demand for understanding materials' behavior at ever smaller scales. Physicists at the U.S. Department of Energy's Ames Laboratory are building a unique ...

Study calculates the speed of ice formation

August 3, 2015

Researchers at Princeton University have for the first time directly calculated the rate at which water crystallizes into ice in a realistic computer model of water molecules. The simulations, which were carried out on supercomputers, ...

Small tilt in magnets makes them viable memory chips

August 3, 2015

University of California, Berkeley, researchers have discovered a new way to switch the polarization of nanomagnets, paving the way for high-density storage to move from hard disks onto integrated circuits.

Scientists bring order, and color, to microparticles

August 3, 2015

A team of New York University scientists has developed a technique that prompts microparticles to form ordered structures in a variety of materials. The advance, which appears in the Journal of the American Chemical Society ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.