It all comes down to roughness

May 2, 2018 by Pe­ter Rüegg, ETH Zurich
Rough and smooth particles in a suspension: Their mixing ratio influences the time of the sudden increase in viscosity. Credit: SEM image: Chiao-Peng Hsu, ETH Zurich

Lucio Isa and his team of researchers have explained how the surface characteristics of microspheres affect rapid increases in the viscosity of suspensions, thus laying the groundwork for applications such as smoothly flowing cement.

The internet is full of videos of people having fun running over white slime. It almost looks as if they were walking on water. But when they stand still, they slowly begin to sink. The slime in question is usually a concentrated comprising cornstarch and water. Although colloquially known as "oobleck" after the children's book by Dr. Seuss, materials scientists use the term "non-Newtonian fluid". In contrast to a "normal" (Newtonian) fluid, non-Newtonian fluids can become more viscous when acted upon by a large, rapidly changing force. For a brief moment, the material behaves like a solid. However, if the force is constant and weak, the material flows like a normal liquid.

"This phenomenon appears in all suspensions with high particle density, such as cement," says Lucio Isa, Professor of Interfaces, Soft matter and Assembly at ETH Zurich. If cement is pumped through a pipe on a construction site too quickly, the pipe will clog.

Higher friction due to rough surfaces

This is partially due to the surface characteristics of the solid particles in the suspension. "If a force is applied suddenly,the are unable to move out of the way quickly enough. Instead, they come into contact, rubbing against and blocking each other", Explains Isa. The rougher the particle surface, the higher the friction.

Researchers use these properties to control the sudden increase in viscosity in a dense suspension in a targeted manner. Instead of cornstarch, Isa and his colleagues "played" with uniform, micrometre-sized silicate particles with a rough surface. The particles look like tiny raspberries and had already been used by the researchers in earlier studies. Chiao-Peng Hsu, a doctoral student working under Isa and ETH professor Nicholas Spencer, has developed a method that allows him to quickly generate a library of these raspberry-shaped particles with different roughnesses.

Higher viscosity despite fewer particles

The researchers used these particles to create suspensions that they could test for sudden increases in viscosity under stress. The results showed that the rougher the particles, the fewer had to be added to the suspension to achieve sudden solidification. In contrast, if smooth particles were used, a greater amount had to be added to the suspension before the researchers were able to observe the sudden thickening.

The researchers showed that the use of rough particles could save material: their portion of the total volume in a suspension can be substantially lower in order to generate the same effect.

When the researchers mixed rough and smooth particles in a single suspension, solidification also occurred earlier than in suspensions with exclusively smooth particles. The ETH researchers discovered that as little as 6 percent of smooth particles in a mixture was enough to reduce the increase in viscosity significantly. "It's like mixing up ball bearings and gearwheels," says Isa. "The gearwheels link together relatively easily to create a stable chain but the ball bearings easily break those chains and enable flow."

In order to research the friction between individual , Hsu and his colleague Shivaprakash Ramakrishna attached a single, half-micrometre particle on a cantilever of an atomic force microscope. The researchers then moved the particle across various rough model surfaces by shifting the cantilever a few hundred nanometres while measuring the angle of tilt. The stronger the friction, the greater the angle. "Working with this kind of particle on a cantilever was extremely difficult, since the dimensions are incredibly small," emphasises Hsu. "We were the first group to accomplish this."

Applications in bullet-proof vests

Whether the results will be integrated into real-life applications remains to be seen. The study is primarily pure research. "Our goal was to investigate the ways in which we can change nano and microstructures in order to influence material behaviour on a macroscopic level, and we succeeded," says Isa. The findings could be used in everyday applications such as cement. "By adjusting the surfaces of granules and mixing them into cement in a similar way to our experiment, one could optimise cement's flow characteristics."

But viscous suspensions with abrupt solidification properties are also used for other purposes; for example, an American manufacturer uses viscous suspensions to develop bullet and stab-proof safety vests. "Our study can help improve these kinds of applications," says Isa.

Explore further: Study shows how rough microparticles can cause big problems

More information: Roughness-Dependent Tribology Effects on Discontinuous Shear Thickening. PNAS, April 30th, 2018. DOI: 10.1073/pnas.1801066115

Related Stories

Study shows how rough microparticles can cause big problems

October 12, 2017

New research from North Carolina State University, MIT and the University of Michigan finds that the surface texture of microparticles in a liquid suspension can cause internal friction that significantly alters the suspension's ...

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 ...

Team models sudden thickening of complex fluids

January 16, 2014

(—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 ...

Tame your Oobleck: Researcher able to control thickening

September 14, 2016

Shear thickening – the increase of a liquid's viscosity through applied force – is a well-known phenomenon. Mix equal parts corn starch and water and you come up with "Oobleck," a liquid that turns solid the more vigorously ...

Universal stabilisation

June 14, 2017

ETH researchers led by Lucio Isa have developed microparticles with a rough, raspberry-like surface that stabilise emulsions following a new principle.

Recommended for you

How a particle may stand still in rotating spacetime

May 25, 2018

When a massive astrophysical object, such as a boson star or black hole, rotates, it can cause the surrounding spacetime to rotate along with it due to the effect of frame dragging. In a new paper, physicists have shown that ...

Scientists discover new magnetic element

May 25, 2018

A new experimental discovery, led by researchers at the University of Minnesota, demonstrates that the chemical element ruthenium (Ru) is the fourth single element to have unique magnetic properties at room temperature. The ...

Long live the doubly charmed particle

May 25, 2018

Finding a new particle is always a nice surprise, but measuring its characteristics is another story and just as important. Less than a year after announcing the discovery of the particle going by the snappy name of Ξcc++ (Xicc++), ...


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.