It all comes down to roughness

May 02, 2018

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 suspension 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 solid particles 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 (see ETH News 14 June 2017). 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 surface 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 ."

Growing "raspberries" on cantilevers

In order to research the friction between individual particles, 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.

Hsu C-P, Ramakrishna SN, Zanini M, Spencer ND, Isa L. Roughness-Dependent Tribology Effects on Discontinuous Shear Thickening. PNAS, published ahead of print April 30th, 2018. DOI: 10.1073/pnas.1801066115

ETH Zurich

Related Viscosity Articles from Brightsurf:

Artificial Intelligence has learned to estimate oil viscosity
A group of Skoltech scientists developed machine learning (ML) algorithms that can teach artificial intelligence (AI) to determine oil viscosity based on nuclear magnetic resonance (NMR) data.

Deep magma facilitates the movement of tectonic plates
A small amount of molten rock located under tectonic plates encourages them to move.

When honey flows faster than water
Physicists surprised to find that in specially coated tubes, the more viscous a liquid is, the faster it flows

Bristol scientists shine light on tiny crystals behind unexpected violent eruptions
In a new study of volcanic processes, Bristol scientists have demonstrated the role nanolites play in the creation of violent eruptions at otherwise 'calm' and predictable volcanoes.

Inexpensive, non-toxic nanofluid could be a game-changer for oil recovery
Researchers from the University of Houston have demonstrated that an inexpensive and non-toxic nanofluid can be used to efficiently recover even heavy oil with high viscosity from reservoirs.

Oil-soluble transition metal-based catalysts tested for in-situ oil upgrading
The results of the study showed that the good catalytic properties of the new transition metal catalysts, as well as their low cost and easy accessibility, make them a potential solution in the aquathermolysis reaction and heavy oil recovery.

Key technology for mass-production of lignin-bio-aviation fuels for reducing greenhouse gas
The team, led by Dr. Jeong-Myeong Ha of the Clean Energy Research Center at the Korea Institute of Science and Technology(KIST), has developed a technology that can be used to mass-produce aviation-grade fuels from wood wastes.

Airborne chemicals could become less hazardous, thanks to a missing math formula
Purdue researchers have figured out a way to calculate surface viscosity just by looking at a stretched droplet as it starts to break.

Nickel-based catalysts tested at Boca de Jaruco oilfield in Cuba
In this publication, the authors studied transformations of asphaltenes, the compounds determining the viscosity of petroleum.

SUTD developed a simple method to print planar microstructures of polysiloxane
SUTD developed the embedded ink writing (EIW) method, enabling the direct writing of polysiloxane which helps in the fabrication of microfluidic devices, flexible wearables, and soft actuators.

Read More: Viscosity News and Viscosity Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to