Impact-activated solidification

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2012

Researchers develop an experimental approach hitting a suspension of cornstarch in water with an aluminum rod.

KEY CONCEPTS
• Applying sufficiently fast shear or sudden impact to a dense suspension in water leads to the formation of a solid-like state. No satisfactory mechanism has been proposed for this phenomenon until now.
• An experimental study hitting a suspension of cornstarch in water with an aluminum rod was used to study this phenomenon.
• According to researchers, a propagation front of solidification is formed when the rod strikes the suspension, which is similar to how a snow plow operates.

SUSPENSIONS HAVE BECOME AN ATTRACTIVE APPROACH to facilitate the use of solid-based lubricants such as molybdenum disulfide and polytetrafluoroethylene in lubricant applications. It is very challenging to incorporate these types of materials in both oil and water media, so preparation of suspensions has become a viable option.

Access to high-viscosity base oils is becoming a concern due to the slow reduction in the use of Group I base oils globally. This has led to the search for development of thickeners that can boost the viscosity of Group II and III base oils, which are becoming more widely used.

There are several interesting phenomena associated with suspensions. For example, in a previous TLT article, the coffee-ring effect was discussed (1). Evaporation of water from aqueous suspensions prompts water to flow from the middle of an individual drop to the edge. Suspended particles also move to the edge forming a ring. The name of the effect was coined from the use of coffee particle suspended in water. Researchers found that changing the shape of the suspended particles from spherically to ellipsoidal will stop the effect from occurring.

A second example involves the application of stress to a dense suspension of particles in water. This stress, during sufficiently fast shear or sudden impact, results in the transformation of the suspension into a solid-like state that eventually reverts back to a liquid-like suspension over time once the stress is removed.

This effect has been known since the 1930s, but no satisfactory mechanism has been proposed to explain how it occurs. Heinrich Jaeger, the William J. Friedman and Alicia Townsend professor of physics at the University of Chicago in Chicago, says, “Two approaches have been proposed to explain the rapid solidification of a suspension under stress. The first starts with pure liquid and then hypothesizes that as particles are added, thus becoming organized into clusters. The problem with this explanation is that theoretical modeling only predicts a modest increase in the viscosity and so far has not been able to explain the huge stresses encountered during sudden impact.”

The second approach starts at the other extreme, where particles are present in the absence of liquid. Jaeger explains, “As a very dense packing of particles is forced to flow, with or without added liquid, the packing must first expand or dilate a little bit. The analogy is that people in a jam-packed room have to have some space for individuals to move. If this dilation cannot occur and no extra space can be created, the result is that all particles remain jammed.”

This process is known as frustrated dilation and can lead to a very large stress increase. However, for situations such as local impact, where the suspension should be free to flow to the sides and dilate as needed, this picture predicts very little stress increase and, thus, also falls short.

A new approach is needed to explain how the solidification occurs during impact. Research has now led to a new hypothesis, which better explains this phenomenon.

PROPAGATION FRONT OF SOLIDIFICATION
Jaeger, in collaboration with his associate Scott Waitukaitis, devised an experimental approach to investigate how the solid is formed. They prepared a dense suspension of cornstarch in water and then hit it with an aluminum rod.

Jaeger says, “We prepared a dense suspension using approximately 50% cornstarch by volume in water. The cornstarch particles have diameters ranging between 10 and 20 microns. This suspension is liquid-like as the particles, while not highly mobile, do exhibit some Brownian motion.”

The researchers used a large suspension volume of 25 liters to avoid any problems with boundaries. The rod pushed down onto the suspension, as shown in the experimental setup in Figure 2.


Figure 2. Hitting down on a suspension of cornstarch in water with an aluminum rod leads to the formation of a solid-like state that is caused by a propagation front of solidification. (Courtesy of The University of Chicago)

Upon impact with the aluminum rod, the suspension quickly forms a solid. Jaeger says, “Once the rod hit the suspension, the particle phase quickly becomes compressed as the particles get closer together. This starts a chain reaction in which a rapidly growing solid is formed that grows below the impact site.”

This phenomenon is described by the researchers as a propagation front of solidification. The stress where the rod strikes can be in the megapascal range, ruling out the models associated with cluster formation or frustrated dilation. The extent of this solid is proportional to the distance the rod has moved, with each centimeter that the rod has moved leading to solidification of approximately 10 centimeters of suspension below the surface.

Jaeger equates the phenomenon to the operation of a snow plow. He adds, “As a plow picks up snow, a solid- like column grows in front of the shovel. This makes it harder to push the shovel.”

The solid formed is not permanent and over time disperses back into particles. Suspension concentration is a very important factor to ensure that this effect is seen. Jaeger says, “Roughly, you need to use a concentration of cornstarch between 45% and 55% to generate the solid. If the percentage of cornstarch drops below about 45%, then it will not occur.”

Changing the viscosity of the fluid won’t affect solid formation as long as the cornstarch concentration is maintained. Jaeger says, “Adding glycerin to boost the viscosity by more than a factor of 10 does not affect the impact, although it does slow the sinking of the aluminum rod into the suspension afterwards.”

Jaeger believes that this effect will occur for a wide variety of liquid-particle combinations and future work involves looking at different particles in other solvent systems such as mineral oil. It is possible that use of a specific solid lubricant additive suspended in mineral oil may increase viscosity and be a potential option for the lubricant formulator for a high-viscosity basestock.

Further information can be found in a recent article (2) or by contacting Jaeger at h-jaeger@uchicago.edu.

REFERENCES
1. Canter, N. (2011), “Eliminating the ‘Coffee Ring Effect,’” TLT, 67 (11), pp. 8-9.
2. Waitukaitis, S. and Jaeger, H. (2012), “Impact-Activated Solidification of Dense Suspensions via Dynamic Jamming Fronts,” Nature, 487 (7406), pp. 205-209.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.