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Imagine a liquid that flows freely one moment, then stiffens into a near-solid the next, and then can switch back with a simple change in temperature. Researchers at the University of Chicago Pritzker School of Molecular Engineering and NYU Tandon have now developed such a material, using tiny particles that can change their shape and stiffness on demand.

Their research paper, "Tunable shear thickening, aging, and rejuvenation in suspensions of shape-memory endowed liquid crystalline particles," in Proceedings of the National Academy of Sciences, demonstrates a new way to regulate how dense suspensions—mixtures of solid particles in a fluid—behave under stress.

These new particles are made from liquid crystal elastomers (LCEs), a material that combines the structure of liquid crystals with the flexibility of rubber. When heated or cooled, the particles change shape: they soften and become round at higher temperatures, and stiffen into irregular, angular forms at lower ones. This change has a dramatic effect on how the suspension flows.

From smooth to stiff and back again

Dense suspensions are found in everyday products like paints, toothpaste, and cement. Under certain conditions, these materials can thicken unpredictably under force, a behavior known as shear thickening. In some cases, the thickening becomes so extreme that the material jams and stops flowing altogether. This can cause problems in processing and manufacturing, where smooth, consistent flow is essential.

The research team, co-led by UChicago PME professor of Molecular Engineering Stuart Rowan and Juan de Pablo, formerly at UChicago and now Executive Vice President for Global Science and Technology at NYU and Executive Dean of the NYU Tandon School of Engineering, designed LCE particles whose shapes can be programmed during synthesis. They found that suspensions made from more irregular, "potato-shaped" particles thickened much more under stress than those made from smoother, "pea-shaped" ones.

But the key breakthrough came with temperature control. At lower temperatures, the potato-shaped particles were rigid and irregular, and their suspensions exhibited strong shear thickening—resisting flow when stress increased. But as the temperature rose past 45–50°C, the particles transformed into softer, rounder shapes, and the suspension became much easier to stir or pump. The researchers showed that this change could be repeated over and over again.

"The basic behavior is akin to what one observes with corn starch and water, where under small shear the material is a liquid, but when submitted to high shear it is a solid. There are several factors that play a role in such shear behavior, including shape and stiffness of the particles in the suspensions. Here we show that it is possible to design stimuli-response particles that allow access to suspensions with tunable flow behavior," said Rowan.

Chuqiao Chen, first author of the study and a Ph.D. candidate in the University of Chicago's Pritzker School of Molecular Engineering at the time of the research, added, "In a narrow temperature window, we saw a full transition from a jammed, thick state to a freely flowing one. It's like flipping a switch on how the fluid behaves."

A suspension with a memory

Over time, even in the absence of flow, the particle suspensions tend to settle into more solid-like states in a process known as "aging." The particles clump together and form structures that resist movement. This behavior, common in dense materials, can make them hard to work with after storage.

However, the LCE-based suspensions have a built-in solution. When the aged suspensions were heated above their shape-transition temperature, the particles relaxed into spherical forms and the clusters broke apart. The suspension returned to a fluid state, effectively resetting itself. This transformation did not require stirring or mixing, just a brief heating and cooling cycle.

The ability to control both particle shape and stiffness with temperature gives researchers an entirely new handle on how dense fluids behave. Traditionally, tuning the flow properties of suspensions required adjusting how many particles were present or modifying the fluid's chemistry. With this approach, the same suspension can be adjusted simply by changing the temperature.

The potential uses are wide-ranging. In additive manufacturing (3D printing), for example, preventing jamming and controlling flow are major concerns. In industrial mixing, being able to "switch off" thickening behavior could help improve efficiency. The team's findings suggest that even modest heating or cooling could achieve this.

The research opens a path toward materials that can flow, jam, and unjam on cue—not by changing their contents, but by altering how their parts are arranged and how they interact.