A colloid is formed by evenly dispersing tiny particles in a liquid. Simple examples include corn starch suspended in water, or microscopic glass beads dispersed in glycol. A simple tabletop demonstration reveals a startling property these two colloids possess: gently push your fingers into the colloid and it flows like liquid but strike it with your fist and it suddenly solidifies.
This abrupt liquid-to-solid transition is known as discontinuous shear thickening (DST). As the name implies, the dynamic response of the colloid abruptly transitions from a liquid to a solid when the applied shear force exceeds a critical value. The general consensus among materials scientists is that inter-particle friction is responsible for DST. But surprisingly little experimental evidence directly supports this hypothesis.
To clarify this issue, researchers recently used X-ray photon correlation spectroscopy (XPCS) to observe the dynamics of glassy colloids subjected to varying shear forces. The results showed that when a strong shearing force is applied, it induces a congested network of colloidal particles governed by friction. The work was performed at beamline 8-ID-I of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory.
Characterizing the underlying basis of DST should allow scientists to tune the dynamic behavior of many complex fluids, with real-world applications such as reducing the energy cost for mixing materials, as well as improved braking devices and body armor.
Fluids can be categorized as either Newtonian or non-Newtonian. These two categories are distinguished by reaction to shear, which is the force applied to drive fluid motion. The viscosity (ease of flow) of a Newtonian fluid does not change as shear is applied. Water and vegetable oil are both considered Newtonian fluids, since their viscosity remains unchanged when stirred.
Non-Newtonian fluids are quite different. Some non-Newtonian fluids actually become thinner (viscosity decreases) under shear as the fluid's interior structure breaks down during shearing. In contrast, viscosity that rises slightly or moderately with shear is called continuous shear thickening (CST), while thickening that yields a solid-like state constitutes discontinuous shear thickening, or DST.
Scientists study the dynamic behavior of fluids using a device called a rheometer, which consists of two concentric cylinders as depicted in Fig. 1a. After the gap between the cylinders is filled with a colloid, the inner cylinder spins which applies a shear force. A torque sensor then measures the force/torque. What sets this particular experiment apart from other fluid-shear experiments is that XPCS data was collected simultaneously with the rheology torque measurements.
Three distinct colloids (A, B, and C) were examined, each consisting of uniformly sized silica particles dispersed in polyetheylene glycol. Particles in sample A measured 200 nanometers across and took up 60.5% of the colloid's volume (called the volume fraction). Sample B possessed particles 360 nanometers across with a 56% volume fraction, and sample C had 360 nm particles with a 60.5% volume fraction.
Each colloid was placed in a cylindrical shear cell and then sufficient stress was applied so that the colloid approached a state nearing either CST or DST. Upon reaching equilibrium, the shear was abruptly stopped, and each colloid was monitored via XPCS for at least an hour, producing a series of speckle patterns as shown in Fig. 1b. These speckle patterns revealed how each colloid's movements (particle velocities) changed over time.
The most interesting discovery by the team was the observation of a slowly evolving beat pattern (or heterodyne signal) in the XPCS data that occurred with both the CST-type and DST-type fluid behavior. Such a “heterodyne signal” only arises when different particles move at different speeds within the X-ray scattering volume. These particular heterodyne signals indicated the movement of mobile colloid particles against an aggregated, or jammed, network of particles produced by shear thickening.
In summary, the XPCS data showed that both CST and DST arise in highly stressed colloids due to the creation of a stagnated network of particles interacting via friction with nearby mobile particles. Moreover, after each colloid reached equilibrium, its internal stresses plunged quickly, while the internal structure and particle motion dissipated much more slowly. The researchers note that these results may also provide new insights into other systems with slowly evolving dynamics, such as the compaction of granular particles under vibration or the compaction of crumpled sheets under stress. – Philip Koth
Read the Center for Nanoscale Materials science highlight here.
See: J.P. Howarth1, H. He1, J. Lee1, Z. Jiang1, S. Chakraborty1, Q. Zhang1, E.M. Dufresne1, M. Sutton2, A. Sandy1, S. Narayanan1, X-M Lin1, “Heterogenous dynamics in shear thickening colloids revealed by intrinsic heterodyne correlation spectroscopy,” Phys. Rev. Lett. 134, 178202 (2025)
Author affiliations: 1Argonne National Laboratory; 2McGill University.
Work was performed at the Center for Nanoscale Materials and Advanced Photon Source, both are U.S. Department of Energy Office of Science User Facilities, supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The work is also partially supported by the LDRD program of Argonne National Laboratory.
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