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Researchers at ETH Zurich have made a significant advance in the understanding of rolling friction in tiny, micrometre-sized particles, marking a pivotal step towards enhancing the properties of everyday materials such as concrete.
Suspensions—liquids that contain tiny, insoluble solid particles uniformly dispersed within them—are commonplace in a variety of contexts, from industrial products like paints and concrete to food items such as ketchup and orange juice. When the concentration of solid particles in a suspension reaches a critical level, some counterintuitive behaviors occur that challenge common perceptions about how liquids function. These materials can exhibit non-Newtonian fluid characteristics, where they become more viscous when subjected to significant forces, temporarily behaving like solids.
This phenomenon of sudden thickening in suspensions stems from the arrangement of particles within the liquid. During deformation, the particles must reposition themselves; rolling past one another is energetically more favorable. However, when they become jammed, they must slide against each other, a process that requires considerably more force, resulting in greater viscosity on a macroscopic scale.
The microscopic interactions that take place among the particles are crucial to understanding and predicting the overall flow behavior of a suspension. To refine these suspensions and tailor their flow properties, it is essential for scientists to grasp the frictional forces at play between individual particles.
Under the guidance of Professor Lucio Isa, a team of materials researchers at ETH Zurich devised a novel technique to measure these frictional forces among particles that are only a few micrometres in diameter. They employed an atomic force microscope, with doctoral student Simon Scherrer crafting a minuscule holder designed to capture a single spherical particle. This holder enables the movement of the particle across a flat surface that mimics its own characteristics, effectively simulating the interaction between two particles and facilitating the measurement of the forces involved.
The particles studied by the team are notably tiny, measuring just 12 micrometres in diameter, which poses unique challenges for developing effective measurement techniques. “It took me countless iterations—about 50 versions—before I created a holder that met our stringent criteria,” Scherrer noted.
To gain insights into how the properties of these minute particles affect the behavior of suspensions, the researchers fabricated various types of particles. They found that particles with smooth, slippery surfaces easily slid past each other, regardless of the pressure applied. In contrast, rough or sticky particles interacted firmly, akin to gears, rolling with minimal resistance. Additionally, the researchers stabilized the particles in the holder to measure their sliding friction, which was found to be significantly higher than the rolling friction, thus providing a scientific explanation for the considerable thickening observed in suspensions.
From their experiments, the team was able to derive coefficients for both rolling and sliding friction of the examined particles, which can now be incorporated into computer models to simulate suspensions with high particle densities. This modeling helps determine the optimal flow characteristics for such mixtures, paving the way for advancements in various sectors.
This research holds particular promise for industries such as concrete manufacturing and microelectronics. For instance, microelectronics companies utilize dense suspensions containing metallic particles to assist in soldering components onto circuit boards. The solder paste must be expelled through narrow nozzles, and excessive pressure can lead to sudden thickening that potentially clogs the system.
“To avoid such complications and enhance the performance of these suspensions, understanding the microscale behavior of particles and the resulting forces is crucial,” Isa explained, highlighting the broader implications of this research on practical applications in both industry and daily life.
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