For the first time, ETH Zurich Materials Scientists have measured the rolling friction of microscopic, micrometer-sized particles. These measures allow them to better understand common items like concrete. The results were reported in the journal PNAS.
The measuring tip of an atomic force microscope with a specially designed holder in which a spherical particle is “trapped”. Image Credit: Simon Scherrer / ETH Zurich
Suspensions are used in a variety of industries and daily activities, including lacquers, paint, concrete, and even ketchup or orange juice. Materials scientists define a suspension as a liquid in which small, insoluble solid particles are equally dispersed.
If the particle concentration is extremely high, such a mixture may exhibit characteristics that defy the conventional idea of a liquid. For instance, when a powerful force is applied to these so-called non-Newtonian fluids, they abruptly become more viscous. The liquid acts like a solid for a short time.
The particles in the suspension produce this abrupt thickening. If the suspension is distorted, the particles must reorganize. When it comes to energy, it is better if they roll past one another whenever they can.
When this is no longer feasible, such as when numerous particles become jammed, they must slide relative to one another. Conversely, sliding requires significantly more force, making the liquid feel viscous on a macro scale.
Microscopically small interactions affect the entire system and decide how a suspension moves. To optimize the suspension and precisely impact its flow characteristics, scientists must first analyze the amount of the frictional forces between individual particles.
What Did the Scientists Investigate?
ETH Zurich Materials Researchers, led by Lucio Isa, Professor of Interfaces and Soft Matter, have discovered a method for measuring frictional forces between individual particles as small as a few micrometers in diameter.
The researchers conducted their measurements using an atomic force microscope. Simon Scherrer, a doctoral student, first created a microscopically small device capable of capturing a single spherical particle.
The “trapped” particle is then moved over a flat surface with the same properties as the particle using an atomic force microscope. In this technique, the researchers could simulate two particles passing each other and detect the small tensions between the surfaces.
Why is This So Crucial?
The particles analyzed are extremely small, measuring only 12 micrometers (12 millionths of a metre) in diameter. Creating a proper measurement technique for the particle's rolling friction was challenging. Making an appropriate holder proved to be quite difficult.
I must have developed 50 versions until I found one that met the requirements.
Simon Scherrer, Doctoral Student, ETH Zurich
The researchers created several particles to better understand how the tiny particles' surface influences the suspension's behavior.
Scherrer added, “Particles with a smooth or very slippery surface simply slid past each other regardless of how firmly we pressed them together.”
The situation with rough or sticky particles was very different, since they interacted like gearwheels and rolled with no resistance. Finally, the researchers placed the particles in the holder and measured their sliding friction. This friction is many times more than rolling friction, which explains the enormous thickness of the suspensions.
What Purpose Does This Serve?
The researchers could immediately calculate the coefficients for rolling and sliding friction of the various particles based on their data. These figures can be used in computer models to simulate suspensions with a large fraction of particles, for example, and thus find the best flow properties. These insights into the microscopic mechanisms that induce thickening are leading to new techniques for optimizing suspensions for use in industry, building, and everyday life.
Beneficiaries could include, for example, the concrete sector or microelectronics producers. The latter already use dense suspensions of metallic, conductive particles to solder components to circuit boards. Solder paste is squeezed through thin nozzles. If the pressure is too high, the paste may suddenly thicken and block the nozzle.
In order to prevent this behavior and optimize such suspensions, we have to know precisely how particles behave on a microscale and what forces occur in the process.
Lucio Isa, Professor and Study Lead, ETH Zurich
Journal Reference:
Scherrer, S., et al. (2025) Characterizing sliding and rolling contacts between single particles. Proceedings of the National Academy of Sciences (PNAS). doi.org/10.1073/pnas.2411414122