Move furniture in the micro-world

Schematic illustration of the evolution of the moiré pattern under the influence of external forces and torques for a circular shaped colloidal cluster interacting with a periodically structured surface. The regions where the cluster particles are close to the bottom of the wells of the patterned surface appear as dark colored regions in the respective moire patterns. The width of the arrows represents the amount of force and/or torque needed to overcome the static friction between the cluster and the surface. Credit: Andrea Silva and Xin Cao

When moving furniture, heavy objects are easier to move if you rotate them while pushing them. Many people do this intuitively. An international research team from Konstanz (Germany), Trieste and Milan (Italy) has now studied on a microscopic scale the reduction of static friction caused by simultaneous rotation.

In their recent study, to be published in Physical examination X on June 15, researchers found that reducing static friction of a microscopic object on a crystal surface can be described by moire patterns, which occur when periodic patterns overlap. Based on this concept, the researchers predict an unusual state, in which microscopic objects can be rotated by applying a minimal amount of torque. In the future, this could allow the construction of micro-machines with very low statics friction against rotation.

Set objects in motion

To set an object in motion, it must be pushed to overcome its static friction with the underlying surface. This is true even if the surfaces in contact are very smooth. Everyday experience teaches us that static friction is much lower when the object is not only pushed, but simultaneously turned. Even though renowned researchers, such as Leonardo da Vinci, already studied friction phenomena more than 500 years ago, the relationship between static frictional forces and torques is still not fully understood. This is quite remarkable, given that rotational friction arises from the same interaction between an object and the underlying surface as the well-explored translational friction.

The complex relationship between translational and rotational static friction becomes even more intriguing on the microscopic scale, where flat contacts only involve a few hundred to a few thousand atoms. “For example, such micro-contacts occur in tiny mechanical devices – known as micro-electromechanical systems (MEMS) – whose behavior is dominated by frictional effects,” explains Professor Clemens Bechinger, head of the research team and professor of experimental physics at the University of Konstanz, providing an example where friction effects play an important role at the microscopic scale. Rotational friction and its interaction with translational friction for such small contacts has remained rather unexplored, since it is technically very difficult to apply well-controlled torques to microscopic rotating objects.

Moiré patterns are the key

In their recent study, combining experimental and theoretical approaches, the researchers from Konstanz, Trieste and Milan overcame this challenge and investigated rotational friction and its interaction with translational friction for microscopic contacts. “For our experiments, we created crystal clusters consisting of micron-sized magnetic spheres and brought them into contact with a structured surface with regularly repeating pits,” said Dr. Xin Cao, the a lead author of the study and a Humboldt Fellow in the work. group of Clemens Bechinger, describes the starting point of the experiments. He continues: “This decor imitates the contact area between two atomically flat surfaces.

The two-dimensional clusters – with surface contacts made up of 10 to 1000 spherical particles – were then set into rotation using a highly controllable rotating magnetic field. The minimum torque required to rotate the respective cluster corresponds to the static rotational friction, similar to the static translational friction, which characterizes the minimum force required to obtain translational movement of the cluster.

In their study, the researchers found that the interplay of rotational and translational friction can be understood through the properties of so-called moiré patterns. These patterns appear when two or more periodic structures overlap. “Optical moire patterns can be observed, for example, when a fine-mesh curtain creases and individual layers of the curtain overlap,” says Dr. Andrea Silva, the study’s second lead author and physicist at the International School for Advanced Studies (SISSA). ) in Trieste. “The resulting patterns are extremely sensitive to minute relative motions and exhibit higher-level geometric structures that are not present in the overlapping structures themselves.”

The advantage of simultaneous rotation

Returning to the experiments, Andrea Silva describes: “The contact between the cluster of particles and the underlying surface in areas where the periodicities of the structure of the two objects match can be compared to eggs in an egg box.” Without applying external forces or torques, this area of ​​structural overlap is at a maximum, meaning that a large number of cluster particles are close to the bottom of the pattern wells. surfacecausing high static friction.

When a force is applied to the cluster to push it in a particular direction, the structural overlap zone moves towards the edge of the contact patch. As a result, it becomes smaller. However, a large number of particles remain “stuck” in the wells of the substrate, so that a relatively large force is required to overcome the resistance of the cluster to movement and to detach it from the substrate. If, on the contrary, the cluster is twisted with a torque, the overlapping area shrinks symmetrically. “This makes it much easier to push the cluster and set it in motion, as the structural overlapping area has already been greatly reduced by the applied torque,” ​​says Xin Cao, explaining how pushing and rotating simultaneously reduces friction. static.

Based on the properties of the observed moiré patterns, physicists were not only able to explain why additional rotation facilitates the translation of microscopic objects, but also to make predictions about the dependence of static friction on rotations on the band size: when this exceeds a certain threshold, the static friction against rotations decreases sharply, resulting in an ultra-low state of static friction for very large clusters. “Such a low-friction state can be very relevant for the fabrication and operation of small mechanical devices – from atomic to micro-scale – bringing us closer to realizing smaller and more efficient machines,” concludes Clemens Bechinger.

More information:
Xin Cao et al, moiré pattern evolution couples rotational and translational friction at crystal interfaces. arXiv:2204.12336v1 [cond-mat.soft],

Quote: Moving furniture in the micro-world (June 10, 2022) retrieved June 12, 2022 from

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