Small self-propelling toys used to study movement of microscopic organisms and molecular motors

Researchers at the Institute of Physics at the University of Amsterdam have found a unique way to study the movement of microscopic organisms and molecular motors inside cells. They connected small, self-propelling Hexbug Nano v2 microbots in a chain using elastic silicon rubber, creating an “elastoactive” structure that returns to its original shape after being deformed. The active microbots constantly push the structure in a certain direction, resulting in a range of movement types depending on the size of the chain links and whether the chains were fixed at one or both ends. The researchers discovered an interplay between activity and elasticity, which leads to self-oscillation and synchronization of the chains when activity dominates. These findings are significant for the development of new autonomous robots that mimic the features of biological machines. The study is published in Physical Review Letters.

The self-oscillatory behaviour of an elastoactive chain pinned at one end, the self-synchronising behaviour of two chains coupled by a stiff rod, and the self-snapping behaviour of a chain pinned at both ends. Credit: Ellen Zheng

Self-oscillation, self-synchronization and self-snapping

Self-oscillation occurs when a structure bends back and forth without any external force. In the elastoactive chains, the microbots initially bend the chain to the left, but the elastic links resist this movement, causing the bots to push and bend the chain to the right. This back-and-forth movement continues as the bots keep reorienting themselves.

Synchronization happens when two elastoactive chains are connected by a stiff rod, causing them to oscillate with the same frequency. This is similar to how sea grasses move in the same way in response to waves.

Lastly, when both ends of a single elastoactive chain are pinned down, it exhibits “self-snapping” behavior. This means that the chain repeatedly snaps back and forth between bending to the left and bending to the right, similar to how a playing card can snap to the other side when bent forcefully. The elastoactive chains demonstrate these behaviors on their own, allowing researchers to study the nonlinear phenomena that are also present in biological machines.

Instructive play

Initially, our research was simply based on experimenting with microbot toys. However, our main objective was to investigate materials that are not in a state of equilibrium. Although active fluids have been extensively studied in soft matter for the past 25 years, their solid counterparts have not received the same attention. This was the motivation behind our work, as stated by Coulais.

Our future plans involve examining the elastoactive properties of colloidal systems, which are made up of small particles suspended in a fluid. Although these systems are still considered model systems, they are closer to biological systems due to their comparable length scales and the presence of fluid. At any scale, it would be interesting to utilize intelligent design to incorporate multiple self-oscillations into a single structure to produce more complex movement patterns. Our goal is to gain a deeper understanding of self-oscillations, with the hope of developing new types of independent robots.

Source: University of Amsterdam

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