If it walks like a particle, and talks like a particle… it should still not be a particle. A topological soliton is a special style of wave or dislocation which behaves like a particle: it might probably move around but cannot opened up and disappear like you’ll expect from, say, a ripple on the surface of a pond. In a brand new study published in Nature, researchers from the University of Amsterdam reveal the atypical behaviour of topological solitons in a robotic metamaterial, something which in the longer term could also be used to regulate how robots move, sense their surroundings and communicate.
Topological solitons will be present in many places and at many various length scales. For instance, they take the shape of kinks incoiled telephone cords and huge molecules corresponding to proteins. At a really different scale, a black hole will be understood as a topological soliton in the material of spacetime. Solitons play a very important role in biological systems, being relevant forprotein folding andmorphogenesis — the event of cells or organs.
The unique features of topological solitons — that they’ll move around but all the time retain their shape and can’t suddenly disappear — are particularly interesting when combined with so-called non-reciprocal interactions. “In such an interaction, an agent A reacts to an agent B otherwise to the best way agent B reacts to agent A,” explains Jonas Veenstra, a PhD student on the University of Amsterdam and first creator of the brand new publication.
Veenstra continues: “Non-reciprocal interactions are commonplace in society and sophisticated living systems but have long been neglected by most physicists because they’ll only exist in a system out of equilibrium. By introducing non-reciprocal interactions in materials, we hope to blur the boundary between materials and machines and to create animate or lifelike materials.”
TheMachine Materials Laboratory where Veenstra does his research specialises in designing metamaterials: artificial materials and robotic systems that interact with their environment in a programmable fashion. The research team decided to check the interplay between non-reciprocal interactions and topological solitons almost two years ago, when then-students Anahita Sarvi and Chris Ventura Meinersen decided to follow up on their research project for the MSc course ‘Academic Skills for Research’.
Solitons moving like dominoes
The soliton-hosting metamaterial developed by the researchers consists of a series of rotating rods which can be linked to one another by elastic bands. Each rod is mounted on somewhat motor which applies a small force to the rod, depending on the way it is oriented with respect to its neighbours. Importantly, the force applied will depend on which side the neighbour is on, making the interactions between neighbouring rods non-reciprocal. Finally, magnets on the rods are attracted by magnets placed next to the chain in such a way that every rod has two preferred positions, rotated either to the left or the appropriate.
Solitons on this metamaterial are the locations where left- and right-rotated sections of the chain meet. The complementary boundaries between right- and left-rotated chain sections are then so-called ‘anti-solitons’. That is analogous to kinks in an old-fashioned coiled telephone cord, where clockwise and anticlockwise-rotating sections of the cord meet.
When the motors within the chain are turned off, the solitons and anti-solitons will be manually pushed around in either direction. Nonetheless, once the motors — and thereby the reciprocal interactions — are turned on, the solitons and anti-solitons routinely slide along the chain. They each move in the identical direction, with a speed set by the anti-reciprocity imposed by the motors.
Veenstra: “Plenty of research has focussed on moving topological solitons by applying external forces. In systems studied to this point, solitons and anti-solitons were found to naturally travel in opposite directions. Nonetheless, if you must control the behaviour of (anti-)solitons, it is advisable to drive them in the identical direction. We discovered that non-reciprocal interactions achieve exactly this. The non-reciprocal forces are proportional to the rotation attributable to the soliton, such that every soliton generates its own driving force.”
The movement of the solitons is comparable to a series of dominoes falling, each toppling its neighbour. Nonetheless, unlike dominoes, the non-reciprocal interactions be sure that the ‘toppling’ can only occur in a single direction. And while dominoes can only fall down once, a soliton moving along the metamaterial simply sets up the chain for an anti-soliton to maneuver through it in the identical direction. In other words, any variety of alternating solitons and anti-solitons can move through the chain without the necessity to ‘reset’.
Motion control
Understanding the role of non-reciprocal driving is not going to only help us to raised understand the behaviour of topological solitons in living systems, but can even result in technological advances. The mechanism that generates the self-driving, one-directional solitons uncovered on this study, will be used to regulate the motion of several types of waves (often known as waveguiding), or to endow a metamaterial with a basic information processing capability corresponding to filtering.
Future robots can even use topological solitons for basic robotic functionalities corresponding to movement, sending out signals and sensing their surroundings. These functionalities would then not be controlled from a central point, but fairly emerge from the sum of the robot’s lively parts.
All in all, the domino effect of solitons in metamaterials, now an interesting statement within the lab, may soon begin to play a job in several branches of engineering and design.
