Science1 publisher2 min readPublished
Unequal colloid sizes turned mutual attraction into self-propelled pairs at Tokyo University of Science
Polystyrene spheres of 1 and 1.5 microns in an alternating field pull on each other unequally, pair off with the big one in front, and keep building clusters that break apart again over more than an hour.
The Scientist · Science desk
What happened
- Tokyo University of Science physicists suspended polystyrene spheres of 1 and 1.5 micron radius in water between indium tin oxide-coated electrodes and drove them with an alternating electric field.
- The electrohydrodynamic flows around each particle strengthened sharply with size, so the larger spheres attracted the smaller ones more strongly than the smaller ones pulled back.
- Unequal spheres paired up on their own into structures with a front and a tail, and those pairs moved through the suspension as self-propelled units even though no single particle can propel itself.
- The pairs gathered into larger clusters, and those clusters repeatedly fragmented, rearranged and reformed instead of growing without limit.
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Why it matters
- capability An active material can in principle be assembled from passive colloids and a field, because the directed motion comes from the size asymmetry between two beads rather than a motor inside either one.
- constraint Anything built this way stays tied to its driving cell, since the flows that produce the motion are induced by the applied field, so the activity belongs to the apparatus and cannot be packaged as a material on its own.
- precedent Sumino's own condition for applications is external control of the interactions. Demonstrating field programming is the next test this line of work has to pass.
The pair interaction looks like a breach of Newton's third law, because the larger sphere pulls the smaller one harder than the smaller one pulls back [6]. Sumino says momentum conservation is not violated: momentum is transferred to the surrounding fluid through the induced flows and is ultimately dissipated through friction with the substrate [9]. The beads themselves cannot propel themselves at all [10]. The asymmetry therefore belongs to the fluid coupling and the driving field.
Set the result against the single-size version of the same system. "Instead of forming increasingly large crystal-like aggregates, as is the case for particles with the same size, the system remained highly dynamic," Sumino said [8]. Mixing two sizes, he said, "completely changed the collective behaviour of these particles" [7].
Why the clusters stop growing is a matter of shape, in the team's account. The researchers said that "the larger particle tends to be at the front of these moving pairs" and that "this head-heavy size asymmetry, together with excluded-volume interactions, promotes the fragmentation of larger clusters and thereby prevents continuous coarsening" [12].
Minutes of video are long enough to watch pairs form and clusters appear. Separating a state that keeps reorganizing from one that is coarsening slowly takes longer. Earlier studies of this kind ran a few minutes; this one tracked more than 10,000 particles and quantified their dynamics for more than an hour [4].
What is on the record is one size pair: radii of 1 and 1.5 microns [3]. That is a radius ratio of 1.5, and for spheres a volume ratio of about 3.4 [17]. The account does not report varying that ratio, so how much asymmetry the effect needs is unestablished [19]. Numerical simulations ran alongside the experiments and identified non-reciprocal pair motion as the microscopic origin of the behaviour [13]. Simulations built to match one experiment support a mechanism; generalising it is a separate demonstration.
The broader claim from Sumino and Yoshii is that non-reciprocal interactions can be a general design principle for active materials whose structure continuously reorganizes [14]. Sumino said: "self-propulsion does not need to be built into each individual particle: it can emerge collectively from non-reciprocal interactions between particles that do not self-propel on their own" [15]. He told Physics World that if such interactions can be controlled externally, they could offer a way to design microscopic systems that collectively gather, transport, fragment or mix materials [16].
What to watch
- A published size-ratio sweep, including nearly matched particles, would show whether the non-reciprocal effect has a threshold.
- Whether the flow asymmetry can be modulated in real time by field frequency or amplitude, the external control Sumino set as the condition for applications.
- Whether the continuously reorganizing state survives at higher particle densities or in fluids other than water between ITO electrodes.