Steady states of floating crystals

Steady states of floating crystals

Animation of the growth of a floating crystal from N = 3 to N = 19 particles. The particles are added one by one to the surface of the liquid under the magnetic field. The attractive capillary forces are balanced by the repulsive magnetic forces. The assembly shows symmetry typical of atomic crystals. Credit: University of Liège/In Fandewal

A research team led by GRASP – the Research and Applications Group in Statistical Physics – at the University of Liège (Belgium), demonstrates how to manipulate the network of floating crystals, their shape and symmetry by wandering, in a controlled manner, between their static lattices. States. This study was published in the journal Scientific Reports.

Multiple-particle systems are of interest in many areas of physics. Its structure is governed by their interactions. In particular, in the presence of attractive interactions, such systems tend to self-assemble, reducing their energy. This phenomenon is present at all levels, and governs the formation of molecules and planetary systems. Depending on the complexity of the interactions, the particles can form simple periodic structures (crystals), or more complex structures such as protein chains.

Allow magnetic interactions between particles Self-assembly of crystals floating along liquid surfaces. For a fixed number of particles, different states with different symmetric properties, called unstable states, coexist. Several pioneering works have observed unstable states in floating crystals.

Due to the coexistence of different states, it is difficult to control the formation of specific structures. However, controlling the formation of unsteady states is a key component to activating such assemblies, paving the way for eg self-assembling micro-robots. The method of controlling the state of the floating crystal has not been studied before.

“Self-assembly has attracted interest from academia and industry due to its use for the fabrication of small structures,” says Nicholas Vandewal, Professor of Physics and Director of GRASP. “In fact, some structures are too large to be prepared by chemical synthesis and too small to be assembled by robotic methods. In particular, the millimeter-micrometer scale is typically the bottleneck between standard bottom-up and top-down fabrication methods.”

A major characteristic of self-assembled systems is that due to the large number of degrees of freedom there are often many local minimums in addition to the global minimum energy state. These unstable states can be observed at all levels, in molecular levelin colloids, on a mesoscopic and on a macroscopic scale.

Steady states of floating crystals

Different combinations of N molecules on the surface of the liquid. For every N number of particles, two different clusters are shown against each other, which indicates the stability of the clustering. Credit: University of Liège/In Fandewal

Recently there has been increased interest in how these unstable states can be exploited for active structuring. Therefore, the fundamental question, which the researchers addressed in this study, is to determine the conditions for hopping between different metastases.

“In the study we just published, we studied magnetic filament self-assemblies of 3 to 19 particles,” says Jelona Collard, a researcher at GRASP and first author of the article. “.

The researchers proposed two different, but complementary, experimental methods of navigating in a controlled manner between these different states. The first allows changing the state of a fixed number of particles. This is achieved by applying a horizontal magnetic field that deforms the assembly.

After relaxing, the pool will have changed its state with a certain probability. The second technique controls assembly growth by selecting the desired state of pooling N (number) + 1 beads from a pool of N beads. infrared laser It is applied to Surface of the water To generate thermal capillary flows, and to control the path of the new bead added to the system.

“Models have been proposed to study the frequency of occurrence of different states of aggregation as they are created,” explains Nicholas Vandewal, modeling the two experimental techniques. The simulations agree very well with the experimental results. An analogy between these lattice magnetic assemblies, which can be minimized to a smaller size, and colloidal crystals has been proposed to broaden the horizons of this work. “

This work is already very relevant to the fabrication of microscopic structures such as electronic circuits, micro-robots, or new materials with new physical properties.

Initiating self-assembly at a microscopic scale using light and heat

more information:
Yeluna Collard et al., Controlled transitions between unstable states of crystals with 2D magnetic filaments, Scientific Reports (2022). DOI: 10.1038 / s41598-022-20035-8

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