Entropic trapping in colloidal clusters
How do a small number of larger colloidal particles behave when mixed with many smaller ones inside spherical droplets? We found that entropy alone drives the larger particles to the droplet surface, where they consistently settle at the twelve vertices of an icosahedral structure formed by the smaller particles. Using a combination of experiment and simulation, we showed that this trapping effect is robust and results from the system’s tendency to maximize free volume during self-assembly. These findings offer new insight into how complex structures can form without external guidance and support the design of programmable materials through simple physical principles.

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Praveen K. Bommineni, Junwei Wang, Nicolas Vogel, Michael Engel
Entropic Trapping of Hard Spheres in Spherical Confinement
Physical Review Letters 134, 198201 (2025)