Synopsis: Survival of the Fitter

Hard spheres may favor one structural arrangement over others that are less mechanically stable during assembly.
Synopsis figure
S. Heitkam et al., Phys. Rev. Lett. (2012)

A layer of dispersed spheres tends to pack in a triangular array. But to stack them in three dimensions, another layer can register with either of two sets of holes in the first layer. A third layer can then align with either the unused holes or with the spheres of the first layer. Repeating the first choice leads to a face-centered-cubic (fcc) structure, while the second choice leads to hexagonal close packing (hcp). Because the relationship of neighboring spheres is identical, neither arrangement (nor others) has an energy advantage. But in practice, spheres spontaneously adopt the fcc structure more frequently, as explained by Sascha Heitkam at the University of Paris-Sud in Orsay, France, and co-workers in Physical Review Letters.

The researchers simulated the assembly of a close-packed structure of spheres that either sink or float in a surrounding fluid, and also did corresponding experiments with buoyant bubbles and hard spheres. They identified a critical phase when a sphere appears on one layer before the previous layer is complete. If the spheres are in the hcp arrangement, incoming spheres push those in the next layer sideways, potentially ejecting them into unfilled regions. In contrast, in the fcc arrangement the force pushes the earlier spheres directly toward others on the layer below, so this structure can persist and seed further growth. Although the results depend on the detailed conditions, the authors suggest that this difference in stability may be important in many types of growth. – Don Monroe


Features

More Features »

Announcements

More Announcements »

Subject Areas

Fluid DynamicsMaterials Science

Previous Synopsis

Biological Physics

Group Mentality

Read More »

Next Synopsis

Nanophysics

A Noisy Junction

Read More »

Related Articles

Focus: <i>Video</i>—Fluid Video Contest Winners
Fluid Dynamics

Focus: Video—Fluid Video Contest Winners

Swimming starfish larvae, dripping paint, and swirling gas jets are featured in the APS Division of Fluid Dynamics’ winning videos. Read More »

Synopsis: How to Make Superhydrophobicity Last
Fluid Dynamics

Synopsis: How to Make Superhydrophobicity Last

Researchers find tricks to prolong the typically short-lived water repellency of a superhydrophobic surface. Read More »

Focus: Drops Falling in Clouds Make More Drops
Fluid Dynamics

Focus: Drops Falling in Clouds Make More Drops

Experiments with a simplified version of the atmosphere show that falling drops seed many smaller droplets in their wake. Read More »

More Articles