Twisting to Pack

One conventional method for packing granular materials is to tap them repeatedly, but a new study suggests that twisting is a faster method. Researchers poured thousands of dice into a cylinder, then twisted the container back and forth. When the acceleration caused by the change in direction exceeded a threshold value, the dice worked their way into ordered layers of concentric rings over ~10,000 cycles. Reaching this maximum packing density through tapping requires a specially designed method where the tapping characteristics change over time. With twisting, shear forces transmitted from the walls of the container tend to align the flat surfaces of the dice, providing an efficient method of ordering if the acceleration is large enough. The researchers hope methods like this may be useful where tapping works poorly, such as in microgravity. (Image and research credit: K. Asencio et al.; via APS Physics; submitted by Kam-Yung Soh)

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