Streams of falling water tend to clump into droplets as surface tension attracts globules of liquid in midair. Even though solid objects are presumed not to have surface tension, the same phenomenon has been observed with grains or sand or tiny beads. But so far, no one has been able to explain exactly what and how much force is at work drawing these objects together.

A new high-speed video outfit, designed by University of Chicago physics graduate student John Royer, has been able to capture these nanoscale forces for the first time, reports a paper published today in Nature (Scientific American is part of Nature Publishing Group).

Using an atomic force microscope, Royer and the research team were also able to measure the attracting force and found that even though the falling solids might look like tumbling liquid, surface tension that attracts the pieces is as much as 100,000 times less than liquid surface tension.

Watch the video for a demonstration.

Image and video courtesy of Helge F. Gruetjen (Supported by Germany-U.S. Fulbright Scholar Program), John R. Royer, Scott R. Waitukaitis, and Heinrich M. Jaeger, The University of Chicago