In order to understand how the particles flow, Cai Riqiang designed a set of unique experimental methods, which can be divided into two parts: "particles" and "containers". In terms of "particles", Cai Riqiang used medical materials (PDMS) to make many granular soft balls with a hardness close to an eraser and a size of about one centimeter.
Why use "soft" balls to represent particles? In the past, in the theories of physicists, particles were often regarded as "rigid bodies". However, the photo retouching assumption of a rigid body not only fails to show that the particle body is compressed in theory; it is also easy to run into trouble experimentally. If hard particles are used for experiments, when they are closely arranged, it is easy to "stuck", not only completely unable to move, but also likely to damage precious instruments.
In order to allow the particles to be closely arranged without being completely stuck and making the instrument unable to move, the particle soft balls that can be squeezed and deformed have become the best experimental materials. The following video is for laboratory demonstration. In actual operation, the liquid will submerge the granular soft ball, keeping the refractive index of the liquid and the soft ball consistent, so that the mo