A Simple Method to Mix Granular Materials

We show that a mixture of two species of granular particles with equal sizes but differing densities can be either segregated or mixed by adjusting the granular temperature gradient and the magnitude of the gravitational force. In the absence of gravity, the dense, heavy particles move to the colder regions. If the temperature gradient is put into a gravitational field with the colder regions above the hotter, a uniform mixture of light and heavy particles can be attained. This situation can be realized in a container of finite height with a vibrating bottom, placed in a gravitational field. We present a relation between the height of the container, the particle properties, and the strength of gravity required to minimize segregation.