Sand pictures

Fine sand moves down the sand dial with a certain speed and not faster, not slower. The friction between the sand grains works on a similar principle to cohesion force in a water drop: the pyramid of sand at the bottom of the sand dial has a unique but specified height.

It is similar with sand: the sand pours for long minutes, and each time the picture is different, especially when the sand grains are of different colour. Such compositions form between two glass plates, with a thin layer of liquid between them. A layer of sand is trapped at the bottom and at the top there is a small layer of air. When you turn the frame, liquid finds its way to the bottom very quickly, and the sand is trapped over the layer of air. The surface friction in water makes it difficult for sand to move down. A canal has to be formed in the layer of air to let the sand pour slowly to the bottom.

A similar canal is formed in atmosphere and an electrical discharge travels through it during a storm.