Gas-liquid transition in a two-dimensional system of millimeter-sized like-charged metal balls

Metal balls with a diameter of 1.59 mm, gently rubbed against a dielectric surface using a shaker, are seen to spontaneously exhibit a two-dimensional liquidlike order with macroscopic dimensions, viz., interball distances of several millimeters. This liquidlike order transforms to a gaslike order t hrough coexistence upon decreasing the area fraction of the balls. The meas ured pair interaction of like-charged balls surprisingly exhibits a long-ra nge attractive term analogous to that in charged colloids.