Two polystyrene beads are suspended in water inside a sealed
PHYSICS · Subjective · Fermi University Physics · Biophysics & Medical Physics · Physical foundations of the living cell · Physical foundations of the living cell
Life at Thermal Energy, Ch Random walks, diffusion and the crowded cell
Problem
Two 1μm polystyrene beads are suspended in water inside a sealed, level glass cell with a cover slip. Under the microscope the two beads, only a few micrometres apart, are seen to jitter randomly for hours, each moving independently of the other: at any instant one may move left while the other moves right, up while the other moves down, and so on. A student proposes three possible causes of the jitter: (i) convection currents in the fluid, (ii) evaporation of water at an exposed surface, (iii) mechanical vibration of the microscope stage. For each proposed cause, state the one feature of the observation described above that rules it out, and explain briefly why.