Streams with moving contact lines: Complex dynamics due to contact-angle hysteresis

We have investigated experimentally the spatiotemporal behavior of a narrow water stream flowing between two parallel inclined Plexiglass plates bounded laterally by air. Complex broadband dynamics occurs as a result of the velocity dependence of the contact angle at the water-air-Plexiglass contact line, which introduces critical values into the pressure required for contact line motion over the Plexiglass plates. This mechanism causes a stick-slip type of motion similar to that appearing in other systems such as flows in granular materials, models of earthquake dynamics, and charge-density-wave motion in solids. We find that above a certain flow rate Qs, a stationary spatial modulation of the width of the stream appears. At higher flow rates, a hysteretic transition occurs in which the contact line begins to move, and the pattern slips down the plates with broadband dynamics. The statistical properties of the motion are presented.