The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Marangoni Driven Dewetting of Thin Liquid Layers in a Cylinder

by Haichao Wu

Abstract

We investigated the dynamics of thin films driven by Marangoni stress on the inner surface of a glass cylinder. The surface of the cylinder is rigid and stationary. The axis of the cylinder is horizontal. The inner wall of the cylinder was coated with a very thin layer of 1 wt% aqueous poly(acrylamide) solution (PA). Surfactant drops were then placed on the PA either on the bottom or top inner surface of the cylinder, and their spreading was monitored. As the drops spread on the PA, the radius of the surfactant front, the radius of the contact line, and spreading area were recorded. Different radii of cylinders were also tested. We found that gravity plays an important role in the spreading process. For the surfactant front, gravity increases the spreading velocity down the sides of the wall (arc direction) and decreases the velocity along the horizontal direction (axis direction). Gravity also increases the total spreading area of the contact line when the surfactant drop is added at the top of the cylinder as compared to a drop added at the bottom. However, gravity does not influence the spreading area of the surfactant front. Therefore, we believe that total spreading area of the surfactant front may be determined by surfactant inventory alone.

We also compared the spreading of a surfactant drop placed on a flat plate with spreading of a drop placed at the bottom of a horizontal cylinder. In this case, the spreading area of the contact line and the surfactant front are the same, however, the spreading pattern of the surfactant front changes significantly. On a flat plate, the drop spreads in a circle, however, for a small radius cylinder, the spreading in the axis direction moves much more quickly than in the arc direction.

With the observation of spreading on a flat plate and in a cylinder, we conclude that the spreading mainly has three features: the contact line, the surfactant front, and a ridge of accumulated liquid around the contact line. We believe that this third feature is the liquid dewetting from the contact line area and that it is composed primarily of solvent from the surfactant drop.

We also explored the influence of solvent inventory and surfactant inventory on spreading. We found that increased the surfactant inventory will increase the spreading area of the contact line and the surfactant front. However, increased solvent inventory does not have influence on the spreading area of the contact line or the surfactant front.