When a bubble slides underneath a horizontal surface, the ensuing surface tension has an effect on the angle produced between the air-water boundary and the solid support (see Fig. 8). The contact angle produced by the boundary is an outcome of buoyancy and surface tension forces. For a stationary case, the contact angle, which can easily be quantified, is usually a property of the two fluids (gas and liquid) as well as of the solid. Therefore, for a static challenge the contact angle can be used for the development of numerical models. If the 12 bubble is sliding, the contact angle varies with shape, angle and fluid properties. Due this dynamic nature, it is practically impossible to have an appropriate numerical model. In a numerical model, the dynamic contact angle between the solid and liquid phase is needed to be specified as a boundary condition, which is not known. This fact means that the predicted result from the model that uses a fixed contact angle (as a boundary condition) may have an effect on the outline and dynamics of the bubble in a manner that is not representative of the real physical occurrence. The model presumes that the entry of a small volume of air into the wall of an adjacent cell causes the interface to be in contact with the wall. At this point, the model employs the imposed contact angle to calculate the shape of the bubble. As a result, the model tries to induce a direction to the borderline. On the other hand, experimental observations show that a very thin layer of liquid isolates the bubble from the solid as the bubble slides along the surface.

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