1. Surface Tension Clarification The magnitude of the force due to surface tension σ is always. The direction of this force is always parallel to the surface of the liquid-gas interface. –This direction is analogous to something you might have encountered before: tension in a two-force member When a liquid drop contacts a solid surface, the drop will deform; surface tension will act at the line of interface between the liquid surface, solid surface, and gas. –The magnitude of the force is still –The direction of the force between the fluid/gas interface is still parallel to the liquid surface, –at the solid it acts perpendicular to the contact line: the direction at the contact line is expressed by the contact angle, θ c –Note: the component of force parallel to the solid surface is F s = σL The red arrow indicates the force of the solid on the liquid surface due to F s. F s = σL The blue arrow indicates the force of the liquid surface on the solid surface due to surface tension F s

2. Two class examples Example 1. Drop on glass Magnitude of force on the slide on the bubble and vice-versa: The horizontal (outward) “stretching force” we calculated in class should then be written: …or, for a drop with contact circle of radius r, this is a horizontal “compressing” or “stretching” force inward or outward on the slide (by the drop) of magnitude (note: whether or not the surface tension from the drop will pull outward away from the drop center – and upward – on the slide, or pull inward – and upward – on the slide, will depend on the sign of cosine, i.e., whether θ c is greater than or less than 90°). Which is it for Hg? for H 2 O?

3. Two class examples Example 2. Capillary action θcθc 2r