Fig. 1.

Extraction of tension by measurement of deformation due to a drop placed under a solid film. (A) Schematic of experimental setup for surface tension experiments. The droplet is placed on the underside of a circular thin film that is 8 mm in diameter, with thickness in the range of 9–40 μm. (B) Schematic drawing shows the balance of forces near the axis of symmetry and at the triple line. Laplace pressure is denoted by P. Note that deformation is mainly in the direction of P, whereas all previous measurements of deformation due to a drop show the formation of a cusp in the direction of surface tension, opposed to P. θ is the internal angle between the film surface and the liquid drop, and ϕ is the angle of the deflected shape. (C) Comparison of measured and theoretical surface profiles. There is virtually no difference in the calculated shape of the deformed membrane near the axis of symmetry with or without bending and gravity, consistent with the assumption that the membrane essentially supports only in-plane tension. (D) Profile of the top surface of an 11.0-μm thick membrane with a drop on the underside showing a bulge of tens of microns.