Sessile Drop or Bubble Method

The cases of the sessile drop and bubble are symmetrical, as illustrated in Fig. n-16. The profile is also that of a meniscus 0 is now positive and, as an 

The usual experimental situation is that of a sessile drop and, as with the pendant drop, it is necessary to determine a shape parameter and some absolute length. Thus /3 may be determined by profile fitting, and Ze measured, where Ze is the distance from the plane at = 90 to the apex. If the drop rests with 

The case of very large drops or bubbles is easy because only one radius of curvature (that in the plane of the drawings) is considered. Equation 11-12 then becomes 

Since h denotes the distance from the apex to the equatorial plane, then aty - h, p , and Eq. 11-33 becomes 

This very simple result is independent of the value of the contact angle because the configuration involved is only that between the equatorial plane and the apex. 

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The static methods are based on studies of stable equilibrium spontaneously reached by the system. These techniques yield truly equilibrium values of the surface tension, essential for the investigation of properties of solutions. Examples of the static methods include the capillary rise method, the pendant and sessile drop (or bubble) methods, the spinning (rotating) drop method, and the Wilhelmy plate method. 

Methods based on the shape of static drops or bubbles include the pendant drop method and the sessile drop or bubble method. The general procedure is to make certain measurements of the dimensions or profile. It is accurate to a few tenths of a percent. 

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