THE SURFACE TENSON OF PURE SUBSTANCES Welcome 对纯物质的表面张力欢迎.pptVIP

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THE SURFACE TENSON OF PURE SUBSTANCES Welcome 对纯物质的表面张力欢迎.ppt

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THE SURFACE TENSON OF PURE SUBSTANCES Welcome 对纯物质的表面张力欢迎

THE SURFACE TENSION OF PURE SUBSTANCES INTRODUCTION Introduction Surface tension ? is the contractile force which always exists in the boundary between two phases at equilibrium Its actually the analysis of the physical phenomena involving surface tension which interests us Our topics primarily concern on Surface tension as a force Surface tension as surface free energy Surface tension and the shape of mobile interfaces Surface tension and capillarity Surface tension and intermolecular forces Surface Tension As A Force: The Wilhelmy Plate The surface of a liquid appears to be stretched by the liquid it encloses Example of this are: the beading of water drops on certain surfaces; the climbing of most liquids in glass capillaries The force acts on the surface and operates perpendicular and inward from the boundaries of the surface, tending to decrease the area of the surface Noted Equation above defines the units of surface tension to be those of force per length or dynes per centimeter in the cgs system The apparatus shown resembles a two-dimensional cylinder/piston arrangement, so its analogous to a two dimensional pressure A gas in the frictionless, three-dimensional equivalent to the apparatus of the figure would tend to expand spontaneously. For a film however the direction of spontaneous change is contraction A quantity that is closely related to surface tension is the contact angle ?, defined as the angle (measured in the liquid) that is formed at the junction of three phases, as shown in figure 6.1b Although the surface tension is a property of two phases which form the interface, ? requires that three phases be specified for its characterization The Wilhelmy Plate Figure 6.2 The Wilhelmy plate method for measuring ?. In (a) the base of the plate does not extend below the horizontal liquid surface. In (b) the plate is partially submerged to buoyancy must be considered Figure 6.2 represent a thin vertical plate suspended at a liquid surface from the arm of t