l10 - CIVE 2700 Civil Engineering Materials, Winter 2007 1...

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Unformatted text preview: CIVE 2700 Civil Engineering Materials, Winter 2007 1 Surface Properties Professor George Hadjisophocleous, Ph.D., P.Eng. Carleton University Department of Civil and Environmental Engineering Ottawa, Ontario CIVE 2700 Civil Engineering materials, Winter 2007, 9.2 Outline Surface tension Wetting Capillary effects Cohesion and adhesion Surfactants Colloids CIVE 2700 Civil Engineering Materials, Winter 2007 2 CIVE 2700 Civil Engineering materials, Winter 2007, 9.3 Surface Tension Atoms at the surface have larger spacing than atoms in the interior of the liquid. CIVE 2700 Civil Engineering materials, Winter 2007, 9.4 Surface Tension . bulk surface s U U ! " # : is energy in difference The This surface energy is viewed as surface tension. CIVE 2700 Civil Engineering Materials, Winter 2007 2 CIVE 2700 Civil Engineering materials, Winter 2007, 9.3 Surface Tension Atoms at the surface have larger spacing than atoms in the interior of the liquid. CIVE 2700 Civil Engineering materials, Winter 2007, 9.4 Surface Tension . bulk surface s U U ! " # : is energy in difference The This surface energy is viewed as surface tension. CIVE 2700 Civil Engineering Materials, Winter 2007 3 CIVE 2700 Civil Engineering materials, Winter 2007, 9.5 Surface Tension In terms of forces, a molecule in the interior of a liquid is under attractive forces in all directions and the vector sum of these forces is zero. A molecule at the surface of a liquid is acted on by a net inward cohesive force that is perpendicular to the surface. Hence it requires work to move molecules to the surface against this opposing force, and surface molecules have more energy than interior ones. CIVE 2700 Civil Engineering materials, Winter 2007, 9.6 Surface Tension The surface tension of a liquid is the work that must be done to bring enough molecules from inside the liquid to the surface to form one new unit area of that surface (J/m 2 = N/m). CIVE 2700 Civil Engineering Materials, Winter 2007 4 CIVE 2700 Civil Engineering materials, Winter 2007, 9.7 Surface Tension Historically surface tensions have been reported in handbooks in dynes per centimetre (1 dyn/cm = 0.001 N/m). Because of surface tension, the liquid behaves like a stretched elastic membrane. There is a natural tendency for liquids to minimize their surface area. For this reason, drops of liquid tend to take a spherical shape in order to minimize surface area. CIVE 2700 Civil Engineering materials, Winter 2007, 9.8 SURFACE TENSION For a small droplet, surface tension will cause an increase of internal pressure p in order to balance the surface force. CIVE 2700 Civil Engineering Materials, Winter 2007 5 CIVE 2700 Civil Engineering materials, Winter 2007, 9.9 Surface Tension The amount of pressure by which the pressure inside a liquid droplet of radius r, exceeds the pressure of the surrounding vapour/air is found...
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This note was uploaded on 11/13/2011 for the course CIVE 2*** taught by Professor - during the Spring '11 term at Carleton CA.

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l10 - CIVE 2700 Civil Engineering Materials, Winter 2007 1...

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