![Surface Tension
Surface tension, σs: the force resulting from molecular
attraction at liquid surface [N/m]
surface tension varies with temperature
Fs= σs L (check your book, pg.21)
Fs= surface tension force [N]
σs = surface tension [N/m]
L = length over which the surface tension acts [m]](https://image.slidesharecdn.com/fluidmechanics-fluidproperties-part4-250328142647-a5fb9a2a/85/Fluid-Mechanics-Fluid-Properties-Part-4-ppt-5-320.jpg)
Fluid Mechanics-Fluid Properties - Part 4.ppt
- 1. Fluid Mechanics INTRODUCTION-FLUID PROPERTIES-2 Mr.S.V.Kumbhar, Assistant Professor, Automation and Robotics, Sharad Institute of Technology, College of Engineering, Yadrav
- 2. Newtonian vs. Non-Newtonian Fluids Newtonian fluid: shear stress is proportional to shear strain – Slope of line is dynamic viscosity Shear thinning: ratio of shear stress to shear strain decreases as shear strain increases (toothpaste, catsup, paint, etc.) Shear thickening: viscosity increases with shear rate (glass particles in water, gypsum-water mixtures).
- 3. Surface tension What’s happening here? – Bug is walking on water Why is this possible? – It doesn’t weigh much – It’s spreading its weight out – The downward forces are less than the effects of surface tension
- 4. Surface Tension • A molecules in the interior of a liquid is under attractive force in all direction. • However, 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 • Higher forces of attraction at surface • Creates a “stretched membrane effect”
- 5. Surface Tension Surface tension, σs: the force resulting from molecular attraction at liquid surface [N/m] surface tension varies with temperature Fs= σs L (check your book, pg.21) Fs= surface tension force [N] σs = surface tension [N/m] L = length over which the surface tension acts [m]
- 6. Capillarity Rise and fall of liquid in a capillary tube is caused by surface tension. Capillarity depends on the relative magnitudes of the cohesion of the liquid to walls of the containing vessel. When the adhesive forces between liquid and solid are larger than the liquid's cohesive forces, the meniscus in a small diameter tube will tend to be concave If adhesive forces are smaller than cohesive forces the meniscus will tend to be convex, for example mercury in glass. water mercury concave convex
- 7. Differences between adhesive & Cohesive A distinction is usually made between an adhesive force, which acts to hold two separate bodies together (or to stick one body to another) and a cohesive force, which acts to hold together the like or unlike atoms, ions, or molecules of a single body.
- 8. Vapor Pressure Vapor pressure: the pressure at which a liquid will boil. Vapor pressure ↑ when temperature increases At atmospheric pressure, water at 100 °C will boil Water can boil at lower temperatures if the pressure is lower When vapor pressure > the liquid’s actual pressure
- 9. Thank you