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Frigus
- 337
- 160
Why direction of surface tension is tangential to the surface and not perpendicular downwards since it is caused by molecules in the bulk?
Charles Link said:To answer your first question, it might be worthwhile to look at an analogous but simpler system=a rope that is wrapped around a cylinder. The tension ## T ## in the rope is along the tangential path, but it exerts a force on the cylinder that is perpendicular to the path, because of the curvature. The normal force on the cylinder per unit length is ## f_l=\frac{T}{r} ##, where ##r ## is the radius of the cylinder.
Charles Link said:The surface tension will create pressure ##P ## inside a spherical droplet. The book "Equilibrium Thermodynamics" by Adkins pp. 39-40 treats this particular case very well. Defining the surface tension ## \gamma=\frac{dW}{dA} ##, we can write ## dW=\gamma \, dA =8 \pi \gamma r \, dr##. We also have ## dW=P \, dV=P \, 4 \pi r^2 \, dr ##. Equating these two expressions for ## dW ## gives ## P=\frac{2 \gamma}{r} ##. ## \\ ## This expression for pressure ## P ## is similar to the expression of the force per unit length in the rope, where, in both cases, the radius ## r ## appears in the denominator.
Sir but how can I understand thatCharles Link said:Pressure is a force per unit area that acts normal to a surface. The ## W ## above is work or energy. Surface tension ## \gamma ## is defined as the work required to increase the area of the surface by one unit, thereby ## \gamma=\frac{dW}{dA} ##.
Sir but from these equations how can I understand that surface tension is tangetial.Charles Link said:Pressure is a force per unit area that acts normal to a surface. The ## W ## above is work or energy. Surface tension ## \gamma ## is defined as the work required to increase the area of the surface by one unit, thereby ## \gamma=\frac{dW}{dA} ##. ## \\ ## Meanwhile, the above area ## A=4 \pi r^2 ##, thereby ## dA=8 \pi r \, dr ##.
Surface tension is a physical phenomenon that occurs when the molecules of a liquid are more strongly attracted to each other than to the molecules of the surrounding air or other materials. This creates a force at the surface of the liquid, causing it to behave like a stretched membrane.
This is because the molecules at the surface of a liquid are only attracted to each other and not to the air or other materials below them. This creates a force that is parallel to the surface, resulting in a tangential force rather than a downward force.
Surface tension affects the behavior of liquids in various ways. It allows insects to walk on water, causes droplets to form instead of spreading out on a surface, and enables some objects to float on the surface of water.
Surface tension can be affected by temperature, pressure, and the presence of other substances. For example, surface tension decreases with increasing temperature and can be reduced by adding surfactants or other chemicals to the liquid.
Surface tension is typically measured in units of force per unit length, such as newtons per meter (N/m). It can be measured using a variety of methods, including the drop weight method, the capillary rise method, and the du Noüy ring method.