How does surface tension influence the behavior of fluids in capillary tubes?
Added by Joseph S.
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It is caused by the cohesive forces between the molecules of the fluid, which tend to minimize the surface area of the fluid. Show more…
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Madhur L.
What happens inside a capillary tube when a liquid "wets" the tube? What happens when a liquid does not "wet" the tube?
The diagram at right shows that if the surface tension is high enough, it can hold up a column of liquid in a glass capillary. This happens because some water molecules adhere to the glass, and their neighboring water molecules then hold on to them via surface tension, like links in a chain. The column of liquid is in equilibrium, pulling on the glass with force T. According to Newton's Third Law, the glass pulls on the liquid with the same force. This force T acts at an angle ̘ called the wetting angle (see the figure at right). The surface tension ̳ is defined as the force per unit length acting along the liquid boundary. This can be written as ̳ = T/l, where l is the length of the boundary. Question 1: Using the equilibrium condition, set the vertical component of the tension force T equal to the force of gravity acting on the column of fluid to obtain the following equation. Be sure to show the steps in your derivation. ̳ = ̱ghr / 2 cos ̘ Where ̱ is the density of the fluid, g is the acceleration due to gravity, h is the height of the column of liquid, and r is the inner radius of the capillary tube. Hint: the length of the liquid boundary is the circumference of the capillary inner diameter.
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