9. Capillary Forces

Question 1

What shape are liquid interfaces?

Answer 1

Curved

Question 2

Define capillary pressure

Answer 2

The pressure difference across a curved liquid interface which acts to try to reduce the area of the interface.

Question 3

Give the equation for capillary pressure

Answer 3

∆P = change in pressure
P1 = liquid 1 pressure
P2 = liquid 2 pressure
γ = surface energy/tension
R = principal radius of curvature

Question 4

Give one use of capillary pressure

Answer 4

The production of nanobubbles to treat tumours. They burst at very high pressure, destroying the tumours.

Question 5

Define capillary rise

Answer 5

The process of fluid being drawn up inside a capillary.

Question 6

Why does capillary rise occur?

Answer 6

When liquid is inside a capillary, it forms a curved liquid meniscus. This results in a pressure drop across the interface, causing the fluid to be drawn up inside the capillary.

Question 7

Give the pressure inequality required for capillary rise to occur

Answer 7

Question 8

Give the equation for the height that a liquid rises within a capillary

Answer 8

h = rising height
γ = surface energy
θ = contact angle
ρ = density
g = gravitational field strength
R = capillary radius