Surface tension and capillary rise – travelling microscope lab
PhysicsForces & DynamicsAges 15–16Virtual lab
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A virtual practical to measure surface tension γ with capillary tubes and a travelling microscope: read the two inner walls of a tube for its radius r, the bottom of the meniscus and the flat surface for the rise h, then compute γ = rhρg/(2cosθ). Two more screens show the excess pressure in drops and bubbles (Δp = 2γ/r, 4γ/r) and a steel needle floating on water; temperature and detergent lower γ.
Lesson: Surface tension and capillarity
What it shows
A liquid surface behaves like a stretched skin: along any line on it, surface tension γ pulls with a force of γ per metre. In a narrow glass tube the surface meets the wall at the contact angle θ, and the liquid rises (θ < 90°, water) or is pushed down (θ > 90°, mercury) until h = 2γcosθ/(ρgr). A curved surface also raises the pressure inside it by 2γ/r, or by 4γ/r for a soap film with two surfaces. Heating and detergents lower γ. The model ignores the volume of the meniscus and the buoyancy of the needle.
How to use
In Capillary rise (practical), move the microscope with Microscope – horizontal scale x and Microscope – vertical scale y, or tap the beaker picture. Align the cross-wires with each edge and press Take reading for the four steps; each tube gives a row in the table. Change Liquid, Temperature T and Detergent concentration c and compare. The other tabs explore drops, bubbles and a floating needle.
Parameters you can change
- Screen Capillary rise (practical), Drops and bubbles, Floating needle
- Liquid Water, Ethanol, Mercury
- Temperature 0–80 °C
- Detergent concentration 0–10 mmol/L
- Curved surface Liquid drop, Air bubble in liquid, Soap bubble
- Radius of the drop or bubble 0.1–50 mm
- Needle diameter 0.2–2 mm
Questions to explore
- Why does water rise higher in a narrower tube, while mercury is pushed down?
- How does adding detergent change the capillary rise, and why?
- Why is the excess pressure in a soap bubble twice that in a drop of the same radius?
