Surface Tension, Contact Angle, and Wetting
Surface tension is tangential force per unit length, equivalently reversible surface free energy per unit area for a liquid interface; contact angle is measured through the liquid at equilibrium.
Why this shows up in the exam
Wetting, coating, and detergency · Needles or light objects supported by interfaces · Contact-angle comparison of surface treatments
Learn the idea
Surface tension acts along a liquid interface, while contact angle records the balance of interfacial preferences at a solid. Surface molecules have fewer neighbours and make the interface resist enlargement. A liquid climbs or depresses near a wall according to whether contact with the solid is energetically favoured.
🧠 Memory hook: Surface tension pulls along the interface; contact angle is measured through the liquid.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- T = F/l — surface-tension force per contacted length for one interface; count multiple interfaces explicitly
- dW = T dA — reversible work to create interfacial area dA at fixed temperature and composition
- gamma_SV = gamma_SL + gamma_LV cos(theta) — Young contact-angle relation for an ideal smooth chemically uniform solid surface
How to approach it
- 1Draw a tangent force at every contact line
- 2Count the number of interfaces
- 3Resolve forces before inserting T and contact angle
Common slip-ups that cost marks
- •Drawing surface-tension force normal to the interface
- •Forgetting two surfaces in a soap film
- •Assuming soap increases water's surface tension
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A wire 1 m long and cross-sectional area 2 mm^2 extends by 1 mm under a 200 N load. Find Young modulus.
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