Surface Tension measurement with Force Tensionmeter

Surface tension, γ

A molecule in the bulk of a liquid experiences

an equally attractive force in all

directions, but at the surface there is more

attractive force in the direction of the bulk

than away from it. This reduced attraction

at the liquid-gas boundary gives rise to the

measurable property of surface tension.

At the liquid-liquid phase boundary, it is

referred to as interfacial tension. Surface

tension can be measured with optical,

force, volumetric or bubble tensiometers,

while interfacial tension can be measured

with force and optical tensiometers.

Contact angle, θ

Surface tension alone does not explain

the shape of a drop along a solid. Interaction

between the liquid and solid can be

attractive or repulsive. When attracted

by the solid, the liquid spreads, forming

short drops with low contact angles. If

repelled, the drops are taller, rounder and

have high contact angles. The lotus effect

is a special case of water repelling, or

hydrophobic, behavior. The microstructuring

of a leaf makes the surface superhydrophic,

causing a rain droplet to roll

off by itself.

Surface free energy

Surface free energy is surface tension for

solids. Like in liquids, surface molecules in

solids have more attractive forces towards

the bulk than away from it. This leaves the

surface molecules inclined to attract neighbouring

substances, so the higher the

surface free energy the easier it is to wet a

substrate. In contrast to liquids, the surface

free energy of solids cannot be measured

with tensiometry but via contact angle

measurements with a range of liquids.

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