92168.pdf

Liquid-gas and liquid-liquid interfaces 101

Surface r/ieo/ogy^58 '^157

Surface viscosity is the change in the viscosity of the surface layer

brought about by the monomolecular film. Monolayers in different

physical states can readily be distinguished by surface viscosity

measurements,

A qualitative idea of the surface viscosity is given by noting the

ease with which talc can be blown about the surface. Most insoluble

films have surface viscosities of c. 10~^6 kg s~l to 10~^3 kg s"^1 (for films

1(T^9 m thick this is equivalent to a bulk viscosity range of c. 103 kg

m-1-1to 10 kg m s ). These films can be studied by means of a

damped oscillation method (Figure 4.20). For a vane of length / and a

disc with a moment of inertia /,

(4.34)

where A = Iog 10 of the ratio of successive amplitudes of damped

oscillation, t is the period of oscillation and the subscripts V refer to a

clean surface.

Platinum vane,

plate, ring, etc.

U

Torsion

wire

Inertia

disc

Figure 4.20 Damped oscillation method for measuring surface viscosities

Many insoluble films, particularly those containing protein, exhibit

viscoelastic behaviour (see Chapter 9). A surface rheometer has been