Particle tracking has been shown to be a powerful technique for measuring bulk and interfacial rheology of fluids.
The Brownian motion of microparticles trapped at interfaces is very sensitive to the viscosity of the subphase, and to
the contact angle of the particles. The Stokes-Einstein relation is fulfilled if the friction factor is properly taken into account.
The diffusion coefficient of the latex microparticles spread on surfactant monolayers allows one to calculate the
shear viscosity of the monolayer using Danov’s theory. Good agreement was found with previous results for monolayers
of pentadecanoic acid. The method has also been used to study monolayers of n-dodecanol. Moreover, the shear viscosity
of a polymer monolayer has been calculated by particle tracking, and the results show good agreement with data obtained
by canal viscosimetry. The temperature dependence of the shear viscosity shows the existence of a glass transition for
monolayers of poly(4-hydroxystyrene).