Dynamics in Ultrathin Films: Particle Tracking Microrheology of Langmuir Monolayers

Particle tracking has been shown to be a powerful technique for measuring bulk and interfacial rheology of flu- ids. 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 ac- count. 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).