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Shear rheology of two-dimensional solids
Many soft solids, such as concentrated suspensions, emulsions, foams, behave in a similar way under an applied shear : they exhibit a Maxwell-type relaxation with a characteristic relaxation time that varies inversely with the applied shear rate. When the storage and loss moduli are measured at different frequencies and constant shear rate, the curves obtained can be rescaled [1]. We will show that the behavior in two dimensions can be strikingly similar. We will present data on monolayers of nanoparticles and on mixed layers made with DNA and surfactant, deposited or adsorbed on water. The nonlinear behavior will be also discussed. Depending on the compaction degree, the layers can behave as brittle or plastic solids. We have observed a similar behavior with monolayers of nanoparticles or monolayers of special lipids such as ceramides.
Monolayers of polymers on water can also be solid (glassy) but melt when the temperature is increased. The glass transition temperature Tg of thin layers is in general different than the bulk value. Recent simulations [2] suggest that changes in Tg only occurs with extremely thin films, of thicknesses of a few nm, whereas most existing experimental data correspond to thicker layers. We have investigated the shear rheology of different polymer layers deposited on water. We found that significant Tg variations indeed occur below 2nm, suggesting that Tg decreases when segmental motions are affected by the presence of limiting bulk phases.
[1] H. M. Wyss, K. Miyazaki, J. Mattsson, Z. Hu, D. R. Reichman, D. A. Weitz, Strain-Rate Frequency Superposition : A Rheological Probe of Structural Relaxation in Soft Materials, Phys. Rev. Lett. 2007 98, 238303.
[2] A. V.Lyulin, D. Hudzinskyy, E. Janiaud, A.Chateauminois, Competition of time and spatial scales in polymer glassy dynamics : Rejuvenation and confinement effects. Journal of Non-Crystalline Solids 2011, 357, 567-574.