Mechanical Testing of Monolayers, 2. Substrate Decoupling in the Long-Wave Approximation

The long-wave approximation was used to analyze the coupling between the water subphase and monolayer film during the course of slow dynamic-mechanical deformation. The motion of tracer particles was used to examine experimentally particle velocity gradients over the length and width of the film. For sufficiently slow strain, experimental data and the theory support the contention that the gradient of the particle velocity is negligible over the film. These results imply that the coupling between the monolayer and the water subphase is negligible for the strain frequencies and amplitudes studied. The monolayer behaves mechanically as an autonomous viscoelastic sheet under these conditions. A condition for this behavior is that h/λ₀≪ N_ca where h is the depth of the subphase, λ₀the wavelength, and N_ca the capillary number.