Measured Elastic Constants of a Surface Gel Phase from a Dilute Aqueous Solution of Methylcellulose

Measurements of the time dependence of the surface tension of four dilute solutions (6 × 10^-4, 9 × 10^-4, 12 × 10^-4 and 24 × 10^-4 wt %) of methylcellulose disclose a strong adsorption of the polymer into the surface. Concomitantly with the drop in surface tension, there is an increase in surface elasticity as shown by measurements on three of the solutions. The response of each surface to a shearing stress was modeled as a spring and dash pot in series (Maxwell liquid) for the 9 × 10^-4 wt % solution and as a spring and dash pot in parallel (Kelvin solid) for the 12 and 24 × 10^-4 wt % solutions. The surface of the 24 × 10^-4 wt % solution sustained a shearing stress for 3600 s with almost no relaxation, from which the static shear modulus, µ0, was calculated to be 9.5 dyn/cm. The observed phenomona are interpreted as arising from the formation of a two-dimensional gel phase. The surface of a second 24 × 10^-4 wt % solution was subjected to a small compressional strain in a horizontal trough fitted with a Langmuir balance. The resulting surface stress was sustained for 1 h with no observable relaxation. The directly measured stress divided by the strain gives the unilateral compressional modulus, K’ = (x + 2µ) = 29.5 dyn/cm, where × and µ are the Lame constants. On combining the shear and compressional measurements, we obtain × = 10.5 dyn/cm; static shear modulus, µ_o,= µ = 9.5 dyn/cm; compressional modulus, K, = x + µ = 20 dyn/cm; Young’s modulus, E_t= 4µK/K’ = 25.8 dyn/cm; and Poisson ratio, σ, = λ/K^- = 0.36.