Viscosity and entropy change in creep during water desorption for wood

Summary

The equation of viscosity during water desorption is derived by use of the viscosity equations by Eyring and Doolittle which represent the viscosity in a steady state of moisture. Their equations give the same formulation as is represented as follows:

$$\eta \left( t \right) = \eta _e /\left[ {1 + nF\left( t \right)} \right],$$

where η (t) is the viscosity during water desorption, η_e is the viscosity in a steady state of moisture, n ∝ 1/f; f is the free volume fraction, F(t) is the desorption rate. On the basis of this viscosity equation, the entropy change for the creep in a steady state of moisture and that in a water desorption process are calculated. The difference of the entropy between both the state after an extend time, t, is represented by the following equation:

$$\Delta {\text{S = }}--\sigma _0^2 {\text{KM}}\left( {\text{t}} \right)/{\text{T,}}$$

where ΔS is the difference of the entropy, σ₀ is stress, K is constant, M (t) is the moisture change, T is the absolute temperature. The entropy decrease in a water desorption process is more than that in a steady state of moisture. This equation suggests that molecules or flowing segments in wood substances rearrange more orderly during water desorption. This leads to the conclusion that the excess entropy decrease in a water desorption process is one of factors contributed to the deflection recovery in the subsequent water adsorption process for the mechano-sorptive creep.