Figure 4.
(a) Paper–plastic bilayers are sensitive to humidity. When one extremity is put in contact with a water bath, water invades the texture by capillarity, inducing a change in the curvature (first three pictures in (a)). The drying process is homogeneous, making the curvature of the structure to be the same along the strip (last three pictures in (a)). The first three pictures were taken 0, 9 and 100 min after contact with the water bath. The last ones were taken 8, 12 and 50 min after the beginning of the drying process. (b) Curvature measured along the bilayer, as a function of the distance s to the water bath. s = 0 is the position of the bath. Circle, 0 min; square, 1 min; plus, 2 min; triangle, 5 min; dot, 10 min; star, 40 min; inverted triangle, 119 min. (c) Impregnated length zm of a strip as a function of the time after contact with a water bath. The wicking process saturates due to evaporation. The full line is a fit of the experimental data (filled circles) with equation (3.2). The dotted line corresponds to the Washburn law in the absence of evaporation. (d) Curvature variation of a bilayer as a function of the relative humidity of the atmosphere.