Disks of PS in 1 mm thickness and 25 mm diameter are prepared by pressing between two metal plates on a Carver laboratory press at 180 C. The PA-6 fibers are obtained by drawing from molten pellets at 230 C obtaining thread diameters ranging from 100 to 300 µm. The fibers are cut to 20 mm length and annealed at 80 C for about 24 h in a vacuum. An Optical Shearing System (model Linkam CSS 450) connected to a videocassette recorder and to a Zeiss transmission optical microscope is used. This device enables the sample to be sheared and heated simultaneously with various gap widths under microscopic observation.
The fiber of PA-6 is placed between two films of PS. This sandwiched sample is placed in the shearing device under the microscope. At first, the temperature is elevated and maintained at 200 C for 10 min in order to ensure imbedding without any undesired deformation of the PA-6 fiber (Tm = 216 C). The gap between the two glass walls is then adjusted very slowly to the desired size range (from 0.4 to 2.5 mm). We have found that the nylon thread remains centered between the two walls. First, the initial thicknesses of the two PS plates are the same, and second, the shear forces during the compression act to keep the thread centered. The position of the thread was confirmed several times by changing the focus of microscope from the bottom wall to the thread and then to the top wall. The temperature is then increased to 230 C. To perform observations, images from the microscope are recorded onto S-VHS videotape. The observed images of the capillary instability phenomenon are in the directions parallel to the glass wall (x-z plane, top view in Figure 1).