FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.1 Time lapse photography

The movie films in this presentation are an acceleration of the actual deformation that is recorded over periods of up to 5 days. The length of an experiment is indicated in the caption of individual movies and a scaling factor can be obtained from the time code superimposed on the movie.

The apparatus (Fig. 2.1.1) used in this presentation permits the in-situ recording of the changes in an ice layer during either progressive simple shear (Fig. 2.1.1) or pure shear (Fig. 2.1.2). A rectangular section of ice 25 x 35 mm and 0.8 mm thick has its lower and upper surfaces confined between two fixed glass plates (Fig. 2.1.1a). The two long edges are constrained to remain parallel by a fixed guide along one, and a sliding plate along the other (Fig. 2.1.1b); the travel of the latter is also constrained by a fixed guide. Displacement is applied laterally to one end by a row of ten moveable platelets pushed by a swinging arm which pivots at one end so as to sweep an arc of ~50° (Fig. 2.1.1b).

Figure 2.1.1: In-situ simple shear deformation apparatus. (a) Section showing location of sample with respect to box containing coolant and the microscope objective lens and light source. (b) Plan view of the apparatus at the start of an experiment. At the pivot X the drive moves the swinging arm in a dextral sense.

This arm is driven by a motor which acts through a gearbox and a micrometer. A similar set of platelets at the other end ensures a bulk simple shear of the ice with shear-strain ranging up to ~1. The final shape approximates a parallelogram with a step development in the two ends being due to angular terminations of the sliding platelets. While the drive operates at a constant speed and the load is applied parallel to the shear direction.

If a pure shear deformation is applied then two rigid plates at either end of the sample replace the moveable plates (Fig. 2.1.2). One plate is fixed and the other plate is driven against the sample through the same motor gearbox and micrometer system.

Figure 2.1.2: Plan view of in-situ pure shear deformation apparatus.

The apparatus is enclosed by a sealed box (Fig. 2.1.1a) through which refrigerated silicone oil is pumped to produce a temperature stability of ±0.2°C. The box is then mounted on a Zeiss Invertoscope and microstructural data are recorded from selected areas between crossed polarized light, using a time-lapse system.

The laboratory-made ice used in this investigation was prepared by refreezing a mixture of sieved crushed ice and distilled water [see technical description in Wilson & Russell-Head (1982)]. The samples initially consisted of equant and randomly oriented polygonal grains (mean grain size 2-5 mm) with small (0.1-0.2 mm) spherical air bubbles. The 'artificial' ice was therefore statistically isotropic and measurements showed that it did not display a preferred crystallographic orientation.