Time-lapse video of dehydrating bentonite

Play the video below to watch a colloidal mixture of bentonite shrink as it dries. Bentonite is a type of clay, weathered from volcanic ash. It has a remarkable ability to absorb water, swelling to many times its dry volume.

Fig 2. Bentonite, near Cathedral Valley, Utah, USA
(the 'kernels' are about 2 cm in diameter)

The extreme swelling and shrinking is hard on plants because it tears apart their roots. Figure 2 shows a typical exposure of weathered bentonite (near Cathedral Valley in Utah, USA).

Production notes

Bentonite powder was prepared by lightly grinding a few 'kernels' using a mortar and pestle. The powder was sprinkled slowly into a tea cup containing water about about 0.5cm deep, distributing the powder as evenly as possible. When the bentonite powder grains strike the water, they swell and sink, gradually building up until the water is entirely absorbed. Powder was further applied until there was a faint, thin layer of dry powder. Then the surface was misted with water from a sprayer, to moisten these few surface grains. This preparation method seemed to result in lower density bentonite (ie., more water per bentonite) than simply stirring bentonite powder into water.

Fig 3. Camera, subject, and lighting

The time-lapse video was produced from a sequence of still images (2048 x 1526 pixels) taken over about 15 hours at 3 minute intervals by a tethered Canon A75 digital camera, using Canon's 'Remote Capture' software (part of Canon's 'ZoomBrowser'). The camera was powered by an AC adapter.

Incandescent lighting was used (a 15W bulb, to reduce heat). Custom white balance was set using a white sheet of paper. The camera was set to macro mode, medium zoom, 1/4 second, f/4.8. The photography was done in a dark basement so as to be unaffected by daily light cycles.

A small clock image was added to the bottom-left corner of each image using a Photoshop script (addClock_cs6.jsx) that rotates the clock hands according to time stamps of the image files. Other clock images and hands could be used by simply modifying the base clock image in clock.psd. The Photoshop script language documentation is in a sub-folder where Abode s/w is installed; the script development environment (editor and debugger), called Extended Script Toolkit 2 (part of Photoshop), is helpful.

The images were merged into a video using VirtualDub (1.10.4). Frame rate was set to 30, video compression to 'no compression'. The output is an uncompressed AVI file (considerably larger than the sum of the input images). Compression was done using HandBrake (0.10.2), using an MP4 container and H.264 encoding, at quality level 18.

Jan 2008