Calcite Crystals at the Air-Water Interface

CaCO3 Crystals: CaCO3 precipitates out of solution to form calcite crystals on the surface of water, some of which aggregate into a fractal mat, and others of which arrange themselves into a lattice. Under certain conditions the lattice crystals exhibit amazing behavior, aggregating with and eventually breaking apart the nearby fractal mat. The ability to control the behavior of these crystals would be very useful in water purification systems.

Each 2ml sample is composed of one ml of deionized water combined with one ml of aqueous CaO solution. Before preparing each sample, the solution is heated to a temperature of between 35 and 450C for approximately 5 minutes. Small crystals (~1 micron) can be observed after 15 minutes, and the fractal mat is usually complete after about an hour. Heating the solution seems to slow the completion of the fractal mat. The sample is imaged in one of two ways: with an inverted optical microscope in bright field, or with a CCD camera attached to a zoom lens.

In successful samples, a group of crystals form on the outside of the fractal mat. These crystals are triangular in shape (actually tetrahedral) and range in size from 10 to 30 microns across the longest edge. Instead of aggregating into the fractal mat, they arrange themselves on an almost crystalline lattice called a w-layer, named for its similarity to a 2-D protein structure known as the s-layer. The w-layer is stable for several hours, but the crystals eventually end up aggregating with the nearby fractal mat.

Here's some pictures of the calcite crystals. Image A shows a section of the fractal mat, B is a picture of the w-layer, and C is a close-up of one of the tetrahedral crystals. In image C, even though there is a separation between the larger particle and the smaller particles of its 'tail,' they are stuck together in this position. And you can see from the w-layer in B that the size of the lattice crystals affects the inter-particle separation. You can also see the nice ordering of the lattice crystals -- they are arranged on concentric rings centered around a point located about an inch to the right of the image. Six rings are clearly visible, but the pattern starts to break down by the seventh ring. There is also some hexagonal ordering, where crystals have 6 nearest neighbors.

The ordering seen in image B above appears to arise from a combination of attractive capillary and repulsive electrostatic forces. The crystal structure of the tetrahedral crystals suggests that highly charged faces are submerged in the water while neutral faces are exposed to the air. Scanning Electron Microscopy helped us to test this hypothesis. Click the image for a cool pic generated by a slow scan of the SEM.

Click for a cool video of some hexagonal crystals moving about each other. The movie has been sped up from real-time -- there's one frame every 5 seconds. The normal behavior of aggregating crystals is simply to stick to each other and remain in that orientation until they're broken apart by some sort of shaking motion. But in this case the hexagonal crystals continued to rearrange themselves for a number of minutes before finally settling down.

References:

• Tetrahedral calcite crystals facilitate self-assembly at the air-water interface
  S. M. Hashmi, H. H. Wickman, and D. A. Weitz, Physical Review E 72 041605 (2005).

• Colloid Crystal self-organization and dynamics at the air/water interface
  H. H. Wickman & J. N. Korley, Nature 393 445 (1998).

This web page is maintained by Sara Hashmi, currently a graduate student at Yale University. Her graduate research webpage may be found here.