When water vapor cools in our atmosphere to below its normal freezing point, expect ice crystals and snow to form from supercooled water droplets. However, if the condensation occurs near or below the earth's surface or within a mass of fallen snow, ice crystals may form directly without an intermediate liquid state. This can occur easily during a cold, clear night as heat is radiated away through a cloudless sky. Depending on the specific conditions, crystals so formed are called hoarfrost or depth hoar. These crystals still exhibit an underlying hexagonal structure due to the crystal lattice of frozen water. That lattice, in turn, is generated due to the peculiar molecular structure of water. However, the crystal structures formed are modified by the conditions of condensation much more so than precipitated snow. They may appear layered in keeping with the waves of condensation of supercooled water vapor enveloping the frost crystals. The external stepped form will be seen in some of the images to follow. People sometimes confuse hoarfrost crystals with snow, but their form and formation are quite different.

To study the formation of ice crystals using not only water (light and heavy) but also organic liquids with freezing points within the range of affordable equipment, I designed and constructed a vapor phase condensation unit that would fit on a microscope stage. With the apparatus, I could record the formation of ice, or frozen crystal formation, in three dimensions from pure or mixed organic and/or inorganic vapors at moderately low temperatures. This recording could be done in real-time or in time-lapse formats.

The chamber is shown here on a standard stereo microscope. Please note the following: The cooled chamber is surrounded by a block of styrofoam for insulation except at the top where a warmer temperature is desirable. White LEDs are placed in the block to illuminate the growing crystals with a minimum of heat. Fiber optic illumination will work but heat then is more of a problem. The small chamber sits on a sandwich of refrigerated water-cooled Peltier stages and metal blocks to bring the temperature of the chamber's bottommost surface down to -30° C. Ideally, the microscope objectives used (for stereo or standard microscopes) should have an extended working distance. Barring that, low-power objectives coupled with high-power projection eyepieces will work. The microscope's video camera is directly connected to a video input card in a computer. Alternatively, a vcr or image processor can be interposed. For the highest image quality, standard film cameras should be used. Notice the rotating optical mount that allows tilting the Peltier stage around a horizontal axis for 3D photography.

The chamber is a plastic or glass cylinder 1" in diameter and 2" tall. A clear glass disk is used as a cover. The open base sits on a rectangular block of copper (seen above) thin enough not to overload the heat removal capacity of the Peltier devices. Sitting on the base within the cylinder, a small metal component is used to hold a tungsten wire that serves as the condensation surface for the forming ice. The wire is usually helical or spiral-wound to help prevent ice crystals from obscuring each other as they grow. Internally and near the top, a band of black, absorbent (felt) material is cemented. The band is used to hold the liquid that vaporizes and condenses below. The band can be split if two or more liquids are to interact only in the vapor state. For a better base seal with glass cylinders, when required, I have employed low-temperature melting alloys to form a fairly tight, thermally conductive plug. Sealing greases, if used, could react with vapors when organics are being studied.