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Temperature Gradient Stage: Objective.
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Temperature Gradient Stage: Objective To test the relationship between ice front curvature and the melting-point depression using ice growth in fine glass capillaries across a temperature gradient. Capillaries are placed in a groove of a copper block and a glass slide is then placed over to provide insulation from the air. One end of the copper block is in contact with a solution of water, solute (optional), and ice that is constantly mixed. The other end is connected to an alcohol bath by drilling a cylindrical hole in the copper block and held at a programmed lower temperature. The capillary is filled by dipping the bent end into the solution. Once the capillary is in thermal equilibrium with the block, ice growth is seeded. The equilibrium interface locations are measured with a dissecting microscope using cold light for illumination.
Temperature Gradient Stage: 1st and 2nd Versions Both the first and second versions of gradient stage follows the schematic apparatus to a near exact except that an additional pump is used to circulate the iced water through hydra connectors to maintain a higher temperature. The middle section of the stage in the second version is reduced dramatically since we realized that too much connection between the higher temperature and the lower temperature would result in a difficulty to obtain a controlled temperature gradient.
Temperature Gradient Stage: 3rd Version In the third version, the higher and lower temperature blocks are completely separated in order to achieve the best gradient control and are enclosed in an acrylic box for heat insulation. A specially designed window on the top enables us to conveniently change between a sapphire or glass slide. The sapphire slide offers a wider view, a lower temperature gradient, and more importantly, an exact linear profile between the two blocks, due to its higher thermal conductivity (27 times of a glass slide).
Temperature Gradient Stage: 4th Version This is the electrical version of the stage. It extends an ordinary uniform temperature stage to one with a controlled temperature gradient by varying the resistance of the coating (heating) layer. A certain pattern of the coating layer will lead to a linear temperature profile. Theoretically, the view window for this stage can be as large as one wants. However, the temperature profile is unstable since it is affected by the varying flux of the cooling gas.
Temperature Gradient Stage: 5th Version The Thermoelectric (TE) stage uses the Peltier effect. It has the advantage of a compact structure (an alcohol bath is not required) and increased precision of temperature control (0.1K). Two independent loops are used for controlling both the higher and lower temperatures. This was done via configuration of a conventional dual loop temperature control unit as well as a specific temperature profile software. By using a sapphire slide, the TE stage can offer a temperature gradient as low as 0.0001K/µm. The maximum cooling rate is 300K/min and the working range is between –20C and 20C, if water is used as the coolant.
Temperature Gradient Stage: Ice Formation in Capillary Shown here is an image of an ice front created in liquid potassium permanganate solution within a 20 micron glass capillary tube. Various pictures of the same front were taken and summarized and it seems that a catenary curve is able to give a perfect fitting, which is still not well understood.
