OBJECTIVE: To observe crystal nucleation and growth rate during directional solidification. BACKGROUND: Directional solidification refers to a process by which a liquid is transformed (by freezing) into a solid through the application of a temperature gradient in which heat is removed from one direction. A container of liquid will turn to a solid in the direction the temperature is lowered. If this liquid has a solute present, typically, some of the solute will be rejected into the liquid ahead of the liquid/solid interface. However, this rejection does not always occur, and in some cases, the solute is incorporated into the solid. This phenomenon has many impor-tant consequences for the solid. As a result, solute rejection is studied extensively in solidification experiments.

Fluid flow in the melt can also affect the buildup of the mass boundary layer. On Earth, fluids that expand become less dense. This causes a vertical flow of liquid which will interfere with the mass boundary layer. In space, by avoiding this fluid flow, a more uniform mass boundary layer will be achieved. This, in turn, will improve the uniformity with which the solute is incorporated into the growing crystal.

PROCEDURE: Step 1. Place a small amount of mannite on a microscope slide and place the slide on a hot plate. Raise the temperature of the hot plate until the mannite melts. Caution: Be careful not to touch the hotplate or heated slide. Handle the slide with forceps. Step 2. After melting, cover the mannite with a cover glass and place the slide on a ceramic bread-and-butter plate that has been chilled in a refrigerator. Permit the liquid mannite to crystallize. Step 3. Observe the sample with a micro-projector. Note the size, shape, number, and boundaries of the crystals. Step 4. Prepare a second slide, but place it immediately on the microprojector stage. Permit the mannite to cool slowly. Again, observe the size, shape, and boundaries of the crystals. Mark and save the two slides for comparison using student microscopes. Forty power is sufficient for comparison. Have the students make sketches of the crystals on the two slides and label them by cooling rate.

MATERIALS NEEDED: Bismarck Brown Y** Mannite (d-Mannitol)    HOCH2(CHOH)4CH2OH* Salol (Phenyl Salicylate)    C13H10O3** MicroprojectorI Student microsopes (alternate to microprojector) Glass microscope slides with cover glass Ceramic bread and butter plate Refrigerator Hotplate Desktop coffee cup warmer Forceps Dissecting needle Spatula Eye protection Gloves Marker pen for writing on slides

Step 5. Repeat the procedure for mannite (steps 1-4) with the salol, but do not use glass cover slips. Use a desktop coffee cup warmer to melt the salol. It may be necessary to add a seed crystal to the liquid on each slide to start the crystallization. Use a spatula to carry the seed to the salol. If the seed melts, wait a moment and try again when the liquid is a bit cooler. (If the microprojector you use does not have heat filters, the heat from the lamp may remelt the salol before crystallization is completed.

The chemical thymol (C10H14O) may be substituted for the salol. Avoid breathing its vapors. Do not substitute thymol for salol if student microscopes are used.) Step 6. Prepare a new salol slide and place it on the microprojector stage. Drop a tiny seed crystal into the melt and observe the solid-liquid interface. Step 7. Remelt the salol on the slide and sprinkle a tiny amount of Bismarck Brown on the melt. Drop a seed crystal into the melt and observe the motion of the Bismarck Brown granules. The granules will make the movements of the liquid visible. Pay close attention to the granules near the growing edges and points of the salol crystals. How is the liquid moving?

NOTES ON CHEMICALS USED:

Bismarck Brown Y
Bismarck Brown is a stain used to dye bone specimens for microscope slides. Because Bismarck Brown is a stain, avoid getting it on your fingers. Bismarck Brown is water solulable.

Mannite (d-Mannitol)
HOCH2(CHOH)4CH2OH Mannite has a melting point of approximately 168° C. It may be harmful if inhaled or swallowed. Caution: Wear eye protection and gloves when handling this chemical. Conduct the experiment in a well ventilated area.

Salol (Phenyl Salicylate)
C13H10O3 It has a melting point of 43° C. It may irritate eyes. Caution: Wear eye protection.

QUESTIONS: 1. What happens to crystals when they begin growing from multiple nuclei? 2. Are there any differences in crystals that form from a melt that has cooled rapidly and from one that has cooled slowly? What are those differences? 3. What happens to the resulting crystals when impurities exist in the melt?

4. What caused the circulation patterns of the liquid around the growing crystal faces? Do you think these circulation patterns affect the atomic arrangements of the crystals? 5. How do you think the growth of the crystals would be affected by growing them in microgravity?

FOR FURTHER RESEARCH: 1. Design a crystal growing experiment that could be flown in space. The experiment should be self-contained and the only astronaut involvement that of turning on and off a switch. 2. Design a crystal growing experiment for space flight that requires astronaut observations and interpretations. 3. Research previous crystal growing experiments in space and some of the potential benefits researchers expect from space-grown crystals.