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NUCLEATION

 

NUCLEATION IS AN IMPORTANT STEP IN MANY FIELDS OF SCIENCE AND TECHNOLOGY, FROM PHYSICS TO MEDICINE, METALLURGY TO SNOWMAKING, AND CLOUDS TO BIOLOGY. WITH PROGRESS IN NANOTECHNOLOGY IT WILL BECOME EVER MORE IMPORTANT. IT IS ESSENTIALLY A SURFACE VS. MOLECULE SIZE COMPETITION. BY THIS I MEAN, IF YOU ARE TRYING TO MAKE A SINGLE LARGE CRYSTAL, IT IS PHSICALLY FAR EASIER TO START WITH A SEED CRYSTAL AND HELP IT GROW RATHER THAN WAIT FOR THE LIQUID MOLECULES TO ARRANGE THEMSELVES ONE BY ONE PROPERLY TO FORM A SMALL CRYSTAL AND GROW ON ITS OWN. THE DIFFERENCE IN SCALE BETWEEN MOLECULES AND SEED CRYSTALS IS HUGE.

IN THE JET ENGINE BUSINESS SOME YEARS AGO THEY CHANGED FROM BLADES MACHINED FROM SOLID METAL TO CAST METAL BLADES. THIS ALLOWED THEM TO USE BETTER ALLOYS AND MAKE THE PARTS MORE EFFICIENTLY. THE PROBLEM OF COURSE WAS THAT ORDINARY CASTINGS HAVE SIGNIFICANT FLAWS DUE TO UNEVEN CRYSTALLIZATION, VOIDS, ETC. THE SOLUTION WAS TO CAST THE BLADES AS "SINGLE CRYSTAL" CASTINGS.

TO DO THIS, YOU CAST THE BLADE LEAVING A SMALL CROOKED PASSAGEWAY LIKE AN APPENDIX THAT EXCESS MOLTEN METAL CAN FLOW INTO. AS THE MOLTEN METAL STARTS TO COOL IT REACHES A TEMPERATURE AT WHICH YOU CAN ADD A SMALL PIECE OF THE SOLID METAL WITHOUT MELTING THE PIECE. THIS NUCLEUS STARTS TO GROW AS MULTIPLE CRYSTALS UP THE CROOKED PATH. EACH TIME THE PATH KINKS, THERE ARE FEWER CRYSTALS GROWING. BY THE TIME THE SOLIDIFICATION REACHES THE ACTUAL CASTING CAVITY, THERE IS ONLY ONE CRYSTAL LEFT GROWING AND IT COMPLETELY CRYSTALLIZES THE BLADE AS A "SINGLE CRYSTAL". IF DONE CORRECTLY, THIS PRODUCES A NEARLY FLAWLESS CASTING BETTER THAN IF IT WERE MACHINED FROM SOLID METAL.

NUCLEATION IS ALSO WHAT CAUSES CARBONATED BEVERAGES TO ERUPT IF YOU REMOVE THE CAP SHORTLY AFTER SHAKING THE BOTTLE. IF YOU LOOK AT THE SMALL AMOUNT OF GAS UNDER THE CAP ABOVE THE LIQUID, THIS GAS BECOMES TINY BUBBLES IN THE LIQUID IF YOU SHAKE IT. IF YOU WAIT A FEW MINUTES THEY WILL RISE HARMLESSLY TO THE SURFACE AND REJOIN THE ONE BIG BUBBLE AT THE TOP. BUT IF YOU TAKE OFF THE CAP TOO SOON, ALL THOSE TINY BUBBLES BECOME NUCLEI GIVING THE CO2 A VERY GOOD PLACE TO ESCAPE THE LIQUID NOW THAT THE PRESSURE HAS BEEN RELEASED. YOU KNOW WHAT HAPPENS.

MANY LIQUIDS ARE RATHER RESISTANT TO ORGANIZING INTO THE NEARLY PERFECT ORDER OF A CRYSTAL. THIS MEANS THAT WHEREAS THEY MAY HAVE A VERY PRECISE MELTING POINT, THEIR "FREEZING" POINT MAY VARY SUBSTANTIALLY, PARTICULARLY IF THEY ARE VERY PURE. IF CAREFULLY PURIFIED, WATER CAN COOL TO - 40 FAHRENHEIT (ALSO - 40 CELSIUS) AND REMAIN A LIQUID. THIS IS CALLED "SUPERCOOLING" OR PERHAPS MORE ACCURATELY "SUBCOOLING".

IN SNOWMAKING, NUCLEATION IS ESSENTIAL TO MAKE SNOW EFFICIENTLY AT REASONABLE TEMPERATURES. NUCLEI ARE USED FROM THREE SOURCES: NATURAL PARTICULATES IN THE WATER, TINY ICE CRYSTALS FORMED BY EXPANDING COMPRESSED AIR, AND SYNTHETIC NUCLEATING AGENTS (WHICH ARE NEVER THE LESS OF NATURAL ORIGIN).

IT IS IMPORTANT TO NOTE THAT OF THE THREE TYPES OF NUCLEI, TWO ARE FLOATING INSIDE THE WATER AND ONE (FROM COMPRESSED AIR) IS A CRYSTAL THAT ATTACHES ITSELF TO THE OUTSIDE OF A LARGE DROPLET ONCE ITS SURFACE HAS COOLED TO 32 F (0 C).

DIFFERENT TYPES OF NUCLEI HAVE CHARACTERISTIC TEMPERATURES AT WHICH THEY INITIATE CRYSTALLIZATION. HIGH TEMPERATURE NUCLEI FOUND IN NATURE MIGHT INITIATE AT ABOUT 25 DEGREES F (ABOUT - 4 C). THESE ARE FROM BACTERIA THAT HAVE PROTEIN SEQUENCES THAT MATCH THE CRYSTALLINE ORDER OF WATER. THERE IS AN IMPRACTICAL (UNCOMMERCIALIZED) NUCLEATING AGENT THAT CAN INITIATE WATER CRYSTALLIZATION AT 31 F (-0.5 C).

THE NUCLEATION BACTERIA, PSEUDOMONAS SYRINGAE, WAS DISCOVERED AS THE CAUSE OF AN ABNORMALLY HIGH FREQUENCY OF HAIL IN A MOUNTAIN RANGE IN CANADA. A SCIENTIST FROM NOAA ACTUALLY WENT DIGGING IN SOME ROTTING LEAVES ON THE MOUNTAIN RANGE IN CANADA AND FOUND IT. HE REALIZED THAT WHEN THE WIND BLEW, IT LOFTED BACTERIA COATED LEAF PARTICLES INTO THE AIR, PRODUCING ICE NUCLEI THAT GREW INTO HAIL. THIS IS THE BACTERIA FROM WHICH SNOMAX (TM) IS DERIVED.

EVEN SUCH EXPENSIVE THINGS AS TESTOSTERONE AND CHOLESTEROL ARE GOOD NUCLEATING AGENTS FOR WATER, BUT OF COURSE THE HIGHEST TEMPERATURE NUCLEATION FOR WATER IS FROZEN WATER ITSELF, CAPABLE OF NUCLEATION AT ALMOST PRECISELY 32 F. (0 C.). THE REASON FOR THIS IS THAT AN ICE CRYSTAL HAS THE PRECISE PATTERN ONTO WHICH LIQUID WATER MOLECULES CAN ARRANGE WITH THE LEAST EXPENDITURE OF ENERGY. PLEASE NOTE I SAID "ONTO WHICH" AS THIS IS PRECISELY WHAT HAPPENS WITH CRYSTAL NUCLEATION. ALSO NOTE THAT I POINTED OUT ABOVE THAT ONLY ONE OF THE THREE TYPES OF NUCLEI ATTACHES ITSELF TO THE OUTSIDE OF THE LIQUID WATER DROPLET - THE TINY ICE CRYSTALS PRODUCED BY COMPRESSED AIR.

THE OTHER TYPES OF NUCLEI ARE SUBMERGED WITHIN THE WATER DROPLET. THEY CANNOT BEGIN TO BE OF USE UNTIL THEY REACH THE SURFACE OF THE DROPLET AND THE THICK FILM OF WATER EVAPORATES FORMING A MINIMAL LAYER THAT CAN BE REARRANGED ONTO THE NUCLEUS AS AN ICE CRYSTAL. THIS EXPLAINS WHY WE HAVE SEEN MINIMAL BENEFIT FROM COMMERCIAL NUCLEATING AGENTS IN HIGH HUMIDITY CONDITIONS AND OFTEN VERY WORTHWHILE BENEFITS IN LOW HUMIDITY CONDITIONS. NATURALLY IT COMES DOWN TO A COST VS. BENEFIT ANALYSIS. THE MOST EFFICIENT USE OF EXPENSIVE NUCLEATING AGENTS MAY BE, FOR EXAMPLE, TO NOT USE THEM AT NIGHT WHEN THE HUMIDITY TENDS TO BE HIGH, THEN START INJECTING NUCLEI AS THE SUN AND TEMPERATURE GO UP (AND THE HUMIDITY GOES DOWN). AS ALWAYS, EXPERIENCE MUST BE HAD WITH BOTH FEET ON THE GROUND TO DETERMINE WHAT WORKS BEST UNDER THE GIVEN CONDITIONS.

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