Tham khảo tài liệu 'engineering materials 2e volume2 episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Case studies in phase transformations 91 This is an example of heterogeneous nucleation. The good matching between ice and silver iodide means that the interface between them has a low energy the contact angle is very small and the undercooling needed to nucleate ice decreases from 40 C to 4 C. In artificial rainmaking silver iodide in the form of a very fine powder of crystals is either dusted into the cloud from a plane flying above it or is shot into it with a rocket from below. The powder seeds ice crystals which grow and start to fall taking the silver iodide with them. But if the ice as it grows takes on snow-flake forms and the tips of the snow flakes break off as they fall then the process once started is self-catalysing each old generation of falling ice crystals leaves behind a new generation of tiny ice fragments to seed the next lot of crystals and so on. There are even better catalysts for ice nucleation than silver iodide. The most celebrated ice nucleating catalyst produced by the microorganism Pseudomonas syringae is capable of forming nuclei at undetectably small undercoolings. The organism is commonly found on plant leaves and in this situation it is a great nuisance the slightest frost can cause the leaves to freeze and die. A mutant of the organism has been produced which lacks the ability to nucleate ice the so-called ice-minus mutant . American bio-engineers have proposed that the ice-minus organism should be released into the wild in the hope that it will displace the natural organism and solve the frost-damage problem but environmentalists have threatened law suits if this goes ahead. Interestingly ice nucleation in organisms is not always a bad thing. Take the example of the alpine plant Lobelia teleki which grows on the slopes of Mount Kenya. The ambient temperature fluctuates daily over the range -10 C to 10 C and subjects the plant to considerable physiological stress. It has developed a cunning response to cope with these temperature .
