Nukleasi: Perbedaan antara revisi

'''Nukleasi''' adalah tahap pertama pembentukan [[Fase benda|fase termodinamika]] baru maupun struktur baru melalui {{Ill|swapemasangan|en|Self-assembly}} atau {{Ill|swapengorganisasian|en|Self-organization}}. Nukleasi biasanya didefinisikan sebagai proses yang menentukan durasi seorang pengamat harus menunggu sebelum fase baru atau struktur hasil swapengorganisasian muncul. Misalnya, jika sejumlah volume air didinginkan (pada tekanan atmosfer) di bawah 0&nbsp;° C, ia akan cenderung membeku menjadi es. Volume air yang didinginkan hanya beberapa derajat di bawah 0&nbsp;° C sering kali masih bebas dari es dalam waktu lama. Pada kondisi ini, nukleasi es berjalan lambat atau tidak terjadi sama sekali. Namun, pada suhu yang lebih rendah, kristal-kristal es muncul lebih cepat. Pada kondisi ini nukleasi es berlangsung cepat.<ref name="pruppacher_book">H. R. Pruppacher and J. D. Klett, ''Microphysics of Clouds and Precipitation'', Kluwer (1997).</ref><ref name="sear_review">{{cite journal |last=Sear |first=R.P. |title=Nucleation: theory and applications to protein solutions and colloidal suspensions |journal=Journal of Physics: Condensed Matter |year=2007 |volume=19 |page=033101 |doi=10.1088/0953-8984/19/3/033101 |url=http://personal.ph.surrey.ac.uk/~phs1rs/review.pdf |issue=3 |bibcode=2007JPCM...19c3101S |citeseerx=10.1.1.605.2550}}</ref> Nukleasi umumnya adalah cara [[transisi fase]] orde pertama dimulai, dan kemudian memulai proses pembentukan [[Fase benda|fase termodinamika]] baru. Sebaliknya, fase baru pada transisi fase secara berkesinambungan berlangsung segera.

Nukleasi seringkalisering kali ditemukan sangat peka terhadap ketakmurnian dalam sistem. Ketakmurnian ini mungkin terlalu kecil untuk dilihat dengan mata telanjang, tetapi tetap dapat mempengaruhi laju nukleasi. Oleh karena itu, seringkalisering kali penting untuk membedakan antara nukleasi heterogen dengan nukleasi homogen. Nukleasi heterogen terjadi di ''situs nukleasi'' pada permukaan sistem.<ref name=pruppacher_book/> Nukleasi homogen terjadi jauh dari permukaan.

For example, in the nucleation of ice from supercooled water droplets, purifying the water to remove all or almost all impurities results in water droplets that freeze below around - 35 C <ref name="pruppacher_book" /><ref name="sear_review14" /><ref name=":0" />, whereas water that contains impurities may freeze at - 5 C or warmer<ref name="pruppacher_book" />.

This observation that heterogeneous nucleation can occur when the rate of homogeneous nucleation is essentially zero, is often understood using [[classical nucleation theory]]. This predicts that the nucleation slows exponentially with the height of a [[Gibbs free energy|free energy]] [[Activation energy#Relationship with Gibbs energy|barrier]] &Delta;G*. This barrier comes from the free energy penalty of forming the surface of the growing nucleus. For homogeneous nucleation the nucleus is approximated by a sphere, but as we can see in the schematic of macroscopic droplets to the right, droplets on surfaces are not complete spheres and so the area of the interface between the droplet and the surrounding fluid is less than a sphere's <math> 4\pi r^2 </math>. This reduction in surface area of the nucleus reduces the height of the barrier to nucleation and so speeds nucleation up exponentially.<ref name="sear_review">{{cite journal |last=Sear |first=R.P. |title=Nucleation: theory and applications to protein solutions and colloidal suspensions |journal=Journal of Physics: Condensed Matter |year=2007 |volume=19 |page=033101 |doi=10.1088/0953-8984/19/3/033101 |url=http://personal.ph.surrey.ac.uk/~phs1rs/review.pdf |issue=3 |bibcode=2007JPCM...19c3101S |citeseerx=10.1.1.605.2550}}</ref>

* [[Clouds]] form when wet air cools (often [[Lapse rate|because the air rises]]) and many small water droplets nucleate from the supersaturated air.<ref name="pruppacher_book">H. R. Pruppacher and J. D. Klett, ''Microphysics of Clouds and Precipitation'', Kluwer (1997).</ref> The amount of water vapor that air can carry [[Relative humidity|decreases with lower temperatures]]. The excess vapor begins to nucleate and to form small water droplets which form a cloud. Nucleation of the droplets of liquid water is heterogeneous, occurring on particles referred to as [[cloud condensation nuclei]]. [[Cloud seeding]] is the process of adding artificial condensation nuclei to quicken the formation of clouds.