Surface layering

Surface layering is a quasi-crystalline structure at the surfaces of otherwise disordered liquids, where atoms or molecules of even the simplest liquid are stratified into well-defined layers parallel to the surface. While in crystalline solids such atomic layers can extend periodically throughout the entire dimension of a crystal, surface layering decays rapidly away from the surface and is limited to just a few near-surface region layers. Another difference between surface layering and crystalline structure is that atoms or molecules of surface-layered liquids are not ordered in-plane, while in crystalline solids they are.{{cite book | last=Croxton | first=Clive A. | title=Liquid State Physics–A Statistical Mechanical Introduction | publisher=Cambridge University Press | location=Cambridge | year=1974 | isbn=978-0-511-75348-0 | doi=10.1017/cbo9780511753480 | page=}}

Surface layering was predicted theoretically by Stuart Rice at the University of Chicago in 1983 {{cite journal | last1=D’Evelyn | first1=Mark P. | last2=Rice | first2=Stuart A. | title=A study of the liquid–vapor interface of mercury: Computer simulation results | journal=The Journal of Chemical Physics | publisher=AIP Publishing | volume=78 | issue=8 | date=1983-04-15 | issn=0021-9606 | doi=10.1063/1.445376 | pages=5081–5095}} and has been experimentally discovered by Peter Pershan (Harvard) and his group, working in collaboration with Ben Ocko (Brookhaven) and Moshe Deutsch (Bar-Ilan) in 1995 in elemental liquid mercury{{cite journal | last1=Magnussen | first1=O. M. | last2=Ocko | first2=B. M. | last3=Regan | first3=M. J. | last4=Penanen | first4=K. | last5=Pershan | first5=P. S. | last6=Deutsch | first6=M. | title=X-Ray Reflectivity Measurements of Surface Layering in Liquid Mercury | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=74 | issue=22 | date=1995-05-29 | issn=0031-9007 | doi=10.1103/physrevlett.74.4444 | pages=4444–4447| pmid=10058508 | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:10357485 | url-access=subscription }} and liquid gallium{{cite journal | last1=Regan | first1=M. J. | last2=Kawamoto | first2=E. H. | last3=Lee | first3=S. | last4=Pershan | first4=P. S. | last5=Maskil | first5=N. | last6=Deutsch | first6=M. | last7=Magnussen | first7=O. M. | last8=Ocko | first8=B. M. | last9=Berman | first9=L. E. | title=Surface Layering in Liquid Gallium: An X-Ray Reflectivity Study | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=75 | issue=13 | date=1995-09-25 | issn=0031-9007 | doi=10.1103/physrevlett.75.2498 | pages=2498–2501| pmid=10059327 | url=https://dash.harvard.edu/bitstream/1/10357490/1/Regan_Surface_1995.pdf }} using x-ray reflectivity techniques.

More recently layering has been shown to arise from electronic properties of metallic liquids, rather than thermodynamic variables such as surface tension, since surfaces of low-surface tension metallic liquids such as liquid potassium are layered,{{cite journal | last1=Shpyrko | first1=Oleg | last2=Huber | first2=Patrick | last3=Grigoriev | first3=Alexei | last4=Pershan | first4=Peter | last5=Ocko | first5=Ben | last6=Tostmann | first6=Holger | last7=Deutsch | first7=Moshe | title=X-ray study of the liquid potassium surface: Structure and capillary wave excitations | journal=Physical Review B | publisher=American Physical Society (APS) | volume=67 | issue=11 | date=2003-03-14 | issn=0163-1829 | doi=10.1103/physrevb.67.115405| arxiv=cond-mat/0406585 | page=115405| s2cid=17613642 | url=http://liquids.deas.harvard.edu/peter/2003.pdf/k_paper.pdf }} while those of dielectric liquids such as water, are not.{{cite journal | last1=Shpyrko | first1=Oleg | last2=Fukuto | first2=Masafumi | last3=Pershan | first3=Peter | last4=Ocko | first4=Ben | last5=Kuzmenko | first5=Ivan | last6=Gog | first6=Thomas | last7=Deutsch | first7=Moshe | title=Surface layering of liquids: The role of surface tension | journal=Physical Review B | publisher=American Physical Society (APS) | volume=69 | issue=24 | date=2004-06-30 | issn=1098-0121 | doi=10.1103/physrevb.69.245423|arxiv=cond-mat/0406579 | page=245423| s2cid=53491473 | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:10354246 }}