{{Short description|Aspect of fluid dynamics}} The '''exclusion zone''' is a large stratum (typically on the order of a few microns to a millimeter) observed in pure liquid water, from which particles of other materials in suspension are repelled. It is observed next to the surface of solid materials, e.g. the walls of the container in which the liquid water is held, or solid specimens immersed in it, and also at the water/air interface. Several independent research groups have reported observations of the exclusion zone next to hydrophilic surfaces.<ref name="Zheng2006"> {{cite journal |first1 = Jian-ming |last1 = Zheng |first2 = Wei-Chun |last2 = Chin |first3 = Eugene |last3 = Khijniak |first4 = Gerald H. |last4 = Pollack |journal = Advances in Colloid and Interface Science |title = Surfaces and interfacial water: Evidence that hydrophilic surfaces have long-range impact |date = 2006 |pages = 19–27 |volume = 127 |issue = 1 |doi = 10.1016/j.cis.2006.07.002 |pmid = 16952332 |bibcode = 2006AdCIS.127...19Z }} </ref><ref name="Chen2011"> {{cite journal |first1 = Chi-Shuo |last1 = Chen |first2 = Wei-Ju |last2 = Chung |first3 = Ian C. |last3 = Hsu |first4 = Chien-Ming |last4 = Wu |first5 = Wei-Chun |last5 = Chin |journal = Journal of Biological Physics |title = Force field measurements within the exclusion zone of water |date = 2011 |number = 1 |pages = 113–120 |volume = 38 |doi = 10.1007/s10867-011-9237-5 |pmid = 23277674 |pmc = 3285724 }}</ref><ref name="Bunkin2013"> {{cite journal |first1 = Marco |last1 = Bischof |first2 = Emilio |last2 = Del Giudice |journal = Molecular Biology International |title = Communication and the Emergence and of Collective and Behavior in and Living Organisms: A Quantum and Approach |date = 2013 |volume = 2013 |article-number = 987549 |doi = 10.1155/2013/987549 |pmid = 24288611 |pmc = 3833029 |doi-access = free }} </ref><ref name="Elton2020"> {{cite journal |first1 = Daniel C. |last1 = Elton |first2 = Peter D. |last2 = Spencer |first3 = James D. |last3 = Riches |first4 = Elizabeth D. |last4 = Williams |journal = International Journal of Molecular Sciences |title = Exclusion Zone Phenomena in Water - A Critical Review of Experimental Findings and Theories |number = 14 |page = 5041 |volume = 21 |date = 2020-07-17 |doi = 10.3390/ijms21145041 |pmid = 32708867 |pmc = 7404113 |doi-access = free }} </ref> Some research groups have reported the observation of the exclusion zone next to metal surfaces.<ref name="Pedroza2015">{{cite journal |first1 = Luana S. |last1 = Pedroza |first2 = Adrien |last2 = Poissier |first3 = M.-V. |last3 = Fernández-Serra |journal = The Journal of Chemical Physics |title = Local order of liquid water at metallic electrode surfaces |date = 2015 |number = 3 |page = 034706 |volume = 142 |doi = 10.1063/1.4905493 |pmid = 25612724 |bibcode = 2015JChPh.142c4706P |osti = 1228123 }}</ref><ref name="Chai2012"> {{cite journal | first1 = B | last1 = Chai | first2 = AG | last2 = Mahtani | first3 = GH | last3 = Pollack | journal = Contemporary Materials | title = Unexpected presence of solute-free zones at metal-water interfaces | date = 2012 | number = 1 | pages = 1–12 | volume = 3 | doi = 10.7251/COM1201001C | pmid = 23807904 | pmc = 3692373 }} </ref> The exclusion zone has been observed using different techniques, e.g. birefringence, neutron radiography, nuclear magnetic resonance, and others,<ref name="Elton2020" /> and it has potentially high importance in biology, and in engineering applications such as filtration and microfluidics.

==Historical background== The first observations of a different behavior of water molecules, close to the walls of its container, date back to late 1960s and early 1970s, when Walter Drost-Hansen, upon reviewing many experimental articles, came to the conclusion that interfacial water (sometimes described as "vicinal water") shows structural difference.<ref name="DrostHansen69"> {{cite journal | author = Drost-Hansen, Walter | journal = Industrial & Engineering Chemistry | title = Structure of water near solid interfaces | date = 1969 | number = 11 | pages = 10–47 | volume = 61 | doi = 10.1021/ie50719a005 }} </ref><ref name="DrostHansen73"> {{cite journal | author = Drost-Hansen, Walter | journal = Annals of the Lyceum of Natural History of New York | title = Phase transitions in biological systems: manifestations of cooperative processes in vicinal water | date = 1973 | volume = 204 | issue = 1 | doi = 10.1111/j.1749-6632.1973.tb30773.x | pages = 100–112 | pmid = 4513148 | bibcode = 1973NYASA.204..100D | s2cid = 35243683 }} </ref>

In 1986, Boris Derjaguin and his colleagues observed an exclusion zone next to the walls of cells.<ref name="Deryagin1986"> {{cite journal | first1 = BV | last1 = Deryagin | first2 = MV | last2 = Golovanov | journal = Colloid Journal of the USSR | title = Electromagnetic nature of forces of repulsion forming aureoles around cells | date = 1986 | number = 2 | pages = 209–211 | volume = 48 }} </ref>

In 2006 the group of Gerald Pollack reported their observation of what they called an ''exclusion zone''. They observed that the particles of colloidal and molecular solutes suspended in aqueous solution are profoundly and extensively excluded from the vicinity of various hydrophilic surfaces.<ref name="Zheng2006" /> The exclusion zone has been observed and characterized by several independent groups since those early observations.<ref name="Chen2012"> {{cite journal | first1 = Chi-Shuo | last1 = Chen | first2 = Wei-Ju | last2 = Chung | first3 = Ian C | last3 = Hsu | first4 = Chien-Ming | last4 = Wu | first5 = Wei-Chun | last5 = Chin | journal = Journal of Biological Physics | title = Force field measurements within the exclusion zone of water | date = 2012 | number = 1 | pages = 113–120 | volume = 38 | doi = 10.1007/s10867-011-9237-5 | pmid = 23277674 | pmc = 3285724 }}</ref><ref name = "Huszar2014"> {{cite journal | first1 = István N | last1 = Huszár | first2 = Zsolt | last2 = Mártonfalvi | first3 = András József | last3 = Laki | first4 = Kristóf | last4 = Iván | first5 = Miklós | last5 = Kellermayer | journal = Entropy | title = Exclusion-zone dynamics explored with microfluidics and optical tweezers | date = 2014 | number = 8 | pages = 4322–4337 | volume = 16 | doi = 10.3390/e16084322 | bibcode = 2014Entrp..16.4322H | doi-access = free }} </ref><ref name="Elton2020" />

==Theoretical models== Since the early observations, several theoretical models have been proposed, to explain the experimental observation of the exclusion zone.

===Mechanical model: Change in molecular structure===

Some researchers suggest that the exclusion zone is due to a change in the molecular structure of water near an adjacent solid hydrophilic or metal surface.<ref name="Zheng2006" /><ref name="Oehr2014"> {{cite journal | first1 = Klaus | last1 = Oehr | first2 = Paul | last2 = LeMay | journal = Entropy | title = The Case for Tetrahedral Oxy-subhydride (TOSH) Structures in the Exclusion Zones of Anchored Polar Solvents Including Water |date = 2014 | number = 11 | pages = 5712–5720 | volume = 16 | doi = 10.3390/e16115712 | bibcode = 2014Entrp..16.5712O | doi-access = free }} </ref> In this model, the water in the exclusion zone has a structure of hexagonal sheets, where the hydrogen atoms are positioned between oxygen atoms. Moreover, hydrogen atoms bond to the oxygen atoms lying in the layer above and below so that in total each hydrogen forms three bonds. This structure can be considered as an intermediate between ice and water. However, the hexagonal sheet hypothesis does not account for all aspects of the exclusion zone, and it is not supported by the majority of physicists.

===Quantum electrodynamical model: quantum confinement===

Another calculation performed describes the molecules of the exclusion zone using quantum mechanics and quantum electrodynamics. In this model the liquid bulk water is in a gaseous state. Then, above a certain density threshold and below a specific critical temperature, those molecules go to another quantum state, with lower energy. In this lower energy, coherent state, the cloud of electrons oscillate between two quantum states: a ground state, and an excited state where one electron per molecule is almost free (the binding energy is about 0.5 eV). In this coherent state the quantum superposition has a component with coefficient 0.9 of the ground state, and a component with 0.1 of the excited state. The electrons in this quantum state oscillate between the ground state and the excited state with a certain frequency, and this oscillation creates an electromagnetic field, which is confined within the super-molecular structure, so that no radiation is observed. The molecules of the structure, together with the confined electromagnetic field, constitute in this model the exclusion zone.<ref name="DelGiudice2013">{{cite journal |last1= V. Elia |first1= R. Germano |last2= C. Hison |first2= E. Del Giudice |date= 2013 |title= Oxhydroelectric Effect in bi-distilled water |journal= Key Engineering Materials |volume= 543 |pages= 455–459 |doi= 10.4028/www.scientific.net/KEM.543.455 |s2cid= 94391774 }}</ref>

==References== {{Reflist}}

Category:Water Category:Fluid dynamics