Editing Soft matter

{{Short description|Subfield of condensed matter physics}}
{{For|the journal|Soft Matter (journal)}}
{{Condensed matter physics}}
'''Soft matter''' or '''soft condensed matter''' is a type of [[matter]] that can be deformed or structurally altered by thermal or mechanical stress which is of similar magnitude to [[thermal fluctuations]].

The science of soft matter is a subfield of [[condensed matter physics]]. Soft materials include [[Liquid|liquids]], [[Colloid|colloids]], [[Polymer|polymers]], [[Foam|foams]], [[Gel|gels]], [[Granular material|granular materials]], [[Liquid crystal|liquid crystals]], [[flesh]], and a number of [[Biomaterial|biomaterials]]. These materials share an important common feature in that predominant physical behaviors occur at an [[energy]] scale comparable with [[room temperature]] [[thermal energy]] (of order of [[KT (energy)|kT]]), and that [[entropy]] is considered the dominant factor.<ref name=":0">{{Cite book |url=https://link.springer.com/10.1007/b97416 |title=Soft Matter Physics: An Introduction |date=2003 |publisher=Springer New York |isbn=978-0-387-95267-3 |editor-last=Kleman |editor-first=Maurice |location=New York, NY |language=en |doi=10.1007/b97416 |editor-last2=Lavrentovich |editor-first2=Oleg D.}}</ref> At these temperatures, [[quantum]] aspects are generally unimportant. When soft materials interact favorably with surfaces, they become squashed without an external compressive force.<ref>{{Cite journal |last1=Carroll |first1=Gregory T. |last2=Jongejan |first2=Mahthild G. M. |last3=Pijper |first3=Dirk |last4=Feringa |first4=Ben L. |date=2010 |title=Spontaneous generation and patterning of chiral polymeric surface toroids |url=http://xlink.rsc.org/?DOI=c0sc00159g |journal=Chemical Science |language=en |volume=1 |issue=4 |pages=469 |doi=10.1039/c0sc00159g |s2cid=96957407 |issn=2041-6520}}</ref>

[[Pierre-Gilles de Gennes]], who has been called the "founding father of soft matter,"<ref>{{Cite journal |date=2005 |title=Soft matter: more than words |url=http://xlink.rsc.org/?DOI=b419223k |journal=Soft Matter |language=en |volume=1 |issue=1 |pages=16 |doi=10.1039/b419223k |pmid=32521835 |bibcode=2005SMat....1...16. |issn=1744-683X}}</ref> received the [[Nobel Prize in Physics]] in 1991 for discovering that methods developed for studying [[Phase transition|order phenomena]] in simple systems can be generalized to the more complex cases found in soft matter, in particular, to the behaviors of [[Liquid crystal|liquid crystals]] and [[Polymer|polymers]].<ref>The Nobel Prize in Physics 1991. NobelPrize.org. Nobel Prize Outreach AB 2023. Mon. 13 Feb 2023. https://www.nobelprize.org/prizes/physics/1991/summary/ </ref>

== History ==
The current understanding of soft matter grew from [[Albert Einstein|Albert Einstein's]] work on [[Brownian motion]],<ref>{{Cite journal |last=Einstein |first=Albert |date=1905 |title=Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen |trans-title=On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat |journal=Annalen der Physik |language=German |volume=322 |issue=8 |pages=549–560 |doi=10.1002/andp.19053220806|bibcode=1905AnP...322..549E |doi-access=free }}</ref><ref>{{Cite journal |last=Mezzenga |first=Raffaele |date=2021-12-22 |title=Grand Challenges in Soft Matter |journal=Frontiers in Soft Matter |volume=1 |pages=811842 |doi=10.3389/frsfm.2021.811842 |issn=2813-0499|doi-access=free }}</ref> understanding that a particle [[Suspension (chemistry)|suspended]] in a [[fluid]] must have a similar thermal energy to the fluid itself (of order of ''[[kT (energy)|kT]]''). This work built on established research into systems that would now be considered colloids.<ref>{{Cite book |last=McLeish |first=Tom |url=https://www.worldcat.org/oclc/1202271044 |title=Soft Matter: a Very Short Introduction |date=2020 |publisher=Oxford University Press |isbn=978-0-19-880713-1 |edition=1st |location=Oxford, United Kingdom |oclc=1202271044}}</ref>

The crystalline optical properties of liquid crystals and their ability to flow were first described by [[Friedrich Reinitzer]] in 1888,<ref>{{Cite journal |last=Reinitzer |first=Friedrich |date=1888 |title=Beiträge zur Kenntniss des Cholesterins |url=http://link.springer.com/10.1007/BF01516710 |journal=Monatshefte für Chemie - Chemical Monthly |language=de |volume=9 |issue=1 |pages=421–441 |doi=10.1007/BF01516710 |s2cid=97166902 |issn=0026-9247}}</ref> and further characterized by [[Otto Lehmann (physicist)|Otto Lehmann]] in 1889.<ref>{{Cite journal |last=Lehmann |first=O. |date=1889-07-01 |title=Über fliessende Krystalle |url=https://www.degruyter.com/document/doi/10.1515/zpch-1889-0434/html |journal=Zeitschrift für Physikalische Chemie |language=en |volume=4U |issue=1 |pages=462–472 |doi=10.1515/zpch-1889-0434 |s2cid=92908969 |issn=2196-7156}}</ref> The experimental setup that Lehmann used to investigate the two melting points of cholesteryl benzoate are still used in the research of liquid crystals as of about 2019.<ref name=":14">{{Cite book |last=DiLisi |first=Gregory A |url=https://iopscience.iop.org/book/978-1-64327-684-7 |title=An Introduction to Liquid Crystals |date=2019 |publisher=IOP Publishing |isbn=978-1-64327-684-7 |doi=10.1088/2053-2571/ab2a6fch1|s2cid=239330818 }}</ref>

In 1920, [[Hermann Staudinger]], recipient of the 1953 [[Nobel Prize in Chemistry]],<ref>Hermann Staudinger – Biographical. NobelPrize.org. Nobel Prize Outreach AB 2023. Mon. 13 Feb 2023. https://www.nobelprize.org/prizes/chemistry/1953/staudinger/biographical/ </ref> was the first person to suggest that polymers are formed through [[Covalent bond|covalent bonds]] that link smaller molecules together.<ref>{{Cite journal |last=Staudinger |first=H. |date=1920-06-12 |title=Über Polymerisation |url=https://onlinelibrary.wiley.com/doi/10.1002/cber.19200530627 |journal=Berichte der Deutschen Chemischen Gesellschaft (A and B Series) |language=en |volume=53 |issue=6 |pages=1073–1085 |doi=10.1002/cber.19200530627 |issn=0365-9488}}</ref> The idea of a [[macromolecule]] was unheard of at the time, with the scientific consensus being that the recorded high molecular weights of compounds like natural rubber were instead due to [[particle aggregation]].<ref>American Chemical Society International Historic Chemical Landmarks. Foundations of Polymer Science: Hermann Staudinger and Macromolecules. http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/staudingerpolymerscience.html  (accessed Feb 13th, 2023).</ref>

The use of [[hydrogel]] in the biomedical field was pioneered in 1960 by [[Drahoslav Lím]] and [[Otto Wichterle]].<ref>{{Cite book |url=https://www.worldcat.org/oclc/1050163199 |title=Hydrogels : recent advances |date=2018 |others=Vijay Kumar Thakur, Manju Kumari Thakur |isbn=978-981-10-6077-9 |location=Singapore |oclc=1050163199}}</ref> Together, they postulated that the chemical stability, ease of deformation, and permeability of certain polymer networks in aqueous environments would have a significant impact on medicine, and were the inventors of the soft [[contact lens]].<ref>{{Cite journal |last1=Wichterle |first1=O. |last2=Lím |first2=D. |date=1960 |title=Hydrophilic Gels for Biological Use |url=https://www.nature.com/articles/185117a0 |journal=Nature |language=en |volume=185 |issue=4706 |pages=117–118 |doi=10.1038/185117a0 |bibcode=1960Natur.185..117W |s2cid=4211987 |issn=0028-0836}}</ref>

These seemingly separate fields were dramatically influenced and brought together by [[Pierre-Gilles de Gennes]]. The work of de Gennes across different forms of soft matter was key to understanding its [[Universality (dynamical systems)|universality]], where material properties are not based on the [[chemistry]] of the underlying [[structure]], more so on the [[Mesoscopic physics|mesoscopic]] structures the underlying chemistry creates.<ref name=":1" /> He extended the understanding of [[Phase transition|phase changes]] in liquid crystals, introduced the idea of [[reptation]] regarding the [[Relaxation (physics)|relaxation]] of polymer systems, and successfully mapped polymer behavior to that of the [[Ising model]].<ref name=":1">{{Cite journal |last1=Joanny |first1=Jean-François |last2=Cates |first2=Michael |date=2019 |title=Pierre-Gilles de Gennes. 24 October 1932—18 May 2007 |journal=Biographical Memoirs of Fellows of the Royal Society |language=en |volume=66 |pages=143–158 |doi=10.1098/rsbm.2018.0033 |s2cid=127231807 |issn=0080-4606|doi-access=free }}</ref><ref>{{Cite journal |last=de Gennes |first=P.G. |date=1972 |title=Exponents for the excluded volume problem as derived by the Wilson method |url=https://linkinghub.elsevier.com/retrieve/pii/0375960172901491 |journal=Physics Letters A |language=en |volume=38 |issue=5 |pages=339–340 |doi=10.1016/0375-9601(72)90149-1|bibcode=1972PhLA...38..339D }}</ref>

== Distinctive physics ==
[[File:Phospholipids aqueous solution structures.svg|thumb|The self-assembly of individual phospholipids into colloids (Liposome and Micelle) or a membrane (bilayer sheet).]]
Interesting behaviors arise from soft matter in ways that cannot be predicted, or are difficult to predict, directly from its [[Atom|atomic]] or [[Molecule|molecular]] constituents. Materials termed soft matter exhibit this property due to a shared propensity of these materials to [[Self-organization|self-organize]] into mesoscopic physical structures. The assembly of the mesoscale structures that form the macroscale material is governed by low energies, and these low energy associations allow for the thermal and mechanical deformation of the material.<ref name=":7">{{Cite journal |last=van der Gucht |first=Jasper |date=2018-08-22 |title=Grand Challenges in Soft Matter Physics |journal=Frontiers in Physics |volume=6 |pages=87 |doi=10.3389/fphy.2018.00087 |bibcode=2018FrP.....6...87V |issn=2296-424X|doi-access=free }}</ref> By way of contrast, in hard [[condensed matter physics]] it is often possible to predict the overall behavior of a material because the molecules are organized into a [[Crystal|crystalline lattice]] with no changes in the pattern at any mesoscopic scale. Unlike hard materials, where only small distortions occur from thermal or mechanical agitation, soft matter can undergo local rearrangements of the microscopic building blocks.<ref name=":8">{{Citation |last1=Spagnoli |first1=A. |title=Fracture in soft elastic materials: Continuum description, molecular aspects and applications |date=2022 |url=https://linkinghub.elsevier.com/retrieve/pii/S0065215621000028 |work=Advances in Applied Mechanics |volume=55 |pages=255–307 |publisher=Elsevier |language=en |doi=10.1016/bs.aams.2021.07.001 |isbn=978-0-12-824617-7 |access-date=2023-02-13 |last2=Brighenti |first2=R. |last3=Cosma |first3=M.P. |last4=Terzano |first4=M.}}</ref>

A defining characteristic of soft matter is the [[mesoscopic scale]] of physical structures. The structures are much larger than the microscopic scale (the arrangement of [[atom]]s and [[molecule]]s), and yet are much smaller than the macroscopic (overall) scale of the material.  The properties and interactions of these mesoscopic structures may determine the macroscopic behavior of the material.<ref name=":9">{{Cite book |last=Jones |first=Richard A. L. |url=https://www.worldcat.org/oclc/48753186 |title=Soft condensed matter |date=2002 |publisher=Oxford University Press |isbn=0-19-850590-6 |location=Oxford |oclc=48753186}}</ref> The large number of constituents forming these mesoscopic structures, and the large [[Degrees of freedom (statistics)|degrees of freedom]] this causes, results in a general disorder between the large-scale structures. This disorder leads to the loss of long-range order that is characteristic of hard matter.<ref name=":5">{{Cite journal |last=Nagel |first=Sidney R. |date=2017-04-12 |title=Experimental soft-matter science |journal=Reviews of Modern Physics |language=en |volume=89 |issue=2 |pages=025002 |doi=10.1103/RevModPhys.89.025002 |bibcode=2017RvMP...89b5002N |issn=0034-6861|doi-access=free }}</ref>

For example, the [[Turbulence|turbulent]] [[vortex|vortices]] that naturally occur within a flowing [[liquid]] are much smaller than the overall quantity of liquid and yet much larger than its individual molecules, and the emergence of these vortices controls the overall flowing behavior of the material.  Also, the bubbles that compose a [[foam]] are mesoscopic because they individually consist of a vast number of molecules, and yet the foam itself consists of a great number of these bubbles, and the overall mechanical stiffness of the foam emerges from the combined interactions of the bubbles.

Typical bond energies in soft matter structures are of similar scale to thermal energies. Therefore the structures are constantly affected by thermal fluctuations and undergo [[Brownian motion]].<ref name=":9" /> The ease of deformation and influence of low energy interactions regularly result in slow [[Dynamics (mechanics)|dynamics]] of the mesoscopic structures which allows some systems to remain out of [[Equilibrium chemistry|equilibrium]] in [[Metastability|metastable]] states.<ref name=":3">{{Cite journal |last1=Chen |first1=Daniel T.N. |last2=Wen |first2=Qi |last3=Janmey |first3=Paul A. |last4=Crocker |first4=John C. |last5=Yodh |first5=Arjun G. |date=2010-08-10 |title=Rheology of Soft Materials |url=https://www.annualreviews.org/doi/10.1146/annurev-conmatphys-070909-104120 |journal=Annual Review of Condensed Matter Physics |language=en |volume=1 |issue=1 |pages=301–322 |doi=10.1146/annurev-conmatphys-070909-104120 |bibcode=2010ARCMP...1..301C |issn=1947-5454}}</ref><ref name=":10">{{Cite book |last=Cantat |first=Isabelle |url=https://www.worldcat.org/oclc/1011990362 |title=Foams: Structure and Dynamics |date=2013 |isbn=978-0-19-966289-0 |edition=1st |location=Oxford |oclc=1011990362}}</ref> This characteristic can allow for recovery of initial state through an external stimulus, which is often exploited in research.<ref name=":2">{{Cite journal |last1=Schmidt |first1=Bernhard V. K. J. |last2=Barner-Kowollik |first2=Christopher |date=2017-07-10 |title=Dynamic Macromolecular Material Design-The Versatility of Cyclodextrin-Based Host-Guest Chemistry |journal=Angewandte Chemie International Edition |language=en |volume=56 |issue=29 |pages=8350–8369 |doi=10.1002/anie.201612150|pmid=28245083 |doi-access=free }}</ref><ref name=":6">{{Cite journal |last1=Shi |first1=Mayue |last2=Yeatman |first2=Eric M. |date=2021-11-23 |title=A comparative review of artificial muscles for microsystem applications |journal=Microsystems & Nanoengineering |language=en |volume=7 |issue=1 |pages=95 |doi=10.1038/s41378-021-00323-5 |issn=2055-7434 |pmc=8611050 |pmid=34858630|bibcode=2021MicNa...7...95S }}</ref>

Self-assembly is an inherent characteristic of soft matter systems. The characteristic complex behavior and hierarchical structures arise spontaneously as a system evolves towards equilibrium.<ref name=":9" /> Self-assembly can be classified as static when the resulting structure is due to a [[Thermodynamic free energy|free energy]] minimum, or dynamic when the system is caught in a metastable state.<ref>{{Cite journal |last1=Whitesides |first1=George M. |last2=Grzybowski |first2=Bartosz |date=2002-03-29 |title=Self-Assembly at All Scales |url=https://www.science.org/doi/10.1126/science.1070821 |journal=Science |language=en |volume=295 |issue=5564 |pages=2418–2421 |doi=10.1126/science.1070821 |pmid=11923529 |bibcode=2002Sci...295.2418W |s2cid=40684317 |issn=0036-8075}}</ref> Dynamic self-assembly can be utilized in the functional design of soft materials with these metastable states through [[Thermodynamic versus kinetic reaction control|kinetic trapping]].<ref name=":7" /><ref name=":15">{{Cite journal |last1=Lin |first1=Qianming |last2=Li |first2=Longyu |last3=Tang |first3=Miao |last4=Uenuma |first4=Shuntaro |last5=Samanta |first5=Jayanta |last6=Li |first6=Shangda |last7=Jiang |first7=Xuanfeng |last8=Zou |first8=Lingyi |last9=Ito |first9=Kohzo |last10=Ke |first10=Chenfeng |date=2021 |title=Kinetic trapping of 3D-printable cyclodextrin-based poly(pseudo)rotaxane networks |url=https://linkinghub.elsevier.com/retrieve/pii/S2451929421003089 |journal=Chem |language=en |volume=7 |issue=9 |pages=2442–2459 |doi=10.1016/j.chempr.2021.06.004|bibcode=2021Chem....7.2442L |s2cid=237139764 }}</ref> 

Soft materials often exhibit both [[Elasticity (physics)|elasticity]] and [[Viscosity|viscous]] responses to external stimuli<ref name=":3" /> such as [[Shearing (physics)|shear]] induced flow or phase transitions. However, excessive external stimuli often result in [[Nonlinear system|nonlinear]] responses.<ref name=":0" /><ref>{{Cite journal |last1=Cipelletti |first1=Luca |last2=Martens |first2=Kirsten |last3=Ramos |first3=Laurence |date=2020 |title=Microscopic precursors of failure in soft matter |url=http://xlink.rsc.org/?DOI=C9SM01730E |journal=Soft Matter |language=en |volume=16 |issue=1 |pages=82–93 |doi=10.1039/C9SM01730E |pmid=31720666 |arxiv=1909.11961 |bibcode=2020SMat...16...82C |s2cid=202889185 |issn=1744-683X}}</ref> Soft matter becomes highly [[Deformation (physics)|deformed]] before [[crack propagation]], which differs significantly from the general fracture mechanics formulation.<ref name=":8" /> [[Rheology]], the study of deformation under [[Stress (mechanics)|stress]], is often used to investigate the [[Bulk property|bulk properties]] of soft matter.<ref name=":3" />

== Classes of soft matter ==
[[File:DNA animation.gif|thumb|A portion of the [[Nucleic acid double helix|DNA double helix]], an example of a [[biopolymer]].]]
[[File:Inclusion complex.png|thumb|Host-guest complex of polyethylene glycol [[oligomer]] bound within an [[Cyclodextrin|α-cyclodextrin]] molecule; a common scaffold used in the formation of gels. The atoms are colored such that red represents oxygen, cyan represents carbon, and white represents hydrogen.]]
[[File:Liquid Crystal.png|thumb|Cartoon representation of the molecular order of crystal, liquid crystal, and liquid states.]]
Soft matter consists of a diverse range of interrelated systems and can be broadly categorized into certain classes. These classes are by no means distinct, as often there are overlaps between two or more groups.

=== Polymers ===
{{Main|Polymer}}
Polymers are large molecules composed of repeating subunits whose characteristics are governed by their environment and composition. Polymers encompass synthetic plastics, natural fibers and rubbers, and biological proteins. Polymer research finds applications in [[nanotechnology]],<ref>{{Cite journal |last1=Mashaghi |first1=Samaneh |last2=Jadidi |first2=Tayebeh |last3=Koenderink |first3=Gijsje|author3-link=Gijsje Koenderink |last4=Mashaghi |first4=Alireza |date=2013-02-21 |title=Lipid Nanotechnology |journal=International Journal of Molecular Sciences |language=en |volume=14 |issue=2 |pages=4242–4282 |doi=10.3390/ijms14024242 |issn=1422-0067 |pmc=3588097 |pmid=23429269|doi-access=free }}</ref><ref>{{Cite journal |last=Hamley |first=Ian W. |date=2003 |title=Nanotechnology with Soft Materials |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200200546 |journal=Angewandte Chemie International Edition |volume=42 |issue=15 |pages=1692–1712 |doi=10.1002/anie.200200546|pmid=12707884 }}</ref> from [[materials science]] and [[drug delivery]] to [[protein crystallization]].<ref name=":2" /><ref name=":11">{{Cite journal |last1=Maimouni |first1=Ilham |last2=Cejas |first2=Cesare M. |last3=Cossy |first3=Janine |last4=Tabeling |first4=Patrick |last5=Russo |first5=Maria |date=2020 |title=Microfluidics Mediated Production of Foams for Biomedical Applications |journal=Micromachines |language=en |volume=11 |issue=1 |pages=83 |doi=10.3390/mi11010083 |issn=2072-666X |pmc=7019871 |pmid=31940876|doi-access=free }}</ref>

=== Foams ===
{{Main|Foam}}
Foams consist of a liquid or solid through which a [[gas]] has been dispersed to form cavities.  This structure imparts a large [[surface-area-to-volume ratio]] on the system.<ref name=":10" /><ref name=":12">{{Cite journal |last1=Jin |first1=Fan-Long |last2=Zhao |first2=Miao |last3=Park |first3=Mira |last4=Park |first4=Soo-Jin |date=2019 |title=Recent Trends of Foaming in Polymer Processing: A Review |journal=Polymers |language=en |volume=11 |issue=6 |pages=953 |doi=10.3390/polym11060953 |issn=2073-4360 |pmc=6631771 |pmid=31159423|doi-access=free }}</ref> Foams have found applications in [[Thermal insulation|insulation]] and [[Textile|textiles]],<ref name=":12" /> and are undergoing active research in the biomedical field of drug delivery and [[tissue engineering]].<ref name=":11" />  Foams are also used in automotive for water and dust sealing and noise reduction. 

=== Gels ===
{{Main|Gel}}
Gels consist of non-solvent-[[Solubility|soluble]] 3D polymer scaffolds, which are [[Covalent bond|covalently]] or physically [[Cross-link|cross-linked]], that have a high solvent/content ratio.<ref>{{Cite journal |last=Ahmed |first=Enas M. |date=2015 |title=Hydrogel: Preparation, characterization, and applications: A review |journal=Journal of Advanced Research |language=en |volume=6 |issue=2 |pages=105–121 |doi=10.1016/j.jare.2013.07.006 |pmc=4348459 |pmid=25750745}}</ref><ref name=":13">{{Cite journal |last1=Qi |first1=Zhenhui |last2=Schalley |first2=Christoph A. |date=2014-07-15 |title=Exploring Macrocycles in Functional Supramolecular Gels: From Stimuli Responsiveness to Systems Chemistry |url=https://pubs.acs.org/doi/10.1021/ar500193z |journal=Accounts of Chemical Research |language=en |volume=47 |issue=7 |pages=2222–2233 |doi=10.1021/ar500193z |pmid=24937365 |issn=0001-4842}}</ref> Research into functionalizing gels that are sensitive to mechanical and thermal stress, as well as solvent choice, has given rise to diverse structures with characteristics such as [[Shape-memory polymer|shape-memory]],<ref>{{Cite journal |last1=Korde |first1=Jay M. |last2=Kandasubramanian |first2=Balasubramanian |date=2020 |title=Naturally biomimicked smart shape memory hydrogels for biomedical functions |url=https://linkinghub.elsevier.com/retrieve/pii/S1385894719318339 |journal=Chemical Engineering Journal |language=en |volume=379 |pages=122430 |doi=10.1016/j.cej.2019.122430|bibcode=2020ChEnJ.37922430K |s2cid=201216064 }}</ref> or the ability to bind [[Host–guest chemistry|guest]] molecules selectively and reversibly.<ref name=":13" />

=== Colloids ===
{{Main|Colloid}}
Colloids are non-soluble particles suspended in a medium, such as proteins in an aqueous solution.<ref name=":16">{{Cite journal |last1=Hamley |first1=Ian W. |last2=Castelletto |first2=Valeria |date=2007-06-11 |title=Biological Soft Materials |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200603922 |journal=Angewandte Chemie International Edition |language=en |volume=46 |issue=24 |pages=4442–4455 |doi=10.1002/anie.200603922|pmid=17516592 }}</ref> Research into colloids is primarily focused on understanding the organization of matter, with the large structures of colloids, relative to individual molecules, large enough that they can be readily observed.<ref>{{Cite journal |last=Manoharan |first=Vinothan N. |date=2015-08-28 |title=Colloidal matter: Packing, geometry, and entropy |journal=Science |language=en |volume=349 |issue=6251 |pages=1253751 |doi=10.1126/science.1253751 |pmid=26315444 |s2cid=5727282 |issn=0036-8075|doi-access=free }}</ref>

=== Liquid crystals ===
{{Main|Liquid crystal}}
Liquid crystals can consist of proteins, small molecules, or polymers, that can be manipulated to form cohesive order in a specific direction.<ref name=":4">{{Cite journal |last1=Bisoyi |first1=Hari Krishna |last2=Li |first2=Quan |date=2022-03-09 |title=Liquid Crystals: Versatile Self-Organized Smart Soft Materials |url=https://pubs.acs.org/doi/10.1021/acs.chemrev.1c00761 |journal=Chemical Reviews |language=en |volume=122 |issue=5 |pages=4887–4926 |doi=10.1021/acs.chemrev.1c00761 |pmid=34941251 |issn=0009-2665}}</ref> They exhibit liquid-like behavior in that they can [[Fluid dynamics|flow]], yet they can obtain close-to-crystal alignment. One feature of liquid crystals is their ability to [[Spontaneous symmetry breaking|spontaneously break symmetry]].<ref>{{Cite journal |last=Tschierske |first=Carsten |date=2018-12-08 |title=Mirror symmetry breaking in liquids and liquid crystals |journal=Liquid Crystals |language=en |volume=45 |issue=13–15 |pages=2221–2252 |doi=10.1080/02678292.2018.1501822 |s2cid=125652009 |issn=0267-8292|doi-access=free |bibcode=2018LoCr...45.2221T }}</ref> Liquid crystals have found significant applications in optical devices such as [[Liquid-crystal display|liquid-crystal displays]] (LCD).

=== Biological membranes ===
{{Main|Biological membrane}}
Biological membranes consist of individual [[phospholipid]] molecules that have self-assembled into a [[Lipid bilayer|bilayer]] structure due to [[Non-covalent interaction|non-covalent interactions]]. The localized, low energy associated with the forming of the [[membrane]] allows for the elastic deformation of the large-scale structure.<ref name=":17">{{Cite journal |last=Brown |first=Michael F. |date=2017-05-22 |title=Soft Matter in Lipid–Protein Interactions |url=https://www.annualreviews.org/doi/10.1146/annurev-biophys-070816-033843 |journal=Annual Review of Biophysics |language=en |volume=46 |issue=1 |pages=379–410 |doi=10.1146/annurev-biophys-070816-033843 |pmid=28532212 |issn=1936-122X}}</ref>

== Experimental characterization ==
Due to the importance of mesoscale structures in the overarching properties of soft matter, experimental work is primarily focused on the bulk properties of the materials. Rheology is often used to investigate the physical changes of the material under stress.<ref name=":3" /> Biological systems, such as protein crystallization, are often investigated through [[X-ray crystallography|X-ray]] and [[neutron crystallography]],<ref name=":18">{{Cite journal |last1=Fusco |first1=Diana |last2=Charbonneau |first2=Patrick |date=2016 |title=Soft matter perspective on protein crystal assembly |url=https://linkinghub.elsevier.com/retrieve/pii/S0927776515300576 |journal=Colloids and Surfaces B: Biointerfaces |language=en |volume=137 |pages=22–31 |doi=10.1016/j.colsurfb.2015.07.023|pmid=26236019 |s2cid=13969559 |arxiv=1505.05214 }}</ref> while [[nuclear magnetic resonance spectroscopy]] can be used in understanding the average structure and lipid mobility of membranes.<ref name=":17" />

=== Scattering ===
{{Main|Scattering}}
[[Scattering]] techniques, such as [[wide-angle X-ray scattering]], [[small-angle X-ray scattering]], [[neutron scattering]], and [[dynamic light scattering]] can also be used for materials when probing for the average properties of the constituents. These methods can determine [[particle-size distribution]], shape, [[crystallinity]] and [[diffusion]] of the constituents in the system.<ref name=":19">{{Cite journal |last=Scheffold |first=Frank |date=2020-09-04 |title=Pathways and challenges towards a complete characterization of microgels |journal=Nature Communications |language=en |volume=11 |issue=1 |pages=4315 |doi=10.1038/s41467-020-17774-5 |issn=2041-1723 |pmc=7473851 |pmid=32887886|bibcode=2020NatCo..11.4315S }}</ref><ref>{{Cite journal |last1=Murthy |first1=N.S. |last2=Minor |first2=H. |date=1990 |title=General procedure for evaluating amorphous scattering and crystallinity from X-ray diffraction scans of semicrystalline polymers |url=https://linkinghub.elsevier.com/retrieve/pii/003238619090243R |journal=Polymer |language=en |volume=31 |issue=6 |pages=996–1002 |doi=10.1016/0032-3861(90)90243-R}}</ref> There are limitations in the application of scattering techniques to some systems, as they can be more suited to [[Isotropy|isotropic]] and dilute samples.<ref name=":19" />

=== Computational ===
{{Main|Computer simulation}}
[[Computational chemistry|Computational]] methods are often employed to model and understand soft matter systems, as they have the ability to strictly control the composition and environment of the structures being investigated, as well as span from microscopic to macroscopic length scales.<ref name=":5" /> Computational methods are limited, however, by their suitability to the system and must be regularly validated against experimental results to ensure accuracy.<ref name=":5" /> The use of [[informatics]] in the prediction of soft matter properties is also a growing field in computer science thanks to the large amount of data available for soft matter systems.<ref>{{Cite journal |last1=Peerless |first1=James S. |last2=Milliken |first2=Nina J. B. |last3=Oweida |first3=Thomas J. |last4=Manning |first4=Matthew D. |last5=Yingling |first5=Yaroslava G. |date=2019 |title=Soft Matter Informatics: Current Progress and Challenges |journal=Advanced Theory and Simulations |language=en |volume=2 |issue=1 |pages=1800129 |doi=10.1002/adts.201800129 |s2cid=139778116 |issn=2513-0390|doi-access=free }}</ref>

=== Microscopy ===
{{Main|Microscopy}}
[[Optical microscope|Optical microscopy]] can be used in the study of colloidal systems, but more advanced methods like [[transmission electron microscopy]] (TEM) and [[atomic force microscopy]] (AFM) are often used to characterize forms of soft matter due to their applicability to mapping systems at the nanoscale.<ref>Wu, H.,  Friedrich, H.,  Patterson, J. P.,  Sommerdijk, N. A. J. M.,  de, N. (2020), "Liquid-Phase Electron Microscopy for Soft Matter Science and Biology". ''Adv. Mater.'' '''32''', 2001582. {{doi|10.1002/adma.202001582}}</ref><ref>{{Cite journal |last=Garcia |first=Ricardo |date=2020-08-17 |title=Nanomechanical mapping of soft materials with the atomic force microscope: methods, theory and applications |journal=Chemical Society Reviews |language=en |volume=49 |issue=16 |pages=5850–5884 |doi=10.1039/D0CS00318B |pmid=32662499 |s2cid=220519766 |issn=1460-4744|doi-access=free }}</ref> These imaging techniques are not universally appropriate to all classes of soft matter and some systems may be more suited to one kind of analysis than another. For example, there are limited applications in imaging hydrogels with TEM due to the processes required for imaging. However, [[Fluorescence microscope|fluorescence microscopy]] can be readily applied.<ref name=":19" /> Liquid crystals are often probed using [[polarized light microscopy]] to determine the ordering of the material under various conditions, such as [[temperature]] or [[electric field]].<ref>{{Cite journal |last1=Miller |first1=Daniel S. |last2=Carlton |first2=Rebecca J. |last3=Mushenheim |first3=Peter C. |last4=Abbott |first4=Nicholas L. |date=2013-03-12 |title=Introduction to Optical Methods for Characterizing Liquid Crystals at Interfaces |journal=Langmuir |language=en |volume=29 |issue=10 |pages=3154–3169 |doi=10.1021/la304679f |pmid=23347378 |pmc=3711186 |issn=0743-7463}}</ref>

== Applications ==
Soft materials are important in a wide range of technological applications, and each soft material can often be associated with multiple disciplines. Liquid crystals, for example, were originally discovered in the biological sciences when the botanist and chemist [[Friedrich Reinitzer]] was investigating [[Cholesterol|cholesterols]].<ref name=":14" /> Now, however, liquid crystals have also found applications as [[Liquid-crystal display|liquid-crystal displays]], [[Liquid crystal tunable filter|liquid crystal tunable filters]], and [[Liquid crystal thermometer|liquid crystal thermometers]]. [[Active fluid|Active liquid crystals]] are another example of soft materials, where the constituent elements in liquid crystals can self-propel.<ref>{{Cite journal |last1=Zhang |first1=Rui |last2=Mozaffari |first2=Ali |last3=de Pablo |first3=Juan J. |date=2021-02-25 |title=Autonomous materials systems from active liquid crystals |url=https://www.nature.com/articles/s41578-020-00272-x |journal=Nature Reviews Materials |language=en |volume=6 |issue=5 |pages=437–453 |doi=10.1038/s41578-020-00272-x |bibcode=2021NatRM...6..437Z |s2cid=232044197 |issn=2058-8437}}</ref> 

Polymers have found diverse applications, from the [[natural rubber]] found in [[latex gloves]] to the [[Sulfur vulcanization|vulcanized rubber]] found in tires. Polymers encompass a large range of soft matter, with applications in material science. An example of this is hydrogel. With the ability to undergo [[shear thinning]], hydrogels are well suited for the development of [[3D printing]].<ref name=":15" /> Due to their stimuli responsive behavior, 3D printing of hydrogels has found applications in a diverse range of fields, such as [[soft robotics]], [[tissue engineering]], and [[flexible electronics]].<ref>{{Cite journal |last1=Zhan |first1=Shuai |last2=Guo |first2=Amy X. Y. |last3=Cao |first3=Shan Cecilia |last4=Liu |first4=Na |date=2022-03-30 |title=3D Printing Soft Matters and Applications: A Review |journal=International Journal of Molecular Sciences |language=en |volume=23 |issue=7 |pages=3790 |doi=10.3390/ijms23073790 |pmid=35409150 |pmc=8998766 |issn=1422-0067|doi-access=free }}</ref> Polymers also encompass biological molecules such as proteins, where research insights from soft matter research have been applied to better understand topics like protein crystallization.<ref name=":18" /> 

3D/4D printing of soft materials is evolving, focusing on various printing techniques, material types, and their broad applications in engineering and technology. Key printing methods are extrusion and inkjet based printing, stereolithography, selective laser sintering, direct ink writing, and VAT photopolymerization. A diversity in soft materials for 3D/4D printing includes elastomers, hydrogels, bio-inspired polymers, conductive and flexible materials, andinkjet-based biomimetic materials for applications in biomedical engineering, soft robotics, wearable devices, textiles, food technology, and pharmaceuticals. Changelings and limitations prevail in design geometric complexity,cost, resolution, material #compatibility, scalability and regulatory concerns.<ref>{{Cite journal |last1=Abdullah |first1=Muhammad Raies |last2=Peng |first2=Zhen |last3=Rajendren |first3=Vignesh babu |last4=Ahmad |first4=Farooq |last5=Shah |first5=Syed Sohail Ahmed |last6=Zia |first6=Abdul Wasy |last7=Ali |first7=Amjad |last8=Qiao |first8=Guanjun |last9=Shehzad |first9=Khurram |date=2025-04-01 |title=Comprehensive review of 3D/4D printing of soft materials, methods and applications |url=https://linkinghub.elsevier.com/retrieve/pii/S2352940725000861 |journal=Applied Materials Today |volume=43 |pages=102667 |doi=10.1016/j.apmt.2025.102667 |issn=2352-9407}}</ref>

Foams can naturally occur, such as the [[Beer head|head on a beer]], or be created intentionally, such as by [[Fire extinguisher|fire extinguishers]]. The physical properties available to foams have resulted in applications which can be based on their viscosity,<ref name=":10" /> with more rigid and self-supporting forms of foams being used as [[Thermal insulation|insulation]] or [[Cushion|cushions]], and foams that exhibit the ability to flow being used in the [[cosmetic industry]] as [[Shampoo|shampoos]] or makeup.<ref name=":10" /> Foams have also found biomedical applications in tissue engineering as scaffolds and [[Biosensor|biosensors]].<ref>{{Cite book |url=https://www.worldcat.org/oclc/872654628 |title=Biomedical foams for tissue engineering applications |date=2014 |publisher=Woodhead Publishing |others=Paulo Netti |isbn=978-1-306-47861-8 |location=Cambridge |oclc=872654628}}</ref>

Historically the problems considered in the early days of soft matter science were those pertaining to the biological sciences. As such, an important application of soft matter research is [[biophysics]], with a major goal of the discipline being the reduction of the field of [[cell biology]] to the concepts of soft matter physics.<ref name=":9" /> Applications of soft matter characteristics are used to understand biologically relevant topics such as membrane mobility,<ref name=":17" /> as well as the rheology of [[blood]].<ref name=":16" />

== See also ==
{{Div col|rules=yes}}
* [[Biological membranes]] 
* [[Biomaterials]]
* [[Colloid]]s
* [[Complex fluids]]
* [[Foam]]s
* [[Fracture of soft materials]]
* [[Gels]] 
* [[Granular material]]s
* [[Liquid]]s
* [[Liquid crystal]]s
* [[Microemulsion]]s
* [[Polymers]]
* [[Protein dynamics]]
* [[Protein structure]]
* [[Surfactants]]
* [[Active matter]]
* [[surface roughness|Roughness]]
{{Div col end}}

==References==
{{Reflist}}

*I. Hamley, ''Introduction to Soft Matter '' (2nd edition), J. Wiley, Chichester (2000).
*R. A. L. Jones, ''Soft Condensed Matter'', Oxford University Press, Oxford (2002).
*T. A. Witten (with P. A. Pincus), ''Structured Fluids: Polymers, Colloids, Surfactants'', Oxford (2004).
*[[Maurice Kleman|M. Kleman]] and O. D. Lavrentovich, ''Soft Matter Physics: An Introduction'', Springer (2003).
*M. Mitov, ''Sensitive Matter: Foams, Gels, Liquid Crystals and Other Miracles'', Harvard University Press (2012).
*J. N. Israelachvili, ''Intermolecular and Surface Forces'', Academic Press (2010).
*A. V. Zvelindovsky (editor), ''Nanostructured Soft Matter - Experiment, Theory, Simulation and Perspectives'', Springer/Dordrecht (2007), {{ISBN|978-1-4020-6329-9}}.
*M. Daoud, C.E. Williams (editors), ''Soft Matter Physics'', Springer Verlag, Berlin (1999).
*Gerald H. Ristow, ''Pattern Formation in Granular Materials'', Springer Tracts in Modern Physics, v. 161. Springer, Berlin (2000). {{ISBN|3-540-66701-6}}.
*de Gennes, Pierre-Gilles, ''Soft Matter'', Nobel Lecture, December 9, 1991
*S. A. Safran, ''Statistical thermodynamics of surfaces, interfaces and membranes'', Westview Press (2003)
*R.G. Larson, "The Structure and Rheology of Complex Fluids," Oxford University Press (1999)
*Gang, Oleg, [https://doi.org/10.1142/11763 "Soft Matter and Biomaterials on the Nanoscale: The WSPC Reference on Functional Nanomaterials — Part I (In 4 Volumes)"], World Scientific Publisher (2020)

==External links==
{{Commons category-inline}}
*[https://www.nobelprize.org/uploads/2018/06/gennes-lecture.pdf Pierre-Gilles de Gennes' Nobel Lecture]
*[http://www.aps.org/units/gsoft/ American Physical Society Topical Group on Soft Matter (GSOFT)]
*[http://www.softbites.org Softbites] - a blog run by graduate students and postdocs that makes soft matter more accessible through bite-sized posts that summarize current and classic soft matter research
*{{usurped|1=[https://web.archive.org/web/20080512085342/http://www.softmatterworld.org/ Softmatterworld.org]}}
*[http://www.softmatterresources.com Softmatterresources.com]
*[http://www.sklogwiki.org/SklogWiki/index.php/Main_Page SklogWiki] - a wiki dedicated to simple liquids, complex fluids, and soft condensed matter.
*[http://soft-matter.seas.harvard.edu/ Harvard School of Engineering and Applied Sciences Soft Matter Wiki] - organizes, reviews, and summarizes academic papers on soft matter.
*[http://softmatter.eng.ufl.edu Soft Matter Engineering] - A group dedicated to Soft Matter Engineering at the University of Florida
*[https://scholar.google.com/citations?view_op=search_authors&hl=en&mauthors=label:soft_matter Google Scholar page on soft matter]

{{Branches of physics}}
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{{DEFAULTSORT:Soft Matter}}
[[Category:Soft matter| ]]
[[Category:Condensed matter physics]]