View source for Foam

{{use mdy dates|date = August 2024}}
{{short description|Form of matter}}
{{about|the substance formed from trapped gas bubbles}}
{{redirect2|Foaming|Foamy|a method of mass killing farm animals|Foam depopulation||Foamy (disambiguation)}}
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{{original research|date=December 2020}}
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[[File:The top of foamy drink (Unsplash).jpg|thumb|Top of a foamy drink]]
[[File:Foam - big.jpg|thumb|[[Soap]] foam bubbles]]
[[File:Sponge-for-washing-1212612.jpg|thumb|Cleaning sponge]]
'''Foams''' are two-phase [[materials science|material]] systems where a gas is dispersed in a second, non-gaseous material, specifically, in which [[gas]] cells are enclosed by a distinct [[liquid]] or [[solid]] material.<ref name = WeaireHutzler1999>{{cite book|author = Weaire, D & Hutzler, S | date = 1999 | title = The Physics of Foams | page = | location = Oxford, England | publisher = Oxford University Press | isbn = 0198510977 | url = https://books.google.com/books?id=mEHlZ9ZodFsC | access-date = 30 August 2024 | quote =  }}  Note, this source focuses only on liquid foams.</ref>{{rp|6}}<ref name = Cantat_etal_1999>{{cite book|author = [[Isabelle Cantat|Cantat, I]]; Cohen-Addad, S; Elias, F; Graner, F; Höhler, R; Pitois, O; Rouyer, F & Saint-Jalmes, A | date = 2013 | title = Foams: Structure and Dynamics | page = | location = Oxford, England | publisher = Oxford University Press | isbn = 978-0199662890 | url = https://books.google.com/books?id=Ndzzl1hUNRoC | access-date = 30 August 2024 | quote =  }}  Note, this source also focuses on liquid foams.</ref>{{rp|4}}<ref>The general use of the term, which includes both noun and verb forms, is both narrower and broader than the one from material science described here. In general use, it is narrower, in that it most often refers to liquid foams; it is broader in that it includes all manners of such, and the actions to produce them, hence, according to [[Merriam-Webster]], the term refers to "a light frothy mass of fine bubbles formed in or on the surface of a liquid or from a liquid", giving the examples of those produced by "salivating or sweating", ones stably produced to fight fires, ones that are the product of gas bubbles introduced during manufacturing, and then the further broad examples of sea foam, and then anything resembling the foregoing. Finally, the general definition includes actions to produce all of the above. See {{cite web|url=http://www.merriam-webster.com/dictionary/foam|title=Foam|work=Merriam-Webster|url-status=live|archive-url= https://web.archive.org/web/20141209213221/http://www.merriam-webster.com/dictionary/foam|archive-date=2014-12-09}}</ref> The foam "may contain more or less liquid [or solid] according to circumstances",<ref name = WeaireHutzler1999/>{{rp|6}} although in the case of gas-liquid foams, the gas occupies most of the volume.<ref name = Cantat_etal_1999/>{{rp|4}} The word derives from the [[Old High German|medieval German]] and otherwise obsolete ''veim'', in reference to the "frothy head forming in the glass once the beer has been freshly poured" (cf. ''ausgefeimt'').<ref name = WeaireHutzler1999/>{{rp|1}}

Theories regarding foam formation, structure, and properties—in [[physics]] and [[physical chemistry]]—differ somewhat between liquid and solid foams in that the former are dynamic (e.g., in their being "continuously deformed"), as a result of gas diffusing between cells, liquid draining from the foam into a bulk liquid, etc.<ref name = WeaireHutzler1999/>{{rp|1-2}} Theories regarding liquid foams have as direct analogs theories regarding [[emulsion]]s,<ref name = WeaireHutzler1999/>{{rp|3}} two-phase [[materials science|material]] systems in which one [[liquid]] is enclosed by another.<ref>{{cite book|title=Compendium of Chemical Terminology (The "Gold Book")|year=1997|publisher=[[Blackwell Scientific Publications]]|location=Oxford|author=IUPAC|chapter-url=http://goldbook.iupac.org/E02065.html|url-status=bot: unknown |archive-url= https://web.archive.org/web/20120310221658/http://goldbook.iupac.org/E02065.html|archive-date=2012-03-10 |doi=10.1351/goldbook.E02065 |chapter=Emulsion|isbn=978-0-9678550-9-7}}</ref>

In most foams, the volume of [[gas]] is large, with thin films of liquid or solid separating the regions of gas.{{fact|date = August 2024}} A [[Sponge (tool)|bath sponge]] and the [[Beer head|head on a glass of beer]] are examples of foams; soap foams are also known as ''suds''.{{citation needed lead|date = August 2024}}<!--Removed bold, until this word appears, with reference, in the main body, to source and explain it.-->

Solid foams can be closed-cell or [[reticulated foam|open-cell]].{{cn|date = August 2024}} In closed-cell foam, the gas forms discrete pockets, each completely surrounded by the solid material. In open-cell foam, gas pockets connect to each other. A bath sponge is an example of an open-cell foam:{{citation needed lead|date = August 2024}} water easily flows through the entire structure, displacing the air. A [[sleeping mat]] is an example of a product composed of closed-cell foam.{{citation needed lead|date = August 2024}}<!-- gas pockets are sealed from each other so the mat cannot soak up water. NOT CRITICAL TO LEAD.-->

Foams are examples of [[dispersed media]].{{citation needed lead|date = August 2024}} In general, gas is present, so it divides into gas [[Bubble (physics)|bubbles]] of different sizes (i.e., the material is [[polydisperse]]{{citation needed lead|date = August 2024}})—separated by liquid regions that may form films, thinner and thinner when the liquid phase drains out of the system [[Interface (chemistry)|films]].<ref>{{cite book |last1=Lucassen|first1=J. |editor1-first=E. H. |editor1-last=Lucassen-Reijnders |title=Anionic Surfactants – Physical Chemistry of Surfactant Action|year= 1981|publisher= Marcel Dekker|location= NY, USA}}{{page needed|date = August 2024}}</ref>{{page needed|date = August 2024}} When the principal scale is small, i.e., for a very fine foam, this dispersed medium can be considered a type of [[colloid]].{{citation needed lead|date = August 2024}}

''Foam'' can also refer to something that is analogous to foam, such as [[quantum foam]].{{citation needed lead|date = August 2024}}

==Structure==
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A foam is, in many cases, a multi-scale system.{{fact|date = August 2024}}

[[File:Order and Chaos.tif|thumb|Order and disorder of bubbles in a surface foam]]
One scale is the bubble: [[materials science|material]] foams are typically [[randomness|disordered]] and have a variety of bubble sizes.{{fact|date = August 2024}} At larger sizes, the study of idealized foams is closely linked to the mathematical problems of [[minimal surface]]s and three-dimensional [[tessellations]], also called [[honeycomb (geometry) |honeycombs]].{{fact|date = August 2024}} The [[Weaire–Phelan structure]] is reported in one primary philosophical source to be the best possible (optimal) [[unit cell]] of a perfectly ordered foam,<ref>{{cite journal|last1=Morgan |date= 2008|issue= 9–10 |first1=F. |title= Existence of Least-perimeter Partitions |arxiv=0711.4228 |journal = Philosophical Magazine Letters |pages = 647–650|volume= 88|doi = 10.1080/09500830801992849 |bibcode= 2008PMagL..88..647M}}</ref>{{better source|date = August 2024}} while [[Plateau's laws]] describe how soap-films form structures in foams.{{fact|date = August 2024}}

At lower scale than the bubble is the thickness of the film for [[metastable]] foams, which can be considered a network of interconnected films called [[Lamella (materials)|lamellae]].{{fact|date = August 2024}} Ideally, the lamellae connect in triads and radiate 120° outward from the connection points,{{says who|date = August 2024}} known as [[Plateau's laws|Plateau borders]].{{fact|date = August 2024}}

An even lower scale is the liquid–air interface at the surface of the film.{{fact|date = August 2024}} Most of the time this interface is stabilized by a layer of [[amphiphilic]] structure, often made of [[surfactant]]s, particles ([[Pickering emulsion]]), or more complex associations.{{fact|date = August 2024}}

==Formation==
{{refimprove section|date = August 2024}}
Several conditions are needed to produce foam: there must be mechanical work, [[Surfactant|surface active components]] (surfactants) that reduce the [[surface tension]], and the formation of foam faster than its breakdown.
To create foam, [[Work (physics)|work]] (W) is needed to increase the [[surface area]] (ΔA):

:<math> W = \gamma \Delta A \,\!</math>

where γ is the surface tension.

One of the ways foam is created is through dispersion, where a large amount of gas is mixed with a liquid. A more specific method of dispersion involves injecting a gas through a hole in a solid into a liquid. If this process is completed very slowly, then one bubble can be emitted from the orifice at a time as shown in the picture below.

One of the theories for determining the separation time is shown below; however, while this theory produces theoretical data that matches the experimental data, detachment due to capillarity is accepted as a better explanation.

[[File:Rising Bubble from Orifice.png|thumb|Rising bubble from orifice]]
The [[buoyancy]] force acts to raise the bubble, which is

:<math> F_b = Vg(\rho_2-\rho_1)\!</math>
where <math>V</math> is the volume of the bubble, <math>g</math> is the acceleration due to gravity, and ρ<sub>1</sub> is the density of the gas ρ<sub>2</sub> is the density of the liquid. The force working against the buoyancy force is the [[surface tension]] force, which is

:<math> F_s = 2r \pi\gamma\!</math>,
where γ is the surface tension, and <math>r</math> is the radius of the orifice.
As more air is pushed into the bubble, the buoyancy force grows quicker than the surface tension force. Thus, detachment occurs when the buoyancy force is large enough to overcome the surface tension force. 
:<math> Vg(\rho_2-\rho_1)> 2r \pi\gamma\!</math>
In addition, if the bubble is treated as a sphere with a radius of <math>R</math> and the volume <math>V</math> is substituted in to the equation above, separation occurs at the moment when
:<math> R^3=\frac{3r\gamma}{2g(\rho_2-\rho_1)}\!</math>

Examining this phenomenon from a capillarity viewpoint for a bubble that is being formed very slowly, it can be assumed that the pressure <math>p</math> inside is constant everywhere. The hydrostatic pressure in the liquid is designated by <math>p_0</math>. The change in pressure across the interface from gas to liquid is equal to the capillary pressure; hence,

:<math> p-p_0=\gamma\left (\frac{1}{R_1}+\frac{1}{R_2}\right)\!</math>
where R<sub>1</sub> and R<sub>2</sub> are the radii of curvature and are set as positive. At the stem of the bubble, R<sub>3</sub> and R<sub>4</sub> are the radii of curvature also treated as positive. Here the hydrostatic pressure in the liquid has to take in account z, the distance from the top to the stem of the bubble. The new hydrostatic pressure at the stem of the bubble is ''p''<sub>0</sub>(''ρ''<sub>1</sub>&nbsp;−&nbsp;''ρ''<sub>2</sub>)''z''. The hydrostatic pressure balances the capillary pressure, which is shown below:

:<math> p-p_0-(\rho_2-\rho_1)gz=\gamma\left (\frac{1}{R_3}+\frac{1}{R_4}\right)\!</math>

Finally, the difference in the top and bottom pressure equal the change in hydrostatic pressure:

:<math> (\rho_2-\rho_1)gz=\gamma\left (\frac{1}{R_1}+\frac{1}{R_2}-\frac{1}{R_3}-\frac{1}{R_4}\right)\!</math>
At the stem of the bubble, the shape of the bubble is nearly cylindrical; consequently, either R<sub>3</sub> or R<sub>4</sub> is large while the other radius of curvature is small. As the stem of the bubble grows in length, it becomes more unstable as one of the radius grows and the other shrinks. At a certain point, the vertical length of the stem exceeds the circumference of the stem and due to the buoyancy forces the bubble separates and the process repeats.<ref name=autogenerated1>Bikerman, J.J. "Formation and Structure" in ''Foams'' New York, Springer-Verlag, 1973. ch 2. sec 24–25</ref>

==Stability==
{{refimprove section|date = August 2024}}
===Stabilization===
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The stabilization of a foam is caused by [[van der Waals force]]s between the molecules in the foam, [[electrical double layer]]s created by [[Dipole|dipolar]] surfactants, and the [[Marangoni effect]], which acts as a restoring force to the lamellae.

The Marangoni effect depends on the liquid that is foaming being impure. Generally, surfactants in the solution decrease the surface tension. The surfactants also clump together on the surface and form a layer as shown below.

For the Marangoni effect to occur, the foam must be indented as shown in the first picture. This indentation increases local surface area. Surfactants have a larger diffusion time than the bulk of the solution—so the surfactants are less concentrated in the indentation.

Also, surface stretching makes the surface tension of the indented spot greater than the surrounding area. Consequentially—since diffusion time for the surfactants is large—the Marangoni effect has time to take place. The difference in surface tension creates a gradient, which instigates fluid flow from areas of lower surface tension to areas of higher surface tension. The second picture shows the film at equilibrium after the Marangoni effect has taken place.<ref>{{cite web|date=January 2009|work=IHC News|title=The Foam|url= http://www.clariant.com/C12575E4001FB2B8/vwLookupDownloads/2009_01_IHC_Newsletter_Defoaming.pdf/$FILE/2009_01_IHC_Newsletter_Defoaming.pdf|access-date=2013-12-09|url-status=dead|archive-url= https://web.archive.org/web/20131212090241/http://www.clariant.com/C12575E4001FB2B8/vwLookupDownloads/2009_01_IHC_Newsletter_Defoaming.pdf/$FILE/2009_01_IHC_Newsletter_Defoaming.pdf |archive-date=2013-12-12}}</ref>

[[Curing (chemistry)|Curing]] a foam solidifies it, making it indefinitely stable at STP.<ref>{{cite book | url=https://books.google.com/books?id=FXAM3eBpZ0kC&dq=info:LfvRsPG2-6QJ:scholar.google.com/&pg=PT10 | title=Foam Engineering: Fundamentals and Applications | isbn=9781119961093 | last1=Stevenson | first1=Paul | date=3 January 2012 | publisher=John Wiley & Sons }}</ref>

===Destabilization===
Witold Rybczynski and Jacques Hadamard developed an equation to calculate the velocity of bubbles that rise in foam with the assumption that the bubbles are spherical with a radius <math>r</math>.

:<math> u=\frac{2gr^2}{9\eta_2}(\rho_2-\rho_1)\left (\frac{3\eta_1+3\eta_2}{3\eta_1+2\eta_2}\right)\!</math>

with velocity in units of centimeters per second. ρ<sub>1</sub> and ρ<sub>2</sub> is the density for a gas and liquid respectively in units of g/cm<sup>3</sup> and ῃ<sub>1</sub> and ῃ<sub>2</sub> is the [[Viscosity#Dynamic viscosity | dynamic 
 viscosity]] of the gas and liquid respectively in units of g/cm·s and g is the [[Gravitational acceleration | acceleration of gravity]] in units of cm/s<sup>2</sup>.

However, since the density and viscosity of a liquid is much greater than the gas, the density and viscosity of the gas can be neglected, which yields the new equation for velocity of bubbles rising as:
:<math> u=\frac{gr^2}{3\eta_2}(\rho_2)\!</math>

However, through experiments it has been shown that a more accurate model for bubbles rising is:
:<math> u=\frac{2gr^2}{9\eta_2}(\rho_2-\rho_1)\!</math>

Deviations are due to the [[Marangoni effect]] and capillary pressure, which affect the assumption that the bubbles are spherical.
For laplace pressure of a curved gas liquid interface, the two principal radii of curvature at a point are R<sub>1</sub> and R<sub>2</sub>.<ref>Wilson, A.J, "Principles of Foam Formation and Stability." ''Foams: Physics, Chemistry, and Structure''. New York, Springer-Verlag, 1989, ch 1</ref> With a curved interface, the pressure in one phase is greater than the pressure in another phase. The capillary pressure P<sub>c</sub> is given by the equation of:

:<math> P_c=\gamma\left (\frac{1}{R_1}+\frac{1}{R_2}\right)\!</math>,
where <math>\gamma</math> is the surface tension. The bubble shown below is a gas (phase 1) in a liquid (phase 2) and point A designates the top of the bubble while point B designates the bottom of the bubble. 

[[File:Bubble for Hydrostatic Pressure.svg|thumb|Bubble for hydrostatic pressure]]
At the top of the bubble at point A, the pressure in the liquid is assumed to be p<sub>0</sub> as well as in the gas. At the bottom of the bubble at point B, the hydrostatic pressure is:
:<math> P_B,1=p_0+g\rho_1z\!</math>
:<math> P_B,2=p_0+g\rho_2z\!</math>
where ρ<sub>1</sub> and ρ<sub>2</sub> is the density for a gas and liquid respectively. The difference in hydrostatic pressure at the top of the bubble is 0, while the difference in hydrostatic pressure at the bottom of the bubble across the interface is ''gz''(''ρ''<sub>2</sub>&nbsp;−&nbsp;''ρ''<sub>1</sub>). Assuming that the radii of curvature at point A are equal and denoted by R<sub>A</sub> and that the radii of curvature at point B are equal and denoted by R<sub>B</sub>, then the difference in capillary pressure between point A and point B is:

:<math> P_c=2\gamma\left (\frac{1}{R_A}-\frac{1}{R_B}\right)\!</math>
At equilibrium, the difference in capillary pressure must be balanced by the difference in hydrostatic pressure. Hence,
:<math> gz(\rho_2-\rho_1)=2\gamma\left (\frac{1}{R_A}-\frac{1}{R_B}\right)\!</math>
Since, the density of the gas is less than the density of the liquid the left hand side of the equation is always positive. Therefore, the inverse of R<sub>A</sub> must be larger than the R<sub>B</sub>. Meaning that from the top of the bubble to the bottom of the bubble the radius of curvature increases. Therefore, without neglecting gravity the bubbles cannot be spherical. In addition, as z increases, this causes the difference in R<sub>A</sub> and R<sub>B</sub> too, which means the bubble deviates more from its shape the larger it grows.<ref name=autogenerated1 />

Foam destabilization occurs for several reasons. First, [[gravitation]] causes drainage of liquid to the foam base, which Rybczynski and Hadamar include in their theory; however, foam also destabilizes due to [[osmotic pressure]] causes drainage from the lamellas to the Plateau borders due to internal concentration differences in the foam, and [[Laplace pressure]] causes diffusion of gas from small to large bubbles due to pressure difference. In addition, films can break under [[disjoining pressure]], These effects can lead to rearrangement of the foam structure at scales larger than the bubbles, which may be individual ([[T1 process]]) or collective (even of the "avalanche" type).

==Mechanical properties==
===Liquid foams===
{{expand section|with=as thorough a treatment as is included in the solid foam subsection|small = no|date = August 2024}}
===Solid foams===
Solid foams, both open-cell and closed-cell, are considered as a sub-class of cellular structures. They often have lower nodal connectivity{{jargon-inline|date=February 2024}} as compared to other cellular structures like honeycombs and truss lattices, and thus, their failure mechanism is dominated by bending of members. Low nodal connectivity and the resulting failure mechanism ultimately lead to their lower mechanical strength and stiffness compared to honeycombs and truss lattices.<ref>{{cite journal|last1=Queheillalt|first1=Douglas T.|last2=Wadley|first2=Haydn N.G.|title=Cellular metal lattices with hollow trusses|journal=Acta Materialia|date=January 2005|volume=53|issue=2|pages=303–313|doi=10.1016/j.actamat.2004.09.024|bibcode=2005AcMat..53..303Q}}</ref><ref>{{cite journal|last1=Kooistra|first1=Gregory W.|last2=Deshpande|first2=Vikram S.|last3=Wadley|first3=Haydn N.G.|title=Compressive behavior of age hardenable tetrahedral lattice truss structures made from aluminium|journal=Acta Materialia|date=August 2004|volume=52|issue=14|pages=4229–4237|doi=10.1016/j.actamat.2004.05.039|bibcode=2004AcMat..52.4229K|s2cid=13958881 }}</ref>

The strength of foams can be impacted by the density, the material used, and the arrangement of the cellular structure (open vs closed and pore isotropy).{{fact|date = August 2024}} To characterize the [[mechanical properties]] of foams, compressive [[Stress–strain curve|stress-strain curves]] are used to measure their strength and ability to absorb energy since this is an important factor in foam based technologies.{{fact|date = August 2024}} 

====Elastomeric foam====
For [[Elastomer|elastomeric]] cellular solids, as the foam is compressed, first it behaves elastically as the cell walls bend, then as the cell walls buckle there is yielding and breakdown of the material until finally the cell walls crush together and the material ruptures.<ref name=":0">{{Cite book |last=Courtney |first=Thomas H. |title=Mechanical Behavior of Materials |publisher=Waveland Press, Inc |year=2005 |isbn=1-57766-425-6 |pages=686–713 |language=English}}</ref> This is seen in a stress-strain curve as a steep linear elastic regime, a linear regime with a shallow slope after yielding (plateau stress), and an exponentially increasing regime. The stiffness of the material can be calculated from the linear elastic regime <ref> [https://hal.science/hal-02344039  M. Liu et al. Multiscale modeling of effective elastic properties of fluid-filled porous materials] International Journal of Solids and Structures (2019) 162, 36-44 </ref> where the [[Elastic modulus|modulus]] for open celled foams can be defined by the equation: 
[[File:Compressive Stress Strain Curve of Elastomeric Foams.jpg|thumb|297x297px|Schematic stress-strain curve of an elastomeric foam, demonstrating the three regions which are linear elastic, cell-wall buckling, and cell-wall fracture, where the area under the curve specified represents the energy per unit volume the foam can absorb.{{cn|date = August 2024}}]]
<math>\left ( \frac{E^*}{E_s} \right )_f = C_f\left ( \frac{\rho^*}{\rho_s} \right )^2</math>

where <math>E_s</math> is the modulus of the solid component, <math>E^*</math> is the modulus of the honeycomb structure, <math>C_f</math> is a constant having a value close to one, <math>\rho^*</math> is the density of the honeycomb structure, and <math>\rho_s</math> is the density of the solid. The elastic modulus for closed cell foams can be described similarly by:

<math>\left ( \frac{E^*}{E_s} \right )_f = C_f\left ( \frac{\rho^*}{\rho_s} \right )^3</math>

where the only difference is the exponent in the density dependence. However, in real materials, a closed-cell foam has more material at the cell edges which makes it more closely follow the equation for open-cell foams.<ref>{{Cite journal |last=Ashby |first=M. F. |date=1983 |title=The Mechanical Properties of Cellular Solids |url=https://doi.org/10.1007/bf02645546 |journal=Metallurgical Transactions |volume=14A |issue=9 |pages=1755–1769|doi=10.1007/bf02645546 |bibcode=1983MTA....14.1755A |s2cid=135765088 }}</ref> The ratio of the density of the honeycomb structure compared with the solid structure has a large impact on the modulus of the material. Overall, foam strength increases with density of the cell and stiffness of the matrix material.

====Energy of deformation====
Another important property which can be deduced from the stress strain curve is the energy that the foam is able to absorb. The area under the curve (specified to be before rapid densification at the peak stress), represents the energy in the foam in units of energy per unit volume. The maximum energy stored by the foam prior to rupture is described by the equation:<ref name=":0" /> 

<math>\frac{W_{max}}{E_s} = 0.05\left ( \frac{\rho^*}{\rho_s} \right )^2\left [0.975-1.4 \left ( \frac{\rho^*}{\rho_s} \right ) \right]</math>

This equation is derived from assuming an idealized foam with engineering approximations from experimental results. Most energy absorption occurs at the plateau stress region after the steep linear elastic regime.{{fact|date = August 2024}}<!--THIS IS NOT SKY-IS-BLUE.-->

====Directional dependence====
The [[isotropy]] of the cellular structure and the absorption of fluids can also have an impact on the mechanical properties of a foam. If there is anisotropy present, then the materials response to stress will be directionally dependent, and thus the stress-strain curve, modulus, and energy absorption will vary depending on the direction of applied force.<ref>{{Cite journal |last1=Li |first1=Pei |last2=Guo |first2=Y. B. |last3=Shim |first3=V. P. W. |date=2020 |title=A rate-sensitive constitutive model for anisotropic cellular materials - Application to a transversely isotropic polyurethane foam |url=https://doi.org/10.1016/j.ijsolstr.2020.08.007 |journal=International Journal of Solids and Structures |volume=206 |pages=43–58 |doi=10.1016/j.ijsolstr.2020.08.007 |s2cid=225310568 |via=Elsevier}}</ref> Also, open-cell structures which have connected pores can allow water or other liquids to flow through the structure, which can also affect the rigidity and energy absorption capabilities.<ref>{{Cite journal |last1=Yu |first1=Y. J. |last2=Hearon |first2=K. |last3=Wilson |first3=T. S. |last4=Maitland |first4=D. J. |date=2011 |title=The effect of moisture absorption on thephysical properties of polyurethane shapememory polymer foams |url=http://dx.doi.org/10.1088/0964-1726/20/8/085010 |journal=Smart Materials and Structures |volume=20|issue=8 |doi=10.1088/0964-1726/20/8/085010 |pmid=21949469 |pmc=3176498 |bibcode=2011SMaS...20h5010Y }}</ref>

==Applications==
===Liquid foams===
Liquid foams can be used in [[fire retardant foam]], such as those that are used in extinguishing fires, especially [[oil fire]]s.{{fact|date = August 2024}}

The dough of leavened [[bread]] has traditionally been understood as a closed-cell foam—[[yeast]] causing bread to rise via tiny bubbles of gas that become the bread [[Pores (bread)|pores]]—where the cells do not connect with each other. Cutting the dough releases the gas in the bubbles that are cut, but the gas in the rest of the dough cannot escape.{{fact|date = August 2024}}<!--One primary source does not support the broad generalisation, traditionally.--> When dough is allowed to rise too far, it becomes an open-cell foam, in which the gas pockets are connected; cutting the dough surface at that point would cause a large volume of gas to escape, and the dough to collapse.{{fact|date = August 2024}}<ref>The open structure of an over-risen dough is easy to observe: instead of consisting of discrete gas bubbles, the dough consists of a gas space filled with threads of the flour-water paste.{{OR|date = August 2024}}{{fact|date = August 2024}}</ref> Recent research has indicated that the pore structure in bread is 99% interconnected into one large vacuole, thus the closed-cell foam of the moist dough is transformed into an open cell solid foam in the bread.<ref>{{cite journal|title=It's a maze: The pore structure of bread crumbs|last1=Wang|first1=Shuo|last2=Austin|first2=Peter|last3=Chakrabati-Bell|first3=Sumana|date=2011|journal=Journal of Cereal Science|volume=54|issue=2|pages=203–210|doi=10.1016/j.jcs.2011.05.004}}</ref>{{primary source inline|date = August 2024}}

The unique property of gas-liquid foams having very high specific surface area is exploited in the chemical processes of [[froth flotation]] and [[foam fractionation]].{{fact|date = August 2024}}

====Depopulation====
[[Foam depopulation]] or foaming is a means of mass killing farm animals by spraying foam over a large area to obstruct breathing and ultimately cause suffocation.<ref>{{Cite web |date=2007-06-10 |title=New weapon in war on bird flu: tiny bubbles |url=https://www.nbcnews.com/id/wbna19036479 |access-date=2024-01-21 |website=NBC News |language=en}}</ref><ref>{{Cite journal |last1=Benson |first1=E.R. |last2=Alphin |first2=R.L. |last3=Dawson |first3=M.D. |last4=Malone |first4=G.W. |date=May 2009 |title=Use of water-based foam to depopulate ducks and other species |url=https://linkinghub.elsevier.com/retrieve/pii/S0032579119389229 |journal=Poultry Science |language=en |volume=88 |issue=5 |pages=904–910 |doi=10.3382/ps.2008-00268|pmid=19359675 }}</ref> It is usually used to attempt to stop disease spread.<ref>{{Cite news |last=Bolotnikova |first=Marina |date=2022-03-08 |title='They're cooking them alive': calls to ban 'cruel' killing methods on US farms |url=https://www.theguardian.com/environment/2022/mar/08/theyre-cooking-them-alive-calls-to-ban-cruel-killing-methods-on-us-farms |access-date=2024-01-21 |work=The Guardian |language=en-GB |issn=0261-3077}}</ref>

===Solid foams===
{{see |Polymeric foam}}
{{multiple issues|section = yes|
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Solid foams are a class of lightweight cellular engineering materials. These foams are typically classified into two types based on their pore structure: open-cell-structured foams (also known as [[reticulated foam]]s) and closed-cell foams. At high enough cell resolutions, any type can be treated as continuous or "continuum" materials and are referred to as [[cellular solid]]s,<ref name=Gibson>{{Cite book|title=Cellular Solids: Structure and Properties|last=Gibson|first=Ashby|publisher=Cambridge University Press|year=1999|isbn=9781316025420|location=Cambridge, UK}}</ref> with predictable mechanical properties.

[[File:Metal Foam in Scanning Electron Microscope, magnification 10x.GIF|thumb|An open-cell metal foam]]
Open-cell foams can be used to filter air. For example, a foam embedded with catalyst has been shown to catalytically convert [[formaldehyde]] to benign substances when formaldehyde polluted air passes through the open cell structure.<ref>{{Cite journal |last1=Carroll |first1=Gregory T. |last2=Kirschman |first2=David L. |date=2023-01-23 |title=Catalytic Surgical Smoke Filtration Unit Reduces Formaldehyde Levels in a Simulated Operating Room Environment |url=https://pubs.acs.org/doi/10.1021/acs.chas.2c00071 |journal=ACS Chemical Health & Safety |language=en |volume=30 |issue=1 |pages=21–28 |doi=10.1021/acs.chas.2c00071 |s2cid=255047115 |issn=1871-5532}}</ref>

Open-cell-structured foams contain pores that are connected to each other and form an interconnected network that is relatively soft. Open-cell foams fill with whatever gas surrounds them. If filled with air, a relatively good insulator results, but, if the open cells fill with water, insulation properties would be reduced. Recent studies have put the focus on studying the properties of open-cell foams as an insulator material. Wheat gluten/TEOS [[Biofoams|bio-foams]] have been produced, showing similar insulator properties as for those foams obtained from oil-based resources.<ref>{{cite journal|last1=Wu|first1=Qiong|last2=Andersson|first2=Richard L.|last3=Holgate|first3=Tim|last4=Johansson|first4=Eva|last5=Gedde|first5=Ulf W.|last6=Olsson|first6=Richard T.|last7=Hedenqvist|first7=Mikael S.|date=2014|title=Highly porous flame-retardant and sustainable biofoams based on wheat gluten and in situ polymerized silica|journal=Journal of Materials Chemistry A|volume=2|issue=48|at=20996–21009|doi=10.1039/C4TA04787G|doi-access=free}}</ref> Foam rubber is a type of open-cell foam.

[[File:Closed cell metal foam with large cell size.JPG|thumb|A closed-cell metal foam]]
Closed-cell foams do not have interconnected pores. The closed-cell foams normally have higher compressive strength due to their structures. However, closed-cell foams are also, in general more dense, require more material, and as a consequence are more expensive to produce. The closed cells can be filled with a specialized gas to provide improved insulation. The closed-cell structure foams have higher dimensional stability, low moisture absorption coefficients, and higher strength compared to open-cell-structured foams. All types of foam are widely used as core material in [[sandwich-structured composite]] materials.

The earliest known engineering use of cellular solids is with wood, which in its dry form is a closed-cell foam composed of lignin, cellulose, and air. From the early 20th century, various types of specially manufactured solid foams came into use. The low [[density]] of these foams makes them excellent as thermal [[thermal insulation|insulators]] and flotation devices and their lightness and compressibility make them ideal as packing materials and stuffings.

An example of the use of azodicarbonamide<ref>{{cite journal|last1=Reyes-Labarta|first1=J.A.|last2=Marcilla|first2=A.|title=Kinetic Study of the Decompositions Involved in the Thermal Degradation of Commercial Azodicarbonamide|journal=Journal of Applied Polymer Science|date=2008|volume=107|issue=1|pages=339–346|doi=10.1002/app.26922|hdl=10045/24682|hdl-access=free}}</ref> as a [[blowing agent]] is found in the manufacture of [[Polyvinyl chloride|vinyl (PVC)]] and [[Ethylene-vinyl acetate|EVA-PE foams]], where it plays a role in the formation of air bubbles by breaking down into gas at high temperature.<ref>{{cite journal|last1=Reyes-Labarta|first1=J.A.|last2=Marcilla|first2=A.|title=Thermal Treatment and Degradation of Crosslinked Ethylene Vinyl Acetate-Polyethylene-Azodicarbonamide-ZnO Foams. Complete Kinetic Modelling and Analysis|journal=Industrial & Engineering Chemistry Research|date=2012|volume=51|issue=28|pages=9515–9530|doi=10.1021/ie3006935}}</ref><ref>{{cite journal|last1=Reyes-Labarta|first1=J.A.|last2=Marcilla|first2=A.|title=Differential Scanning Calorimetry Analysis of the Thermal Treatment of Ternary Mixtures of Ethylene Vinyl Acetate, Polyethylene and Azodicarbonamide|journal=Journal of Applied Polymer Science|date=2008|volume=110|issue=5|pages=3217–3224|doi=10.1002/app.28802|hdl=10045/13312|hdl-access=free}}</ref><ref>{{cite journal|last1=Reyes-Labarta|first1=J.A.|last2=Olaya|first2=M.M.|last3=Marcilla|first3=A.|title=DSC Study of the Transitions Involved in the Thermal Treatment of Foamable Mixtures of PE and EVA Copolymer with Azodicarbonamide|journal=Journal of Applied Polymer Science|date=2006|volume=102|issue=3|pages=2015–2025|doi=10.1002/app.23969|hdl=10045/24680|hdl-access=free}}</ref>

The random or "stochastic" geometry of these foams makes them good for energy absorption, as well. In the late 20th century to early 21st century, new manufacturing techniques have allowed for geometry that results in excellent strength and stiffness per weight. These new materials are typically referred to as engineered cellular solids.<ref name=Gibson/>

====Syntactic foam====
{{Main article|Syntactic foam}}

====Integral skin foam====
<!-- [[Self skin]], [[self-skin foam]] and [[integral skin foam]] redirect here -->
''Integral skin foam'', also known as ''self-skin foam'', is a type of foam with a high-density skin and a low-density core. It can be formed in an ''open-mold process'' or a ''closed-mold process''. In the open-mold process, two reactive components are mixed and poured into an open mold. The mold is then closed and the mixture is allowed to expand and cure. Examples of items produced using this process include [[arm rest]]s, [[baby seat]]s, [[shoe sole]]s, and [[mattress]]es. The closed-mold process, more commonly known as ''[[reaction injection molding]]'' (RIM), injects the mixed components into a closed mold under high pressures.<ref>{{Cite book | last = Ashida | first = Kaneyoshi | title = Polyurethane and related foams: chemistry and technology | publisher = CRC Press | pages = 79–81 | year = 2006 | url = https://books.google.com/books?id=IQUd-3aKSD4C | isbn = 978-1-58716-159-9 | url-status = live | archive-url = https://web.archive.org/web/20170217110121/https://books.google.com/books?id=IQUd-3aKSD4C | archive-date = 2017-02-17 }}</ref>

==Gallery==
<gallery>
<!--Image:Plankton creates sea foam 2.jpg|Close-up of [[sea foam]] ([[decomposition|decomposing]] [[plankton]]) on a [[tide pool]] NO MENTION IN ARTICLE.-->
Image:FoamedPlastic.jpg|[[Micrograph]] of [[temper foam|temper]] (memory) foam
Image:Silikonschaum riesenblase verfuellungsversuch.jpg|[[Silicone foam]] [[Firestop|penetration seal]] {{what|date = August 2024}}
<!--Image:Diet Coke Mentos.jpg|[[Diet Coke and Mentos eruption|Diet Coke and Mentos]] foam "geyser"-->
Image:Foam ball.png|[[Industrial CT scanning]] of a foam ball {{what|date = August 2024}}
Image:Expanded polystyrene foam dunnage.jpg|Polystyrene foam [[cushioning]]
Image:Aluminium foam.jpg|[[metal foam|Foamed]] [[aluminum]] {{what|date = August 2024}}

</gallery>

==Foam scales and properties==
{{Foam_scales_and_properties}}

==See also==
* [[Aluminium foam sandwich]]
* [[Ballistic foam]]
* [[Chaotic bubble]]
* [[Defoamer]]
* [[Foam glass]]
* [[Metal foam]]
* [[Nanofoam]]
* [[Sea foam]]
* [[Reversibly assembled cellular composite materials]]
* [[Foam party]]
* [[Soft matter]]

==References==
{{Reflist|30em}}

==Further reading==
* {{cite book|author = Weaire, D & Hutzler, S | date = 1999 | title = The Physics of Foams | page = | location = Oxford, England | publisher = Oxford University Press | isbn = 0198510977 | url = https://books.google.com/books?id=mEHlZ9ZodFsC | access-date = 30 August 2024 | quote =  }} A modern treatise almost exclusively focused on liquid foams.
* {{cite book|author = Gibson, LJ & Ashby, MF | date = 1997 | title = Cellular Solids: Structure and Properties | page = | series = Cambridge Solid State Science | location = Cambridge, England | publisher = Cambridge University Press | isbn = 0521499119 | url = https://books.google.com/books?id=IySUr5sn4N8C | access-date = 30 August 2024 | quote =  }} A treatise termed a classic by Weaire & Hutzler (1999), on solid foams, and the reason they limit their focus to liquid foams.
* {{cite book|author = [[Isabelle Cantat|Cantat, I]]; Cohen-Addad, S; Elias, F; Graner, F; Höhler, R; Pitois, O; Rouyer, F & Saint-Jalmes, A | date = 2013 | title = Foams: Structure and Dynamics | page = | location = Oxford, England | publisher = Oxford University Press | isbn = 978-0199662890 | url = https://books.google.com/books?id=Ndzzl1hUNRoC | access-date = 30 August 2024 | quote =  }}  Note, this source also focuses on liquid foams.
* Thomas Hipke, Günther Lange, René Poss: Taschenbuch für Aluminiumschäume. Aluminium-Verlag, Düsseldorf 2007, {{ISBN|978-3-87017-285-5}}.
* Hannelore Dittmar-Ilgen: Metalle lernen schwimmen. In: Dies.: Wie der Kork-Krümel ans Weinglas kommt. Hirzel, Stuttgart 2006, {{ISBN|978-3-7776-1440-3}}, S. 74.


==External links==
{{Wiktionary}}
{{Commons category}}
* Andrew M. Kraynik, Douglas A. Reinelt, Frank van Swol [https://web.archive.org/web/20160804103003/http://www.aquafoam.com/papers/Kraynik.pdf Structure of random monodisperse foam]
* {{cite web|last=Moriarty|first=Philip|title=Foam Physics|url=http://www.sixtysymbols.com/videos/foam.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]|year=2010}}

{{Patterns in nature}}

[[Category:Foams| ]]
[[Category:Colloids]]