cience 13 (2008) 134–140www.elsevier.com/locate/cocis

Current Opinion in Colloid & Interface S

Foams and foam films stabilised by solid particles

Tommy S. Horozov ⁎

Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, UK

Received 8 August 2007; accepted 31 October 2007Available online 13 November 2007

Abstract

Recent developments in the field of particle-stabilised aqueous foams and foam films are reviewed. Reports on ultrastable foams stabilised bysolid particles are highlighted and factors responsible for the extraordinary foam stability are discussed in view of the recent experimental andtheoretical results. Mechanisms of foam film stabilisation by solid particles and the role of different factors in the film stability are considered.Link between the film stability and that of particle-stabilised foams is discussed.© 2007 Elsevier Ltd. All rights reserved.

Keywords: Foams; Foam films; Bubbles; Stabilization; Solid particles

1. Introduction

Small solid particles can attach to planar or curved liquidinterfaces and are a subject of considerable attention recently[1••,2•,3•,4]. Their ability to stabilise bubbles and emulsiondroplets was recognised a long time ago, but the potential ofsolid particles alone for stabilising emulsions and foams wasfully demonstrated during the last decade [1••,2•,3•]. Differentaspects of foam stabilisation by solid particles have beenpartially covered in previous reviews [1••,2•,3•]. However,remarkable progress in this field was achieved within the lastcouple of years and several reports dealing with importantaspects of solid-stabilised foams appeared very recently. Anupdate of the current knowledge for the role of solid particles instabilisation of bubbles, aqueous films and foams is given here.The emphasis is on the literature from the last 3 years, thoughsome previous key articles are also referenced for consistency.

2. Ultrastable aqueous foams

Stabilising effect of solid particles in aqueous foams underdynamic conditions at continuous bubble generation iswell known,and its role in froth flotation ofminerals [5,6] or boiling suspensions[7–9] has been extensively investigated. The dynamic foams,

⁎ Tel.: +44 1482 465220; fax: +44 1482 466410.E-mail address: t.s.horozov@hull.ac.uk.

1359-0294/$ - see front matter © 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.cocis.2007.11.009

however, are very unstable and rapidly collapse once the bubblingis terminated.Only very recently it has been demonstrated that solidparticles alone [10••,11••,12] or with appropriate surfactant [13•]are able to stabilise aqueous foams to an extent they can survive forweeks or more even under extremely harsh conditions (e.g. dryingor vacuum treatment). Some of these works reporting ultrastablefoams are worth to be considered in more detail.

Recently, Alargova et al. [10••] have demonstrated thatparticles with non-spherical shape can act as an effective foamstabiliser in the absence of any additives. They used polymerrodlike particles with an average length of 23.5 μm and anaverage diameter of 0.6 μm. Their contact angle at the air–watersurface measured through the water was θ≈80°. It was foundthat fairly dilute microrod suspensions (0.2–2.2 wt.%) in purewater readily produce foams upon shaking. The foams havebeen stable for more than 3 weeks even under drying in an openvessel. It has been extremely difficult to destroy the foam evenat more harsh conditions of fast drying and expansion in avacuum. Microscopic examination revealed that the microrodfoams were made of small (10–100 μm) approximatelyspherical air bubbles. They shape did not change for thewhole period of observation. The bubbles were covered withdense hairy shells of entangled microrods. Similar arrangementof intertwined rods was observed at the surface of single foamfilms. The films were rather thick (∼1–2 μm) and very stablewhich was attributed to the mechanical rigidity of thecontinuous net of overlapping and entangled microrods at the

135T.S. Horozov / Current Opinion in Colloid & Interface Science 13 (2008) 134–140

film surface. The authors concluded that the strong particleattachment to the bubbles, microrod entanglement, formation ofrigid hairy shells, and sustaining of thick films between thebubbles were the main factors for the extraordinary stability ofthe microrod-stabilised foams. This was also supported by theunexpected destabilising effect of a popular foam-formingsurfactant (sodium dodecylsulfate) on the particle-stabilisedfoams. Soon after the surfactant had been introduced, themicrorod particles began to transfer from the foam into theliquid and to sediment at the bottom. As a result more than 70%of the foam had been destroyed within 30 min. The destabilisingaction of the surfactant was attributed to its adsorption on thefairly hydrophobic particle surface, thus making the microrodsmore hydrophilic and less strongly attached to the air–watersurface. These findings suggest that flexible rodlike particleswith high aspect ratio and appropriate hydrophobicity can bevery effective foam stabilisers. Interestingly, hydrophobicfilamentous bacteria, identified in the stable biological foamsin waste water treatment plants, have filament dimensions of∼0.5–2 μm diameter and ∼50–200 μm length [14–16],therefore very similar to those of microrods used in [10••]. Itis believed that the filamentous species attach to and stabilize airbubbles to form thick, stable, persistent and scum-like foam onthe surface of aeration basins and settling tanks, thus causing aserious problem [14–16].

Very stable aqueous foams have also been obtained by meansof spherical micro- or nanoparticles. Binks and Horozov [11••]used near spherical fumed silica nanoparticles (primarydiameter ≈30 nm) hydrophobised to different extents, toinvestigate the effect of particle hydrophobicity on foamstability in the absence of any surfactant. Foams were preparedeither by shaking by hand the system of powdered particlesresting on water (3 w/v%) or by aerating an aqueous dispersionof particles (0.86 w/v%) by using an Ultra Turrax homogeniser.In the latter, the powders had been dispersed in water with theaid of ethanol, which was deliberately removed by severalsedimentation-redispersion cycles in pure water before thefoaming tests. Stable foams were obtained by both methods(either without or with 8.5 mM NaCl in the water) whenparticles with intermediate hydrophobicity were used. Thefoams produced by homogenisation of the aqueous dispersionswere with larger initial volumes than the hand-shaken ones.They also showed a more pronounced maximum with respect tothe hydrophobicity of the particles, with the most effectivebeing those containing at their surface 32% SiOH (about onethird of that at the surface of unmodified hydrophilic particles,100% SiOH). The foams were wet and even after several dayscontained about 60% water. They were very stable to collapse.Similar to the stable micro-rod foams [10••], a slow decrease intheir volume was detected during the first 24 h as a result ofwater drainage and bubble compaction, but not of gas loss.Almost all particles were retained in the white, creamy foam onthe top of a clear liquid below. Optical microscope imagesrevealed that the foam contained micron-sized non-sphericalbubbles (5–50 μm) surrounded by branched particle aggregates.It was suggested that particle aggregation increased theviscosity of the aqueous phase (gelling) which resulted in

slower drainage of the foam films and increased foam stability.The bubble surfaces were rough as a result of ripples similar tothose observed in the case of a planar air–water monolayer ofsilica nanoparticles after compression. It was inferred that thebubbles were covered with dense particle layers compressed to ahigh surface pressure that is close to the surface tension ofwater.

Preparation of stable foams from latex suspensions in theabsence of any additives has been reported recently by Fujiiet al. [12,17]. In contrast to previous studies [18] they usedsterically stabilised latex particles, therefore additional additives(electrolyte or surfactant) were not needed to induce foaming. Aseries of near-monodisperse sterically stabilised and charge-stabilised polystyrene latexes with diameters in the range 0.2–1.6 μm were synthesised by dispersion or emulsion polymer-isation. Foams were prepared from thoroughly purified aqueoussuspensions (1–10 wt.%) either by shaking by hand or bybubbling nitrogen through the suspensions. The most stablefoams were those obtained at the largest concentration ofparticles with biggest size. They were stable to drying with littleor no change in volume. Highly ordered particulate bilayershave been identified in the dry foam by means of SEM andoptical microscopy, thus supporting the previous finding [18]that the films between the bubbles in the wet foam arecomposed of a bilayer of particles separated by water.

Above findings demonstrate that aqueous suspensions ofcertain solid particles with inherent hydrophobicity are able tomake extremely stable foams in the absence of any surfactant.Particle shape, size, concentration and hydrophobicity havebeen identified as the main factors for the foam stabilisation.The optimum particle hydrophobicity has been achieved etherin advance, by appropriate chemical synthesis [10••,12,17] orsurface modification [11••], or after dispersing the particles inthe aqueous phase by adjusting the pH and electrolyteconcentration [18]. Alternatively, an in-situ hydrophobisationof initially hydrophilic particles can be accomplished throughthe adsorption of appropriate amphiphiles on the particlesurface. This approach is widely used in froth flotation [5,6]but its potential and versatility for preparation of ultrastableaqueous foams stabilised by inorganic nanoparticles wasdemonstrated only recently.

Gonzenbach et al. [13•,19,20] applied short-chain amphi-philes (carboxylic acids, alkyl gallates and alkylamines) toconcentrated suspensions (15–45 vol.%) of different oxide andnonoxide particles (e.g. silica, alumina, calcium phosphate etc.)and produced high-volume foams by mechanical frothing of thesuspensions. The foams with air contents between 45 and 90%and average bubble sizes between 20 and 80 μm werecompletely stable against drainage, coalescence and dispropor-tionation for days. This remarkable foam stability was attributedto the strong attachment of particles at the air–water interfaceand the formation of an attractive particle network at the interfaceand throughout the foam lamella. It has also been demonstratedthat the microstructure of these foams can be tailored in a widerange by adjusting the composition of the initial colloidalsuspension [20] that could be useful for various applicationsincluding the fabrication of macroporous ceramics [21].

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It is known that behaviour of solid particles in foam systemscould be remarkably different depending on particle hydro-phobicity. Partially hydrophobic particles with contact angle, θ,close to 90° can act as a foam stabiliser (see above), whereas thehydrophobic particles (θN90°) act in the opposite way and areused as antifoams, usually in combination with oily additives[22,23]. Very hydrophobic particles, however, can stabilisewater droplets in air [24] making them perfectly resistant tocoalescence, therefore a free-flowing powder material contain-ing up to 95% water could be prepared [25]. Binks andMurakami have shown recently [26•] that the transformation ofparticle-stabilised aqueous foam into water-in-air powder(“dry water”), and vice versa, can be achieved in a singlesystem comprised of air, water and fumed silica nanoparticleshydrophobised to different extents. Similar to the phaseinversion in particle-stabilised emulsions [1•] the inversion ofthe air-water-particles system was achieved either by aprogressive change in silica particle hydrophobisity at constantair/water ratio or by changing the air/water ratio at fixed particlewettability. The mechanism behind the phase inversion is stillunclear and is worthy to be further investigated. This interestingwork [26•] clearly demonstrates the similarity in the particlebehaviour in emulsion and foam systems and the ability of solidparticles to stabilise various materials with potential applica-tions in the food, pharmaceutical and cosmetics industries.

Perhaps the most striking finding in the ultrastable particle-stabilised foams is that bubbles do not change their size for avery long time. This does not happen in foams stabilised bysurfactants or proteins, in which coarsening of the bubblesoccurs with time as gas diffuses from smaller to larger bubblesdue to the higher Laplace pressure within the former [2•,27]. Asa result larger bubbles grow at the expense of smaller oneswhich shrink and eventually disappear.

Stabilisation of bubbles against shrinking by solid particleshas been investigated both theoretically and experimentally inseveral papers. Kam and Rossen [28] analyse theoretically thechange of the Laplace pressure during dissolution of a bubblecovered with incompressible particle monolayer by using a two-dimensional model. It is found that the liquid surface around theparticles changes its curvature from convex to flat or evenconcave depending on the reduction of the gas volume. Hencethe Laplace pressure decreases as the bubble dissolves, thusdiminishing or shutting down completely the gas transfer. At thesame time, the incompressible particle armour around thebubble remains intact, thus accumulating a significant stress. Itis shown that particle ejection from the stressed armour isenergetically unfavourable and argued that particles may sinterinto a rigid shell, thus making the bubble shrinking impossible.

These predictions are supported by the results from recentexperiments with single bubbles under a planar air–watersurface [29•–31]. They show that bubbles with radii 50–100 μm generated in aqueous suspensions of hidrophobisedfumed silica nanoparticles in the presence of salt can becompletely stable to shrinking for days, in contrast to the proteinstabilised bubbles which shrink and disappear within two hours[29•,30]. It is demonstrated that particles form a dense layeraround the bubbles [29•]. In addition, formation of a weak gel

with a finite yield stress is observed in some suspensions at highsalt concentration [30,31]. It is argued that both the denseparticle layer around the bubbles and gel in the bulk contributeto the enormous stability of the bubbles to shrinking.

Further progress in understanding stabilisation of bubbles bysolid particles has been made recently due to work of Stone'sgroup in Harvard [32•–35••–37]. A method for targeteddelivery of colloidal particles to the surface of bubbles or dropsby means of a designed three-channel hydrodynamic focusingdevice is reported [32•]. The method allows direct controlledassembly of jammed particle monolayer at the bubble surfacewithout the aid of any additives (electrolyte or surfactant).Armoured bubbles covered with 4 μm polystyrene latex par-ticles have been used to investigate their stability to shrinking[35••]. When the bubble is placed in a drop below the air–watersurface, the particle shell deforms to produce a distinct flat facetin the contact region between the bubble and the water surfaceby forming a bridge between the air phase in the bubble and theatmosphere with a thin layer of water in between. Microscopeobservations reveal that the bubble changes its shape by losingsome gas, thus becoming progressively non-spherical. The non-spherical bubble then remains stable, keeping its volume andshape for at least 2 days. It is concluded that the crumpledparticle shell around the stable bubble is behaving in a solidlikemanner because equilibrium non-spherical shapes of ordinarybubbles with an isotropic surface tension are prohibited.Surfactant added to the water destabilises the non-sphericalbubbles and they progressively shrink by ejecting particles intowater. The shape of the shrinking bubbles is crumpled atintermediate surfactant concentrations or spherical at largerconcentrations near the cmc, thus demonstrating that the particleshell is stressed. Very recent numerical simulations that mimicdissolution of armoured bubbles [37] confirm that these bubblesstabilise in faceted or crumpled shapes. This is accompanied bysimultaneous decrease of the surface energy and Laplace pres-sure until a local energy minimum is reached. This metastableequilibrium state is characterized by a mostly saddle-shapedgas-liquid interface with zero mean curvature, hence vanishingLaplace pressure. The latter can explain the enormous stabilityto shrinking of armoured bubbles observed experimentally. Thesame stabilising mechanism could operate in the ultrastableparticle-stabilised foams reported recently.

3. Mechanisms of foam film stabilisation by solid particles

Thinning and stability of the aqueous films separating thefoam bubbles are crucial for the coalescence and foam collapse.The link between the stability of aqueous foams and that ofisolated foam films stabilised by surfactants or proteins has beenextensively investigated and is well documented [38,39]. Muchless is known about the foam films stabilised solely by solidparticles in the absence of surface-active molecules. Twodistinct cases with respect to the particle location can beconsidered: (i) the particles are present only inside the film butnot at its surfaces and (ii) at least part of the particles are firmlyattached to the film surfaces i.e. to the vapour-water interface. Inthe first case, solid particles at sufficiently high concentrations

Fig. 2. Possible mechanisms of liquid film stabilisation by: (a) a monolayer ofbridging particles; (b) a bilayer of close-packed particles and (c) a network ofparticle aggregates (gel) inside the film.

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can form a layered structure inside the thinning film, thusstabilising it by the so called oscillatory structural force. Thismechanism of film stabilisation by near monodisperse solidparticles and colloids (e.g. surfactant micelles and macro-molecules) has been extensively studied during the last decadeand covered in recent reviews [1••,2•,40]. The second case offoam films with particles at their surfaces in the absence ofsurfactant is considered only in a few recent experimentalreports dealing with microrod particles [10••] and complexmulticomponent particulate systems simulating radioactivewaste slurries [8,9]. Lack of systematic experimental data forwell defined model systems obscures the mechanisms behindthe enormous stability of the surfactant-free particle-stabilisedfoams. To shed some light on this issue, some recent findingsfor emulsion films stabilised by particles can be invoked. Onecan expect that the behaviour of solid particles in the aqueousemulsion films (oil–water–oil, o–w–o) is similar to that in thefoam films (gas–water–gas). Therefore the stabilising mechan-isms identified in aqueous emulsion films could be relevant toaqueous foam films as well.

Recently, Horozov et al. [41•] have reported a systematicstudy on vertical emulsion films with 3 μm silica particlemonolayers at their surfaces in the absence of any surfactant.Water films in octane (o–w–o) are formed by crossing a particlemonolayer at the oil–water interface with a circular glass framefrom the water side. The thick films are forced to thin bysucking water out of the meniscus by a syringe, while beingobserved with a horizontal microscope. It is found that in filmswith dilute monolayers at their surfaces, the partially hydro-phobic particles (θ=65° or 85°) are expelled from the filmcentre toward its periphery, giving a dimple surrounded by aring of particles bridging the film surfaces (Fig. 1). The thickercentral part of the films (the dimple) gradually flattens andeventually ruptures within less than 30 min. Water films withclose-packed particle monolayers at their surfaces have beenmuch more stable. The particle monolayers remain intact duringthe thinning until a spot of bilayer of sticking particles from theopposite film surfaces is formed in the central thinnest film

Fig. 1. (left) Image of a vertical water film in octane (o–w–o) with a ring ofbridging partially hydrophobic silica particles with diameter of 3 μm and contactangle of 65° [41•]. The particles outside the ring sediment with time, as seen inthe lower part of the image. (right) Schematic cross section of the film withbridging particles and a central dimple formed during the fast initial filmthinning. Arrows show the direction of the liquid flow and that of the draggedparticles.

region. Further suction of water out of the film meniscus causessome rearrangement of the particles, thus transforming thebilayer into a monolayer of particles bridging the opposite filmsurfaces in the centre of the film. Stable water films with morehydrophobic particles (θN90°) have not been observed. It isconcluded that the lateral mobility of particles at the filmsurfaces plays a very important role in the film stabilisation bysolid particles. Particles in dilute monolayers cannot resist thehydrodynamic flow inside the thinning film and are draggedaway from the film centre, thus leaving the thinnest part of thefilm unprotected and vulnerable to rupture. In contrast, close-packed particle monolayers at the film surfaces can oppose thedrag, thus slowing down the film thinning and preventing thefilm rupture by a stable bilayer or a bridging monolayer formedat the final stage of thinning. Ordered monolayers of bridgingparticles in foam films in the presence of surfactant have alsobeen observed previously [42]. Similar film stabilisation by abridging monolayer or a bilayer of close-packed particles wasalso observed in a recent, yet unpublished work on surfactant-free foam films with silica particle monolayers at their surfaces(BP Binks, DA Braz, JH Clint, TS Horozov, unpublished data).This suggests that the behaviour of solid particles in aqueousemulsion and foam films could be rather similar.

Several recent works [43–46•] treat the problem of liquidfilm stability by solid particles theoretically, assuming either abridging monolayer or a bilayer of hexagonally close-packedparticles (Fig. 2a,b). The stability of the film in both cases isdetermined by the maximum capillary pressure, Pc

max, whichcan be resisted by the liquid menisci around the particles assuggested earlier [47•]. In order to obtain expressions for Pc

max,

Fig. 4. Possible mechanisms of rupture of a water film stabilised by a bilayer ofparticles: (B–C) direct rupture without rearrangement of the particles [43–46•],(B–M–C) via bilayer-to-monolayer transition [41•] and (B–V–C) via voidformation [51].

138 T.S. Horozov / Current Opinion in Colloid & Interface Science 13 (2008) 134–140

different approximations for the shape of the pores between theparticles are used. The pores are often approximated by a toroidwith a tube radius equal to the particle radius, a, and a holeradius equal to that of a circle inscribed between three close-packed particles [41•,48,49]. A recent review on this subject byKaptay [46•] is worth to consider. There, previous and morerecent theoretical results are analysed and semi-empiricalequations for the dependence of Pc

max on the particle contactangle, θ are obtained with the toroidal pore model. Resultscalculated by these equations are compared in Fig. 3 where thedimensionless maximum capillary pressure Pc

max=Pcmax a/2γ

(γ is the interfacial tension) is plotted versus θ. The trends ofPc

max(θ)in both monolayer and bilayer cases are similar and inagreement with previous findings [47•]. Some distinctdifferences, however, need a comment.

The maximum capillary pressure for rupture of the film witha bridging monolayer drops to zero at θ=90° (Fig. 3, dashedline), thus suggesting that particles with contact angles equal toor greater than 90° should not be able to stabilise water films bya bridging monolayer mechanism [46•,47•]. This is in accordwith the experimental findings [22,23,41•]. However, hydro-phobic particles with contact angles smaller than ∼129° shouldbe able to stabilise aqueous films by a close-packed bilayer,because Pc

max is greater than zero up to that value of θ (solidline) [46•]. Recent numerical calculations suggest that this limitcould be shifted to even higher contact angles of ∼170° [49].These are interesting results, but conclusive data for foam filmsor foams stabilised solely by hydrophobic particles (θ≥90°)have not yet been reported, though there are such data forparticle-stabilised emulsions [50].

Some recent experimental studies with emulsion filmsconcern with the mechanism of rupture of liquid films stabilisedby a bilayer of close-packed particles [41•,51]. They suggestalternative mechanisms to that of direct film rupture (i.e.

Fig. 3. Contact angle dependence of the dimensionless maximum capillarypressure, Pc

max, for rupture of a water film stabilised by spherical hexagonallyclose-packed particles. Pc

max is calculated with the toroidal pore model in thecase of a bridging monolayer (dashed line, Eq. 2d in [46•]) or a bilayer (solidline and circles). The solid line is calculated by eq. 6e (θb90°) and Eq. 6f(θ≥90°) obtained in [46•]. The circles are calculated by the exact analyticalequation (Eq. (2), [41•]).

without rearrangement of the particles in the bilayer) which isassumed in the theory [43–46•]. These experiments show thatat least two other mechanisms exist due to the lateral mobility ofthe film surfaces (Fig. 4). The mechanism of film rupture viabilayer-to-monolayer transition [41•] occurs in two steps: aparticle rearrangement into a film with bridging monolayerfollowed eventually by its rupture. Alternatively, if the particlerearrangement is difficult due to strong cohesion, the film couldbreak via formation of a void (crack) inside the bilayer followedby a rupture of the unprotected by particles region [51]. Bothtwo-step mechanisms of film rupture occur at a lower criticalcapillary pressure than the direct mechanism, therefore theymight be important for the collapse of the particle-stabilisedfoams. Further investigation is needed to clarify this issue.

Another mechanism of foam film stabilisation by a networkof particle aggregates (gel) inside the film has also beendiscussed [46•] (Fig. 2c). It occurs when the excess particles inthe bulk aqueous phase are flocculated and form three-dimensional network (gel). This mechanism has been calledto explain the high stability of particle-stabilised foams andbubbles reported recently [11••,13•19,20,30,31]. This seems tobe the most effective mechanism of stabilisation because theparticle network keeps the bubbles well separated, thuspreventing the coalescence and drainage.

Link between the stability of particle-stabilised aqueousfilms and that of particle-stabilised foams or oil–in–water (o/w)emulsions has also been discussed [46•,47•]. Both models offilm stabilisation by a bridging monolayer and a bilayer predictincrease of the film stability as the contact angle decreases to 0°(see Fig. 3), thus suggesting that very hydrophilic particles(θ≈0°) should stabilise aqueous films better than morehydrophobic ones. This is in apparent contradiction to theexperimental finding that the stability of particle-stabilisedfoams [1••,11••,26•] and o/w emulsions [1••,50] is lowest withhydrophilic particles. It is argued [46•,47•] that the filmstability should be taken in conjunction with the strength ofparticle attachment to the liquid interface when the stability of

139T.S. Horozov / Current Opinion in Colloid & Interface Science 13 (2008) 134–140

foams (emulsions) is considered. Solid particles with smallercontact angles stabilise the film better but attach to the bubbles(drops) weaker, hence maximum foam (emulsion) stabilityshould be reached at some optimum contact angle. Semi-quantitative arguments have been used to estimate the optimumcontact angles for the highest foam (o/w emulsion) stability[46•]. It is found that the optimum contact angles should be∼70 and ∼86° for the bridging monolayer and bilayermechanisms of film stabilisation, respectively.

The situation with the effect of particle size on the film andfoam stability looks similar to that with the contact angle effectoutlined above. Smaller particles should stabilise the film better(Pc

max∼Pcmax/a) but their attachment to the liquid surface is

weaker and vice versa [46•,47•]. Therefore to stabilise the foamparticles should not be very small or too big. The particle sizerange for foam stabilisation deduced from the experimentalfindings is between several tens of nanometres and severalmicrometers [10••,11••,17,20].

4. Conclusions

Remarkable progress in preparation and understanding ofparticle-stabilised foams is achieved within the last couple ofyears. It is demonstrated that certain solid particles alone orwith appropriate surfactant can be very effective stabilisers ofaqueous foams that are completely stable to collapse, coalescenceand disproportionation even under drying or vacuum treatment.Particle shape, size, concentration and hydrophobicity areidentified as the main factors for the foam superstabilisation.The enormous stability of particle-stabilised foams results fromthe interplay between the ability of the particles to form densecoherent particle shells around the bubbles, to stabilise the liquidfilms separating the bubbles and to form a three-dimensionalnetwork in the bulk aqueous phase. To develop methods andapproaches for control of these factors is the main challenge forthe future research. Due to their strong attachment to fluidinterfaces, the non-spherical or biphilic (‘Janus’) particles seempromising candidates for fabrication of ultrastable foams withpossible application in the food, cosmetics and ceramic industries.

Acknowledgements

The author thanks the EPSRC for the Advanced ResearchFellowship grant EP/D07214X/1.

References and recommended reading

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Murray BS, Ettelaie R. Foam stability: proteins and nanoparticles. CurrOpin Colloid Interface Sci 2004;9:314–20. A good review on the effects of

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