Tracking the interfacial dynamics of PNiPAM soft microgels particles adsorbed at the air–water interface and in thin liquid films

Cohin, Yann; Fisson, Maelle; Jourde, Kévin; Fuller, Gerald G.; Sanson, Nicolas; Talini, Laurence; Monteux, Cécile

doi:10.1007/s00397-013-0697-3

Cited by 65 publications

(85 citation statements)

References 53 publications

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“…Equations (6,7,9,10) show that u r /R and u θ /R only depend on the two dimensionless parameters R/L = RE/γ p and γ p /γ ow . This dependence is shown in Figure 4 for u r /R with ν = 1/2.…”

Section: Elastic Deformation Of Neutrally-wetting Spheresmentioning

confidence: 99%

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Adsorption of soft particles at fluid interfaces

2015

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Soft particles can be better emulsifiers than hard particles because they stretch at fluid interfaces. This deformation can increase adsorption energies by orders of magnitude relative to rigid particles. The deformation of a particle at an interface is governed by a competition of bulk elasticity and surface tension. When particles are partially wet by the two liquids, deformation is localized within a material-dependent distance L from the contact line. At the contact line, the particle morphology is given by a balance of surface tensions. When the particle radius R L, the particle adopts a lenticular shape identical to that of an adsorbed fluid droplet. Particle deformations can be elastic or plastic, depending on the relative values of the Young modulus, E, and yield stress, σp. When surface tensions favour complete spreading of the particles at the interface, plastic deformation can lead to unusual fried-egg morphologies. When deformable particles have surface properties that are very similar to one liquid phase, adsorption can be extremely sensitive to small changes of their affinity for the other liquid phase. These findings have implications for the adsorption of microgel particles at fluid interfaces and the performance of stimuli-responsive Pickering emulsions.Emulsions are vital in many fields, including foods, cosmetics, pharmaceuticals, and oil recovery [1][2][3][4]. Emulsions are typically stabilised by molecular surfactants, but they can also be stabilised with microparticles [5,6]. The resulting Pickering emulsions have many benefits over regular emulsions. In particular, they are highly stable and avoid the use of potentially harmful or irritating surfactants [5].A recent development is the use of soft colloidal particles (typically microgels) to make Pickering emulsions [6][7][8][9]. These form easily [6,10,11], and can be tuned in situ, for instance by altering solvent properties, such as temperature and pH [12][13][14][15][16][17]. This has been demonstrated with emulsions created with poly(Nisopropylacrylamide) (pNIPAM) particles. Since pNI-PAM undergoes a reversible swelling/shrinking transition at a temperature close to body temperature, they have potential for stimuli-responsive release of encapsulated active ingrediants for drug delivery [6].A key difference between soft and hard particles as Pickering emulsifiers is that soft particles stretch as they adsorb [8,18]. For example, microgel particles can have a much larger diameter at the interface than they do in the bulk. The extent of stretching depends on the crosslinking ratio of the hydrogel [18,23,24]. Figure 1 shows a side view of a p(NIPAM) microgel at a water/n-decane interface using cryo-SEM after freeze-fracture [25]. While these soft particles have a spherical shape in solution, they are strongly deformed at the interface. The central region of the particle remains somewhat spherical, especially the part exposed to the bulk water phase. However, the particles are pulled strongly outward at the contact line, and a thin film of ...

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Section: Elastic Deformation Of Neutrally-wetting Spheresmentioning

confidence: 99%

“…These form easily [6,10,11], and can be tuned in situ, for instance by altering solvent properties, such as temperature and pH [12][13][14][15][16][17]. This has been demonstrated with emulsions created with poly(Nisopropylacrylamide) (pNIPAM) particles.…”

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confidence: 99%

Adsorption of soft particles at fluid interfaces

2015

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“…the bulk elastic modulus, of the particles PNIPAM shell. The bulk elastic modulus of the particles, in turn, is expected to be proportional to the dilational modulus of fluid interfaces covered by core-shell particles [44]. We found an inverse relation between the dilational response of a particle covered air-water interface and the pH of the bulk phase.…”

Section: Tuning the Dilational Response Of Particle-laden Fluid Intermentioning

confidence: 66%

“…The compressional modulus of the interface is presumably linked to the mechanical properties of the adsorbed material [44]. In the limit of large compressional strains (>> 40% starting from the point where the surface pressure starts to diverge) we thus expected the hard core to influence the response of the core-shell particles.…”

Section: 22mentioning

confidence: 99%

“…Brugger et al could show that charged microgels can form an entangled network at the oil-water interface which yields a viscoelastic interfacial layer. Upon pH induced protonation of charged groups inside the microgels, particles became more dense as well as less connected, their performance as interfacial stabilizer deteriorated [37,44,45]. Upon protonation of charged groups, also the elastic response of the interface as probed by dilational rheology decreased [39].…”

Section: Viscoelastic Properties Of Interfaces Stabilized By Soft Parmentioning

confidence: 99%

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Core-shell particles at fluid interfaces : performance as interfacial stabilizers

Buchcic¹

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There is a growing interest in the use of particles as stabilizers for foams and emulsions. Applying hard particles for stabilization of fluid interface is referred to as Pickering stabilization. By using hard particles instead of surfactants and polymers, fluid interfaces can be effectively stabilized against Ostwald ripening and coalescence. A drawback of the use of hard particles as interfacial stabilizers is that they often experience a pronounced energy barrier for interfacial adsorption and that hard particles are very specific with regard to the type of fluid interface they can adsorb to. Soft particles, on the other hand, are known as good stabilizers against coalescence and they spontaneously adsorb to a variety of different fluid interfaces. The aim of this thesis was to investigate core-shell particles comprising a hard core and soft shell with regard to their interfacial behaviour and their ability to act as sole stabilizers for foams and emulsions. We hypothesised that the presence of the soft shell allows for easier interfacial adsorption of core-shell particles compared to the hard core particles only. To test this hypothesis, we prepared core-shell particles comprising a solid polystyrene (PS) core and a soft poly-N-isopropylacrylamide (PNIPAM) shell. To ascertain the effect of shell thickness, we prepared a range of core-shell particles with different shell thicknesses, containing identical core particles. We found that core-shell particles are intrinsically surface active and can generate high surface pressures at the air-water interface and oil-water interfaces, whereas core particles seemed to experience a large energy barrier for interfacial adsorption and did not lower the surface tension. We also confirmed by microscopy that coreshell particles are actually adsorbing to the fluid interface and form densely packed interfacial layers. Further, we found that a certain critical thickness of the soft shell is necessary in order to ensure facile interfacial adsorption. If the PNIPAM shell on top of the core particles is well above 100nm thick, particle adsorption at the air-water interface was found to be diffusion limited.By gentle hand-shaking we were able to produce dispersion of air bubbles and emulsion droplets solely stabilized by core-shell particles. The resulting bubbles still underwent Ostwald ripening, albeit slowly. For oil-in-water emulsions of hexane and toluene, both of which have a relatively high solubility in the continuous phase, we found that core-shell particles can stop Ostwald ripening. The resulting emulsion droplets adopted pronounced non-spherical shapes, indicating a high elasticity of the interface. The high stability and the remarkable non-spherical shape of the emulsion droplets stabilized by core-shell particles were features we also observed for fluid dispersion stabilized by hard particles. This shows that in terms of emulsion stability core-shell particles behave similar to hard particles as interfacial stabilizer.As to why the differences between the st...

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The Compressibility of pH‐Sensitive Microgels at the Oil–Water Interface: Higher Charge Leads to Less Repulsion

Geisel¹,

Isa²,

Richtering³

2014

Angewandte Chemie

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pH-responsive microgels are unique stabilizers for stimuli-sensitive emulsions that can be broken on demand by changing the pH value. However, recent experiments have indicated that electrostatic interactions play a different role to that in conventional Pickering emulsions. The influence of charges on the interactions between microgels at the oil-water interface is now described. Compression isotherms of microgels with different charge density and architecture were determined in a Langmuir trough, and counter-intuitive results were obtained: Charged microgels can be compressed more easily than uncharged microgels. The compressibility of microgels is thus not determined by direct Coulomb repulsion. Instead, the different swelling of the microgels in the charged and the uncharged states is proposed to be the key parameter.

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Tracking the interfacial dynamics of PNiPAM soft microgels particles adsorbed at the air–water interface and in thin liquid films

Cited by 65 publications

References 53 publications

Adsorption of soft particles at fluid interfaces

Adsorption of soft particles at fluid interfaces

Core-shell particles at fluid interfaces : performance as interfacial stabilizers

The Compressibility of pH‐Sensitive Microgels at the Oil–Water Interface: Higher Charge Leads to Less Repulsion

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