Adsorption of soft particles at fluid interfaces

Abstract

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 ...

Elastocapillarity: Surface Tension and the Mechanics of Soft Solids

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“…Green's function techniques are closely related to transform methods which have proven useful in solving elastocapillarity problems. These include Fourier transforms (2d geometries [33]), Hankel transforms (axisymmetric geometries [14,74]), and Legendre transforms (spherical geometries [131]). …”

In vitro digestion of Pickering emulsions stabilized by soft whey protein microgel particles: influence of thermal treatment

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