3. EMULSION STABILITY AND PARTCILE SOFTNESSOne of the most important applications of particles is the stabilization of liquid interfaces, meaning the stability of emulsions. For this is needed a large interfacial surface area and high surface tension. The most popular way to do so far is with hard particles and these are called Pickering emulsions. These systems are metastable and their mechanisms is to basically adsorb at the fluid interface and can be stabilized or destabilized against coalescence on demand by directly changing specific control parameters , like pH or temperature [13 , 14 , 15].
Emulsions’ instability is a result of the high energy associated to a liquid-liquid interface and coalescence ” the fusion of particles together to reduce high surface energy- and Ostwald ripening “rearrangement of smaller particles to form larger ones since they are more stable thermodynamically- are the most important processes. Despite the fact that their adsorption at the fluid interface is preferable thermodynamically as mentioned already, the actual stabilization process comes with some issues which could be due to an adsorption energy barrier.
At times this energy barrier is so high that emulsions of rigid particles need external stimuli to be dispersed at the interface like strong shaking . As an alternative to solid particles, soft particles can be used also as stabilizing agents. What is rather interesting in these systems is that their polymeric part allows them to spread at the interface in order to maximize their surface area in a way of minimizing the counteractive interactions between the two fluid phases that are not favorable to each other, like water and oil, and finally lowering their surface tension. Also they exhibit fascinating elastic properties. Microgels for example have great deformability because of the polymer network in the inside that can handle external forces. However, they are very challenging to be studied experimentally because imaging is difficult due to their size and the match with the refractive index of the solvent, and apart from this exogenous parameters, like pH or temperature, have strong effect on them [13-15]. In order to study the formation of soft particles at interfaces computer simulation studies were performed. Snoeijer et al.  state that in order to study their shape during adhesion or wetting and how it alters throughout is needed to use a continuum framework based on linear elasticity and also take into consideration the interfacial forces. In their study they claim that the governing parameter is to compare the surface tension іs to their Young modulus E which is known as the elasto-capillary length. So comparing this to the radius of the particle R it is determined whether the particle is hard or soft. This parameter is known the softness of the particle S= іs/ (E€™R). The second parameter studied is the ratio of surface tensions і= іs/іb, so basically the wetting conditions. The particles that they used for their study were consisted of a cross-linked polymeric fluid and could be adsorbed at the liquid-liquid interface. During their study they choose to vary the molecular interactions and the crosslinking density to observe if and how the shape changes with these changes in interfacial tension. For having a full insight of their effect they simulated both soft and rigid particles in two different states, partially and complete wetting (see Figure 3.1). It has been proven that the systems which are exposed to partial wetting are described mainly by linear elasticity, where the solid surface tension (іs) is comparable or larger than that the one on the liquid interface and the shape is more or less no different. Contrary to this when in complete wetting, is favorable for the interfacial forces to cover the entire interface however this is in the case of the soft particle is not possible because is forbidden by the network elasticity. Ideal soft colloidal particles form into a lenticular shape or else known as fried egg when they are exposed to the fluid interface [15, 16]. The shape is determined by the Neumann’s triangle  construction which requires force balance at the contact line. Whereas, rigid particles need to float at the interface because they cannot spread. For further understanding, when a liquid droplet is placed on a solid surface, the liquid forms a contact angle with the solid, which is directly related to the interfacial tensions: solid to liquid, solid to vapor and liquid to vapor like Young’s equation states. Note though that Young’s equation implies only the equilibrium of the horizontal components of the surface tension (as shown in Figure 3.2(a) and (b)) but does not accompany the perpendicular ones . It is believed that this component is the reason for the deformation of the shape. The balance of forces on a liquid substrate (i.e., in a three-phase liquid system) is described by the Neumann’s triangle of forces. [17, 18]. Dufresne et al.  proposed a study where they mapped the elastic deformation of such wetting spheres. In more detail, they studied the two extremes of ideally soft and ideally hard particles to find out that the contact radius is only depended on the ratio of surface tensions: іop/іow and іwp/іow, so between the two liquids (іow: oil-water) and between the particle and each one of them (іop: oil-particle, іwp: water-particle). For mapping the results of particle spreading, they compared the initial contact radius ± with the radius R. Their data are shown in Figure 3.2.4. RHEOLOGICAL BEHAVIORAs for the rheological behavior, it is already familiar that hard particles can be described by a one dimensional phase diagram since they are not depended by temperature but from concentration and shape. The key control parameter is the volume fraction , meaning the fraction of volume occupied by the solid particles. Samples with a larger volume fraction will have a larger viscosity, and this viscosity grows dramatically as increases. Typical value for the glass transition of a monodisperse system is ‰€ 0.58 . Then again, soft particles are proven to have a glass transition at higher effective volume fraction than hard spheres . Moreover, this time temperature plays a significant role this time. Without a doubt, soft particles have a thermos-reversible liquid to solid transition since changing the temperature, you probe changes at the solvent’s quality which affects the size of the particle ” when is bad it will shrink, when is good it will swell ” and therefore the volume fraction . Studies from Richtering et al  state that indeed by increasing the temperature the PNIPAM microgel particles shrink and with it the effective volume fraction drops, having huge effect in their rheological properties since there is a decrease of viscosity. This effect of the temperature on relative viscosity ·rel=·/·s, · is the viscosity of the solution and ·s, the viscosity of the solvent used, is clear in Figure 4.1 where are shown data for various concentrations. The results that they got was in a great agreement with Batchelor’s equation (dashed and dotted lines on Figure 4.1 are fittings of it) for dilute solutions . At higher concentrations the dispersions become viscoelastic and yield stress was observed and again a strong temperature dependence was shown. At the low temperatures, the sample behaves like a viscoelastic solid, meaning that the storage modulus G’ is higher than the loss modulus G throughout the dynamic frequency sweep test (DFS). This is also very clear on Figure 4.2 from Carrier and Petekidis . When the temperature rises, the sample’s status swifts to a viscoelastic liquid. In Figure 4.2 are depicted some rheological data for a concentration of 19.1% glassy sample of microgel PNIPAM. Below the critical temperature of 31 °C, the PNIPAM are swollen and as indicated from Fig 4.2(a) their hydrodynamic radius is large so the system is stabilized sterically via repulsive interactions and the sample is a viscoelastic solid. As they continued heating they show at first a gradual drop of both moduli and hydrodynamic radius. At temperatures around 32oC both loss modulus G” (open square) and storage modulus G’ (full square) display a drop at a minimum with very similar values and also there is a drop in the hydrodynamic radius RH meaning that it is entering its liquid state. What is rather interesting is that after T~ 33 °C, besides the fact that the hydrodynamic radius (RH) is lowering so the particle continues shrinking, both G’ and G increase again and moreover, storage modulus is higher than loss indicating a solid like behavior. This effect is known as reentrance [25,26] and is frequently observed in soft particles due the polymer-colloid nature, since switching from repulsive to attractive interactions as the temperature rises above 33 °C, the solvent worsens for PNIPAM, the particles come to be attractive . In Figure 4.2(b) are plotted the rheological data for dynamic frequency sweep experiments where again is shown that at first T~20oC there is a clear and dominant solid like behavior since G’>G throughout and as we move up at T~28oC the solid like behavior is less pronounced since the G’ value has decreased up until we reach T~31oC and we are able to observe a crossover at around 0.03 Hz. Studies have shown re-entrance phenomena for other soft particle systems like star polymers [27, 28, 29] or more complex like Janus particles [30, 31]. However, data are not shown.5. CONCLUSIONSTo conclude, soft colloidal particles is an alternative for hard spheres since they have exquisite properties and they offer great potential for creating tunable response with external stimuli, like temperature or pH which is due to their nature of both colloids and polymers. Complex and various architectures can be created on demand with the use of state-of-the-art synthesis techniques like ATR polymerization. Their effect on fluid-fluid interfaces is studied mainly with computer simulations since experimentally there are still many issues to overcome. When soft microgels were compared and contrasted with familiar model systems like Pickering emulsions, it was shown how they could alter their size to reduce surface tension since they don’t have the ability to completely spread as a consequence of their inner network. Furthermore, rheological studies prove their effect as viscosity modifiers. Studies with thermos-responsible (PNIPAM) were discussed and interestingly it was shown to have reentrant in their phase diagram. Regarding this, soft colloids are very interesting system that need to be further studied to understand their behavior. They offer custom made particles, stimuli response thus also mechanical properties. Already they are used thoroughly as emulsifying agents in foods, cosmetics and motor oil companies. Also they are used as drug delivery systems and medicine carriers. However these were just a few and there is still plenty of room for potential uses.