(267d) Fundamental Investigation of the Rheology and Structure of Capillary Suspensions Made with Oyster Particles

Ternary particle-liquid-liquid (PL2) suspensions offer different properties from their elementary constituents or components. One quadrant of these systems are the particle-stabilized emulsions also known as Pickering emulsions [1, 2]. Pickering emulsions offer unique features and have found great potential in agrochemical applications, as well as in the oil, food and paint industries while pharmaceutical and cosmetic products are increasingly explored [3,4]. The formation of a stable emulsion requires a mechanically balanced and sufficiently resistant film that will occlude the dispersed phase from the continuous. This is mostly established by the alignment of amphiphilic surfactant molecules on the droplet’s interface. Similarly, particles tend to position themselves on the interface based on their hydrophilic or hydrophobic surface, their wettability profile, size and shape.

The depth of immersion of a particle in the liquid depends on the angle of contact shown between the liquid-particle-liquid interface [5] while the particles’ morphology plays also a role [4]. Previous studies have shown that the tendency to destabilize depends strongly on the degree of surface coverage, which with increasing particle concentration can transition from low surface coverage, to monolayers and multilayers and reach up to three dimensional network structures [6].

In the present investigation we review the evolution of Pickering emulsions and focus on the innovative use of oyster shell particles for the stabilization as our base case study. According to FAO* forecasts the consumption of oysters is expected to rise and with that the untreated volume of shells will also increase. By incorporating oyster shell particles in our formulations, we present a potential solution to the waste accumulation by considering the increased demand for clean label products and emulsifier-free formulations and at the same time we suggest these systems as potential edible particle stabilizers in the food sector.

*Food and Agriculture Organisation

Wet grinded oyster shells were prepared and tested in a cream formulation at different concentrations. The evaluation period lasted eight weeks and the Pickering emulsions were analysed by particle size measurements, instability indexes and characterised using microscopy. Based on the results the formulations containing concentrations ranging from 6 wt-% to 8 wt-% oyster shell particles showed great potential. The formulation showing better performance was then scaled up and compared with other formulations including aerosil and other surfactants and reviewed for a further period of eight weeks. The analytical part was extended, and rheological measurements rounded up our understanding of their stability. Among these, viscosity curves, the yield point and extrapolated long-term storage behaviour were analysed by means of rotation and oscillation measurements (Physica UDS 200; Anton Paar).

Figure 1a depicts the preparation process followed for the Pickering emulsion formulation by using an Ultra-Turrax ®.

Full details of the methods employed, the control experiments and the results obtained will be presented.

Figure 1b shows the results obtained for the viscosity dependence with respect to shear rate for time variations considering the different formulations. These profiles show a decrease in dynamic viscosity with increasing shear rate and the curves are characteristic of shear thinning behaviour. This is to be expected with emulsions because the formation of finely distributed oil droplets occurs and thus the internal friction decreases. The fact that the dynamic viscosity does not change much over time (for eight weeks) indicates that the structure of the emulsions and the oil droplets remain stable.

Figure 1b. Viscosity dependence with respect to shear rate for time variations considering the different formulations.

This research bridges the knowledge gap on the structural transitions of ternary suspensions. Furthermore we also point out opportunities in the current research on the applications of Pickering emulsions as capillary suspensions and suggests future directions for the design of particulate stabilizers and preparation methods for Pickering emulsions and their derived materials. [7]

References

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[2] Pickering SU. Emulsions 1907:2001–21.

[3] Aveyard R, Binks BP, Clint JH. Emulsions stabilised solely by colloidal particles. Advances in Colloid and Interface Science 2003;100-102:503–46.

[4] Huang F, Liang Y, He Y. On the Pickering emulsions stabilized by calcium carbonate particles with various morphologies. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2019;580:123722.

[5] Inoue M, Hashizaki K, Taguchi H, Saito Y. Formation and characterization of emulsions using beta-cyclodextrin as an emulsifier. Chem Pharm Bull (Tokyo) 2008;56(5):668–71.

[6] Schröder A, Sprakel J, Schroën K, Spaen JN, Berton-Carabin CC. Coalescence stability of Pickering emulsions produced with lipid particles: A microfluidic study. Journal of Food Engineering 2018;234:63–72.

[7] Jiang H, Sheng Y, Ngai T. Pickering emulsions: Versatility of colloidal particles and recent applications. Curr Opin Colloid Interface Sci 2020;49:1–15.