Huda A. Jerri

Interfacial viscoelasticity controls buckling, wrinkling and arrest in emulsion drops undergoing mass transfer

Contrary to the notion that ‘oil and water do not mix’, many oils possess a residual diffusive mobility through water, causing the drop sizes in oil-in-water emulsions to slowly evolve with time. Liquid interfaces are therefore typically stabilized with polymeric or particulate emulsifiers. Upon adsorption, these may induce strong, localized viscoelasticity in the interfacial region. Here, we show that shrinkage of oil drops due to bulk mass transfer may render such adsorption layers mechanically unstable, causing them to buckle, crumple and, finally, to attain a stationary shape and size. We demonstrate using two types of model interfaces that this only occurs if the adsorption layer has a high interfacial shear elasticity. This is typically the case for adsorbed layers that are cross-linked or ‘jammed’. Conversely, interfacial compression elasticity alone is a poor predictor of interface buckling or arrest. These results provide a new perspective on the role of interfacial rheology for compositional ripening in emulsions. Moreover, they directly affect a variety of applications, including the rapid screening of amphiphilic biopolymers such as the Acacia gum or the octenyl succinic anhydride modified starch used here, the interpretation of light scattering data for size measurements of emulsion drops, or the formulation of delivery systems for encapsulation and release of drugs and volatiles.

Antimicrobial sand via adsorption of cationic Moringa oleifera protein.

Moringa oleifera (Moringa) seeds contain a natural cationic protein (MOCP) that can be used as an antimicrobial flocculant for water clarification. Currently, the main barrier to using Moringa seeds for producing potable water is that the seeds release other water-soluble proteins and organic matter, which increase the concentration of dissolved organic matter (DOM) in the water. The presence of this DOM supports the regrowth of pathogens in treated water, preventing its storage and later use. A new strategy has been established for retaining the MOCP protein and its ability to clarify and disinfect water while removing the excess organic matter. The MOCP is first adsorbed and immobilized onto sand granules, followed by a rinsing step wherein the excess organic matter is removed, thereby preventing later growth of bacteria in the purified water. Our hypotheses are that the protein remains adsorbed onto the sand after the functionalization treatment, and that the ability of the antimicrobial functionalized sand (f-sand) to clarify turbidity and kill bacteria, as MOCP does in bulk solution, is maintained. The data support these hypotheses, indicating that the f-sand removes silica microspheres and pathogens from water, renders adhered Escherichia coli bacteria nonviable, and reduces turbidity of a kaolin suspension. The antimicrobial properties of f-sand were assessed using fluorescent (live-dead) staining of bacteria on the surface of the f-sand. The DOM that can contribute to bacterial regrowth was shown to be significantly reduced in solution, by measuring biochemical oxygen demand (BOD). Overall, these results open the possibility that immobilization of the MOCP protein onto sand can provide a simple, locally sustainable process for producing storable drinking water.

Fabrication of stable anisotropic microcapsules

The layer-by-layer polyelectrolyte adsorption technique has been combined with the particle lithography technique to produce anisotropic polymer microcapsules each with a single nanoscale patch. The patch region covers roughly 5% of the capsule surface area, while the remaining 95% of the capsule surface is reinforced with a fluorescent nanoparticle shell. The nanoparticle shell maintains the capsule integrity and stability in suspension, while also providing a foundation for further ionic or covalent modification. The microcapsules are pH sensitive to loading and release, which we show by loading Rd6G red fluorescent dye, and they shrink and swell in response to solution pH. Confocal laser scanning microscopy confirms that these dual-functionalized capsules have a single precisely-placed nanoscale red region (revealing the underlying fluorescence) on an otherwise green surface (due to the nanoparticle coating). Even after the capsules are dehydrated, they re-hydrate intact and assume their original spherical morphology, demonstrating a resilient “Lazarus behavior”. The fabrication technique avoids organic solvents, is adaptable, and produces anisotropic microcapsules that are robust to many solution conditions.

Prolonging density gradient stability.

For bottom-up particle fabrication, separation of complex particle assemblies from their precursor colloidal building blocks is critical to producing useable quantities of materials. The separations are often done using a density gradient sedimentation due to its simplicity and scalability. When loading density gradients at volume fractions greater than 0.001, however, an inherent convective instability arises. By translating the Rayleigh-Benard instability from the heat-transfer literature into an analogous mass-transfer problem, the variables affecting the critical stability limit were effectively catalogued and examined. Experiments using submicrometer particles loaded onto sucrose and Ficoll density gradients matched theoretical trends and led to a series of useful heuristics for prolonging density gradient stability. Higher particle loading heights, lower volume fractions, and smaller gradient material diffusion coefficients were found to improve stability. Centrifugation was useful at short times, as particles were removed from top of the gradient where the stable density profile degrades to unstable, and the resulting density inversion arises as the sucrose diffuses upward.