Philipp Erni

Stress- and strain-controlled measurements of interfacial shear viscosity and viscoelasticity at liquid/liquid and gas/liquid interfaces

An interfacial rheometer for both stress- and strain-controlled measurements of shear rheological properties at liquid/liquid and gas/liquid interfaces is presented. The device is based on a rotating or oscillating biconical bob design in combination with a low friction electronically commutated motor system. The interfacial shear stress, viscosity, and dynamic moduli are obtained by solving the Stokes equations (low Reynolds number) along with the Boussinesq–Scriven interfacial stress tensor, which is used for the boundary conditions at the interface. An improved and simple numerical method for the calculation of the velocity distribution in the measuring cell is presented. The scope and limitations of the rheometer are discussed. Results from steady shear and oscillatory experiments as well as creep recovery and stress relaxation tests at both oil/water and air/water interfaces with adsorbed films of a globular protein (ovalbumin) and spread films of a surfactant (sorbitan tristearate) are presented.

Deformation modes of complex fluid interfaces

The rheology of complex fluid interfaces influences the dynamics of emulsion droplets and foam bubbles, vesicles, polymersomes and polymer microcapsules, biological cells, lung alveoli, or thin liquid films. With recent progress in improved and robust measuring techniques, both the shear and the dilatational viscoelastic properties of interfacial adsorption layers have received much attention. Understanding the relation between interfacial rheology, interface structure, and macroscopic material properties of complex fluids remains a challenge. In this article, the role of these links is discussed for the most important interface deformation modes, shear and dilatation, in the context of adsorption layers formed by surfactants, proteins, nanoparticles and for composite interfaces.

Deformation of single emulsion drops covered with a viscoelastic adsorbed protein layer in simple shear flow

The small-deformation behavior of single Newtonian oil drops covered by an adsorbed viscoelastic protein layer is investigated in simple shear flow. Adsorption and network formation of the protein (lysozyme) at the oil/water interface are tracked by interfacial rheology and tension. While uncovered drops deform to the expected steady ellipsoidal shape, protein-covered drops are able to resist the bulk shear stress to a much higher degree, leading to a smaller average deformation and oscillating drop shapes. The results show direct evidence for a commanding role of in-plane interfacial stresses of a viscoelastic protein network on the macroscopic drop deformation as opposed to the equilibrium interfacial tension.

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.

Microrheometry of sub-nanolitre biopolymer samples: non-Newtonian flow phenomena of carnivorous plant mucilage

Sundew plants (Drosera) capture insects using tiny drops of a viscoelastic fluid. These mucilage droplets are typically tens of micrometres in diameter, corresponding to fluid volumes in, or below, the nanolitre range. In contrast to other carnivorous plants, the physical principles and the role of rheology in the capturing mechanism are not yet fully understood. The rather simple chemical composition reported for the capturing fluid (a high molecular weight acidic polysaccharide composed of a D-glucurono-D-mannan backbone with alternating monosaccharide sidegroups) is in stark contrast to the compositional and structural complexity of other biological materials with strong extensional responses, including fibers, silks, or mucins. Here we show that the strategy used by the sundew plant combines the effects of high fluid viscosity, extensional viscoelasticity, capillary thinning and a liquid-to-solid transition driven by solvent mass transfer and aided by extensional strain-stiffening. When stretched to large extensional strains, beads-on-a-string type morphologies develop and remain frozen in place, presumably due to the combined effects of extensional strain stiffening and drying. More generally, we demonstrate how the rheological material functions of microscopic biopolymer samples in the sub-nanolitre range can be measured using an interferometry-based microrheometer for shear deformations and a capillary thinning microrheometer for extensional deformations.

Nonlinear viscoelasticity and shear localization at complex fluid interfaces.

Foams and emulsions are often exposed to strong external fields, resulting in large interface deformations far beyond the linear viscoelastic regime. Here, we investigate the nonlinear and transient interfacial rheology of adsorption layers in large-amplitude oscillatory shear flow. As a prototypical material forming soft-solid-type interfacial adsorption layers, we use Acacia gum (i.e., gum arabic), a protein/polysaccharide hybrid. We quantify its nonlinear flow properties at the oil/water interface using a biconical disk interfacial rheometer and analyze the nonlinear stress response under forced strain oscillations. From the resulting Lissajous curves, we access quantitative measures recently introduced for nonlinear viscoelasticity, including the intracycle moduli for both the maximum and zero strains and the degree of plastic energy dissipation upon interfacial yielding. We demonstrate using in situ flow visualization that the onset of nonlinear viscoelasticity coincides with shear localization at the interface. Finally, we address the nonperiodic character of this flow transition using an experimental procedure based on opposing stress pulses, allowing us to extract additional interfacial properties such as the critical interfacial stress upon yielding and the permanent deformation.

Turning Coacervates into Biohybrid Glass: Core/Shell Capsules Formed by Silica Precipitation in Protein/Polysaccharide Scaffolds

Delivery systems with low-permeability barriers and controllable release are crucial for the encapsulation of cells, pharmaceuticals, vitamins, inks, or fragrance and flavor molecules. Core/shell capsules provide a stable microenvironment and protect sensitive chemicals from degradation, undesired reactions, or evaporation. Traditionally, volatile oils have been encapsulated using synthetic polymers. While there is a strong interest in using capsule wall materials of biological origin, their barrier properties for small-molecularweight, highly volatile active ingredients remain inferior to those of polyurea or aminoplast capsules with walls that are produced synthetically. 3] Herein, we describe core/shell capsules with dense walls composed entirely of a biopolymer scaffold interpenetrated by a network of amorphous silica. We first formed a weakly acidic hydrogel shell around an oil drop. This shell then served as a scaffold to induce protein-directed mineralization of silicon dioxide from a liquid-silica precursor. The precipitation process occurring in the hydrogel scaffold consumes water and forms SiO2, yielding dense shells with very low permeability for volatile organic compounds and adjustable mechanical characteristics. Macroscopic biopolymer layers at the oil/water interface can be formed by a process called complex coacervation. For this to occur, micrometer-sized droplets of a polymer-rich aqueous liquid (coacervates) are first formed by associative phase separation between a protein and a weakly anionic polyampholyte (Scheme 1). Unlike polyelectrolyte multilayers and precipitated complexes, coacervate phases remain in the liquid state and are therefore moldable, providing a wide range of design possibilities for composite materials. If a aqueous dispersion of liquid coacervate droplets is mixed with an oil-in-water emulsion, the interfacial energy balance between the three phases (oil/ solvent/coacervate) causes the polymer-rich phase to deposit at the oil/solution interface. This coating process results in a composite emulsion of oil droplets contained within larger coacervate drops. The protein component of the outer coacervate droplet is then gelled, resulting in a physically cross-linked hydrogel wall. However, these traditional biopolymer shells often remain permeable and provide poor protection for the sensitive core materials even after further covalent cross-linking. Moreover, capsules made from classic hydrogels are soft and mechanically compliant, complicating controlled release, for example, upon chewing or rubbing on the skin. Recent efforts to improve the mechanical and stability profiles of encapsulation systems has focused on the design and synthesis of composite structures and alternative wall Scheme 1. The coacervate/silica scaffold-precipitation process. Step 1) Formation of scaffold capsules: complex coacervate droplets form by associative phase separation of a protein and a polyanion (the coacervation corner is indicated in the protein/polyanion/water phase diagram); the coacervate droplets then deposit and coalesce around the core material (e.g. volatile oil). Physical and covalent cross-linking stabilizes the coacervate shell. Step 2) Silica formation in the coacervate shell: The capsules a) are immersed in a liquid silica precursor (TEOS). The coacervate hydrogel b) serves as a mechanical scaffold shell and provides an acidic microenvironment wherein the silica precursor locally hydrolyzes and condenses to form precipitated silica. Additionally, water consumption compresses and densifies the scaffold, resulting in composite capsules (c) with dense silica/ biopolymer shells (d).

Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems

Complex coacervation is an associative liquid/liquid phase separation resulting in the formation of two liquid phases: a polymer-rich coacervate phase and a dilute continuous solvent phase. In the presence of a third liquid phase in the form of disperse oil droplets, the coacervate phase tends to wet the oil/water interface. This affinity has long been known and used for the formation of core/shell capsules. However, while encapsulation by simple or complex coacervation has been used empirically for decades, there is a lack of a thorough understanding of the three-phase wetting phenomena that control the formation of encapsulated, compound droplets and the role of the viscoelasticity of the biopolymers involved. In this contribution, we review and discuss the interplay of wetting phenomena and fluid viscoelasticity in coacervate/oil/water systems from the perspective of colloid chemistry and fluid dynamics, focusing on aspects of rheology, interfacial tension measurements at the coacervate/solvent interface, and on the formation and fragmentation of three-phase compound drops.

Free Impinging Jet Microreactors: Controlling Reactive Flows via Surface Tension and Fluid Viscoelasticity

We investigate the use of impinging free liquid jets as wall-free continuous microreactors. The collision of two reactant jets forming a free-standing thin liquid sheet allows us to perform rapid precipitation reactions to form colloidal particles, enhance micromixing, and master challenging reactions with very fast kinetics. To control the shape, size, and hydrodynamics of the impingement zone between the two liquid streams, it is crucial to understand the interplay among surface tension, fluid viscoelasticity, and reaction kinetics. Here, we study these aspects using model fluids, each illustrating a different physical effect of surface and bulk fluid properties. First, solutions of sodium dodecyl sulfate below, near, and above the critical micelle concentration are used to assess the role of static and dynamic surface tension. Second, we demonstrate how dilute solutions of high-molecular-weight polymers can be used to control the morphology of the free surface flow. If properly controlled, these effects can enhance the micromixing time scales to the extent that very rapid reactions can be performed with outstanding selectivity. We quantitatively assess the interplay between the free surface flow and reaction kinetics using parallel-competitive reactions and demonstrate how these results can be used to control the particle size in precipitation processes.

Rheology of microgels in single particle confinement

In this work, we investigate the shear rheology of Carbopol 981 microgel particle suspensions, confined between shearing plates with gap separations from 5 to 100 μm. We show that even for confining gaps smaller than that of the gel particle size, the yielding of concentrated microgel suspensions is delayed to stress levels above the bulk yield stress. Furthermore, for stresses below this new yield point, slip is described by elastohydrodynamic lubrication theory as long as the direct confinement of the single gel particles between the shearing surfaces is limited to a Hertzian deformation. For a strong, non-Hertzian particle deformation, the slip layer breaks down and leads to a frictional interaction of the single confined particle with the two shearing surfaces, depending on their surface roughness. Lubrication pressures and friction coefficients have been quantified with in situ normal force measurements on the confined particles, which have also been utilized to unambiguously determine the relevant swollen particle dimensions.