Session G34: Emulsions, Micelles and Vesicles

Ripening foams are not really analogous to glasses

Foams and dense emulsions display complex mechanics, including intermittent rearrangement dynamics, power-law rheology, and slow recovery after perturbation. These effects have long been considered evidence for glassy physics in these and other materials having similar mechanics, such as the cytoskeleton.  Here we observe such anomalous mechanics in a simulated wet foam driven by ripening, and find behavior that is distinctly different from that in glasses and controlled by the strength of viscous damping. For a wide range of intermediate damping values, the system configuration moves continuously over a high-dimensional potential energy landscape devoid of energy minima, at an energetic height typically associated with equilibrium fluids, following a trajectory having a tortuous, fractal character. At the very lowest viscosities, the configuration moves similarly but at lower potential energy, while intermittently hopping between shallow energy minima producing avalanche-like rearrangements, as seen in previous studies. This tortuous motion over a fractal landscape leads the material to display power-law rheology at all but the very highest viscosities. Our model successfully captures the observed slow recovery of perturbed foams while suggesting that it is driven by the configuration being kinetically rather than energetically trapped in the high-energy portions of the landscape. Overall, our findings suggest these systems display a hierarchical fractal landscape which is stratified by energy, where different energy domains give rise to separate foam-like, intermittent, and glassy dynamics.

https://arxiv.org/abs/2301.13400   

Emulsions and Foams Stabilized by Pickering Crystals

Commonly used food, pharma and personal care products are emulsions or foams in which the fluid-fluid interfaces are stabilised by excipients that crystallise at the interface from the continuous phase. We prepare two-dimensional fluid-fluid interfaces stabilised by crystals of monostearate, a commonly used excipient, and quantify the strength and relaxation dynamics of the interfaces as a function of excipient loading by means of interfacial shear rheology. Further, we propose a model that would predict the force required to induce a certain degree of phase separation, with only the microstructure and the interfacial elasticity as the input parameters. The predictions of the proposed model were validated by artificially inducing phase separation by means of centrifugation for the prepared foams and emulsions.   

Double Emulsions Droplets with Thermally Reconfigurable Shells as Micro-Lenses

Complex emulsions, serving as micro-lenses, offer a unique and versatile material platform for innovative applications in optics, photonics and beyond. Multi-phase droplets, with the versatility to manipulate light at microscale dimensions, are ideal candidates for designing tunable lenses. For example, triple-phase emulsions, consisting of three distinct phases with different refractive indices, have demonstrated promise in correcting optical aberrations. Here we introduce a novel kind of tri-phase emulsions with thermally reconfigurable shells. Water-oil-water double emulsions featuring a shell made of hydrocarbon and fluorocarbon components, which exhibit temperature-dependent miscibility, are rendered stable with specially tailored surfactants to form solid-like W/O interfaces, whose viscoelasticity is then characterized with the droplet flow in patterned sinusoidal channels as a rheometer in PDMS devices. Upon temperature change, the two components in the oil shell phase separate; therefore, the droplets reconfigure into a new morphology resembling an egg with double yolks. The morphologies of the droplets are tuned by changing the hydrocarbon-to-fluorocarbon ratio and the small surfactants used in the outer aqueous phase, forming exotic structures. 2D simulations of ray tracing passing through different droplet morphologies are further performed in order to calculate the longitudinal and transverse spherical aberrations for informative fluid lens design and selection.   

The peculiar shapes of air bubbles trapped in ice

Open the freezer and look at an ice cube. Chances are it is not perfectly transparent; maybe it is white; if so, it likely contains air bubbles. Water usually contains dissolved gases, and because freezing is a purifying process these gases are expelled as ice forms. Bubbles appear at the freezing front and are then entrapped into the ice. Such bubbles come in a large range of sizes from microns to millimeters and their shapes are peculiar; never spherical but elongated, and usually fore-aft asymmetric. They result of the interplay between freezing, capillarity and diffusion. More generally, gas-laden liquids yield porous materials when they freeze.

We conduct experiments to characterize the bubbles obtained at different freezing rates; we obtain empirical laws for the dimensions and aspect ratio. Then, we show that the problem can be reduced to a simple yet non-linear ordinary differential equation; it involves only three parameters, related to the freezing rate and the gas saturation. This equation predicts asymptotical regimes which describe the top and the bottom of the bubble; our experiments confirm both regimes. Finally, we provide a method for matching solutions of the differential equation to given experiments. Most bubble shapes correspond to solutions starting with a straightforward initial condition; others require a more subtle one but are still well matched by theory. This matching enables a indirect measurement of the gas concentration and freezing rate under which the bubble is formed.   

Recognizing trends in the impact of properties of the oil phase and emulsifiers on emulsion breakdown time with high-throughput image analysis and machine learning model.

Emulsion stability is a complex problem that has been studied for decades and is of interest to many industries. However, the impact of the oil properties on the overall emulsion stability needs to be better understood. We present a systematic study of the impact of properties of oil phases and emulsifiers on the stability of emulsions over time. A high-throughput image analysis method is utilized to profile the stability of emulsions across a wide range of materials and curate a dataset. Subsequently, machine learning (ML) methods are leveraged to discover the underlying relationships between the molecular properties of the formulation of an emulsion and its stability. An ML model is trained on the collected data to accurately predict emulsion stability from formulation and thus efficiently guide the emulsion design for a desired breakdown time.   

The Effect of Surface Interactions on the Coalescence of Water Droplets in Fuel

The process of coalescence plays a crucial role in treating phase separation in emulsions, especially in industrial filtration applications. Water trapped in fuel commonly causes corrosion of fuel engine parts, which necessitates water removal in fuel applications. This is often achieved using coalescence filters. The interaction between the filter’s fibers and other droplets significantly impacts the coalescence process, but its precise effects remain under-characterized. This presentation investigates how surface interactions affect droplet coalescence using microfluidic platforms. The film drainage times of surfactant-laden water-in-fuel emulsions are measured using a “contact” device that examines the coalescence of droplets in contact with poly(dimethylsiloxane) (PDMS) traps. The results are compared to  a “contactless” Stokes trap device, which studies the coalescence of free droplets in a hydrodynamic cross-slot. It was found that droplets that have multiple contacts, either with other droplets or with the PDMS droplet traps, coalesce more readily and exhibit a lower dimensionless drainage time compared to two free droplets brought together in the Stokes trap. This work serves as a foundational understanding of coalescence, critical in various industrial applications.   

Transfer Kinetics of Cargo Items among Mobile Nanocarriers

Micelles, liposomes, microgels, dendrimers, and nanoparticles represent nanocarriers that deliver cargo items–often drug molecules–to a target. We calculate the kinetics of collision-mediated transfer of cargo items within ensembles of chemically distinct mobile nanocarriers in the Gaussian regime. To this end, the relevant rate equations for collision-mediated transfer of cargo items are expressed in the continuum limit as a set of Fokker-Planck equations and solved analytically. The solutions fully describe the time evolution of an arbitrary initial distribution of the cargo items among the nanocarriers toward equilibrium.   

A Fusion Peptide's Interactions with Cholesterol in a Lipid Bilayer Membrane

Fusion peptides (FPs) are short sequences within larger viral proteins that play a major role in the infection of a cell.  Isolated FPs can also interact directly with the cellular membrane and cause fusion. The 23 amino acid residue peptide from the HIV-1 virus gp41 coat glycoprotein is one such FP. Here, a study of the interaction of a less fusogenic form of the HIV-1 FP with lipid bilayer membranes containing different amounts of cholesterol (Chol) is presented. Chol strongly influences the interaction of the peptide with the lipid bilayer in a concentration-dependent manner. When sufficient Chol is present, fusion can take place. Small-angle neutron scattering and circular dichroism spectroscopy experiments were complemented with molecular dynamics simulations of the systems studied to gain additional insight into how Chol influences the behavior. The results reveal surprising new information about lipid-specific interactions that the peptide has in the bilayer.    

Vesicle explosion under light-induced asymmetric oxidation

Oxidation of lipids by reactive oxygen species (ROS) alters their structural properties, compromising lipid-bilayer integrity, disrupting homeostasis in living cells, and even causing the cell death. Oxidation of a model membranes employing giant unilamellar vesicles (GUVs), leads to the formation of a series of short-lived pores as well as total catastrophic loss of membrane stability. However, the understanding of how the lipid oxidation leads to GUVs destabilization is lacking. In this talk, we will discuss the response of GUVs to the oxidation of lipids caused by photo-generated ROS in presence of a photosensitizer. We show that only under the asymmetric oxidation conditions, the GUVs develop micron-sized short-lived pores or exhibit vesicle explosion (i.e., the sudden catastrophic collapse of vesicles). Utilizing numerical modeling of pore-opening dynamics we link the oxidation-induced conformational changes in lipid to the generated spontaneous curvature, and demonstrate its critical role GUVs explosion. We further discuss the non-monotonic temporal evolution of spontaneous curvature. Finally, we will present a phase diagram delineating vesicle explosion and short-lived pore formation. Insights from our experiments and theoretical model have the potential to enhance the fundamental understanding of the stability of biological membranes in oxidative environments as well as aid in precision  drug delivery.   

How do the dumbbell- and bola-shaped amphiphiles assemble: vesicles with condensed hydrophobic domains or blackberry-type structures with porous surfaces?

Dumbbell-shaped hybrid amphiphiles, with two large, rigid, charged hydrophilic heads and a hydrophobic flexible linker in between, can assemble into hollow, spherical supramolecular structures in polar solvents. The question is if they assemble into conventional vesicles with a condensed hydrophobic domain via hydrophobic interaction, or blackberry-type structures via counterion-mediated attraction with porous assembly surfaces. These two types of assemblies have different driving forces and mechanisms but similar apparent assembly structures; both are possible.

We have demonstrated that some dumbbell-shaped amphiphiles form regular vesicles, while another type of dumbbell hybrid, AC₆₀-AC₆₀, containing two carboxylic acid-functionalized C₆₀ (AC₆₀) polar heads linked by an organic tether, behave as macroions and assemble into blackberry-type structures. We distinguish between vesicles and blackberries by examining the size response of assemblies on solvent polarity and pH of aqueous solutions. In various solutions, the assembly size responds to environmental change with features for blackberry structures. The surface porosity of assemblies is also examined by all-atom simulations. Furthermore, a bolaform macromolecule, AC₆₀-PEG-AC₆₀, with a hydrophilic polyethylene glycol (PEG) chain as the linker, can also self-assemble and show similar pH responses as assemblies from AC₆₀-AC₆₀. Therefore, the nature of these assemblies is blackberry-type structures rather than vesicles.   

Dynamic Mesh-based Simulations of Vesicle Interactions with Anisotropic Nanoparticles

Understanding the intricate interactions between nanoparticles and fluid vesicles is crucial for assessing their impact on biological processes. While previous research has extensively investigated vesicle-particle interactions, the complexities of wrapping processes, particularly involving irregular and anisotropic particles, remain incompletely understood. In this study, we present a force-based, continuum membrane model employing a triangulated membrane representation and discrete differential geometry to study the dynamics of vesicle-particle interactions. Our model accurately captures the morphological transformations of vesicles and the wrapping of spherical nanoparticles by computing forces originating from membrane bending and particle adhesion. To validate our simulation results, we compare them with theoretical predictions of minimal bending energy and the corresponding vesicle shapes.  We then quantitatively explore interactions between individual spherical particles and spherical vesicles by sampling energy landscapes across various different wrapping fractions. Furthermore, we develop two algorithms describing adhesion between the membrane and arbitrarily shaped particles, which are represented by 3D polyhedra meshes. We systematically compare the effects of these adhesion algorithms on the wrapping dynamics and energy profiles. This innovative model significantly enhances our ability to examine the dynamics of vesicle-particle interactions, offering insights into the biological consequence of nanoparticle exposure.   

Deducing size and stability of peptide-induced transmembrane pores from structure and phase behavior of peptide-lipid systems

Transmembrane pore formation induced by peptides is commonplace in biological processes. Examples include the formation of pores by antimicrobial peptides (AMPs) and cell-penetrating peptides (CPPs). Pore formation is susceptible to thermal fluctuations and is difficult to directly observe experimentally, so it is challenging to assess be influence of membrane physicochemical properties and intensive thermodynamic conditions. In contrast, the structure and phase behavior of peptide-lipid systems are relatively straightforward to map out experimentally for a broad range of conditions. In principle, Negative Gaussian Curvature (NGC) is topologically required for the formation of transmembrane pores. Consistent with this, cubic phases are often observed in peptide-lipid systems for pore-forming peptides and proteins. However, it is not clear how to correlate the measured induced curvatures in the cubic phases to the actual sizes of transmembrane pores in the membrane. Here, we present a general method in which a minimal mechanical model, combined with information on structure and phase behavior from Small Angle X-ray Scattering (SAXS) measurements, is used to estimate the size of peptide-induced transmembrane pores. Using this method, we find that the pore radius is a non-monotonic function of the cubic lattice constant; the pore radius initially increases with the cubic phase lattice constant, but eventually saturates and then decreases. Additionally, we find the surprising result that transmembrane pores formed by typical pore formers like AMPs are qualitatively different from small-radius pores induced by CPPs like HIV TAT, which are intrinsically labile and unstable. We confirm this behavior using atomistic simulations and provide an explanation for these effects.   

Many-Body Dissipative Particle Dynamics with the MARTINI "Lego" Approach

MARTINI is a popular coarse-grained force-field that is mainly used in molecular dynamics

(MD) simulations. It is based on the “Lego” approach where intermolecular interactions

between coarse-grained beads representing chemical units of different polarity are obtained

through water–octanol partition coefficients. This enables the simulation of a wide range of

molecules by only using a finite number of parametrized coarse-grained beads, similar to the

Lego game, where a finite number of bricks are used to create larger structures. Moreover, the

MARTINI force-field is based on the Lennard-Jones potential with the shortest possible cutoff

including attractions, thus rendering it very efficient for MD simulations. However, MD simulation

is in general a computationally expensive method. Here, we demonstrate that using the

MARTINI “Lego” approach is suitable for many-body dissipative particle (MDPD) dynamics,

a method that can simulate multi-component and multi-phase soft matter systems in a much

faster time (about 4–7 times) than MD. In this study, a DPPC lipid bilayer is chosen to provide

evidence for the validity of this approach and various properties are compared to highlight the

potential of the method. Thus, we anticipate that our study opens new possibilities for faster

simulations of a wide range of soft matter systems by using the MDPD method