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We report evidence for photocontrolled stability andbreakage ofaqueous foams made from solutions of a cationic azobenzene-containingsurfactant over a wide range of concentrations. Exposure to UV orvisible lights results in shape and polarity switches in the surfactantmolecule, which in turn affects several properties including criticalmicelle concentration, equilibrium surface tension, and the air–waterinterfacial composition (cis isomers are displaced by trans ones).We demonstrate that the trans isomer stabilizes foams, whereas thecis isomer forms unstable foams, a property that does not correlatewith effects of light on surface tension, nor with total surfactantconcentration. Achieving in situ breakage of foam is accordingly ascribedto the remote control of the dynamics of adsorption/desorption ofthe surfactant, accompanied by gradients of concentrations out ofequilibrium. Photomodulation of adsorption kinetics and/or diffusiondynamics on interfaces is reached here by a noninvasive clean trigger,bringing a new tool for the study of foams. [ABSTRACT FROM AUTHOR]

Polymer association at liquid-liquid interfaces is a promising way to spontaneously obtain soft self-healing membranes. In the case of reversible bonding between two polymers, the macromolecules are mobile everywhere within the membrane and they can be absorbed into it at both boundaries due to binding to macromolecules of the other type. In this work, we develop the theoretical model of membrane growth based on these assumptions. The asymptotic dependence of membrane thickness on time as h ∼ t 1/2 , as typically observed experimentally in a stationary regime, reveals an interdiffusion-controlled process, where the polymer fluxes sustain the polymer absorption. The membrane growth rate is mainly determined by the difference in equilibrium compositions at the boundaries, the association constant, the polymer lengths and mobilities. This model is further used to describe the growth of hydrogel membranes formed via H-bonding of polymers at the interface between a solution of poly(propylene oxide) (PPO) in isopropyl myristate and an aqueous solution of poly(methacrylic acid) (PMAA). The film thickness is measured by reflectometric methods. The growth rate slows down about 25 times for 500-nm-thick films at pH = 5.1 compared to the case of pH = 3. The ionization degree of PMAA solutions is studied by potentiometric methods. Even a small change in ionization is found to influence noticeably the growth rate of the film. In the diffusion-controlled regime, the slowdown can be explained by a drop in the composition gradient in the membrane, whereas the process becomes non-stationary if the absorption of PMAA is hindered by an interfacial electrostatic barrier.

We explore the effect of poly(ethylene glycol) (PEG) molar mass on the intrinsic permeability and structural characteristics of poly(ethylene glycol) diacrylate PEGDA/PEG composite hydrogel membranes. We observe that by varying the PEG content and molar mass, we can finely adjust the water intrinsic permeability by several orders of magnitude. Notably, we show the existence of maximum water intrinsic permeability, already identified in a previous study to be located at the critical overlap concentration C * of PEG chains, for the highest PEG molar mass studied. Furthermore, we note that the maximum intrinsic permeability follows a non-monotonic evolution with respect to the PEG molar mass and reaches its peak at 35 000 g mol -1 . Besides, our results show that a significant fraction of PEG chains is irreversibly trapped within the PEGDA matrix even for the lowest molar masses down to 600 g mol -1 . This observation suggests the possibility of covalent grafting of the PEG chains onto the PEGDA matrix. CryoSEM and AFM measurements demonstrate the presence of large micron-sized cavities separated by PEGDA-rich walls whose nanometric structures strongly depend on the PEG content. By combining our permeability and structural measurements, we suggest that the PEG chains trapped inside the PEGDA-rich walls induce nanoscale defects in the crosslinking density, resulting in increased permeability below C *. Conversely, above C *, we speculate that partially trapped PEG chains may form a brush-like arrangement on the surface of the PEGDA-rich walls, leading to a reduction in permeability. These two opposing effects are anticipated to exhibit molar-mass-dependent trends, contributing to the non-monotonic variation of the maximum intrinsic permeability at C *. Overall, our results demonstrate the potential to fine-tune the properties of hydrogel membranes, offering new opportunities for separation applications.

For drug delivery systems, the mechanical properties of drug carriers are suspected to play a crucial role in the delivery process. However, there is a lack of reliable methods available to measure the mechanical properties of drug carriers, which hampers the establishment of a link between delivery efficiency and the mechanical properties of carriers. Lipid nanoparticles (LNPs) are advanced systems for delivering nucleic acids to target cell populations for vaccination purposes (mRNA) or the development of new drugs. Hence, it is crucial to develop reliable techniques to measure the mechanical properties of LNPs. In this article, we used AFM to image and probe the mechanical properties of LNPs which are loaded with two different biopolymers either pDNA or mRNA. Imaging the LNPs before and after indentation, as well as recording the retraction curve, enables us to obtain more insight into how the AFM tip penetrates into the particle and to determine whether the deformation of the LNPs is reversible. For pDNA, the indentation by the tip leads to irreversible rupture of the LNPs, while the deformation is reversible for the mRNA-loaded LNPs. Moreover, the forces reached for pDNA are higher than for mRNA. These results pave the way toward the establishment of the link between the LNP formulation and the delivery efficiency.

Hydrogels are promising systems for separation applications due to their structural characteristics (i.e., hydrophilicity and porosity). In our study, we investigate the permeation of suspensions of rigid latex particles of different sizes through free-standing hydrogel membranes prepared by photopolymerization of a mixture of poly(ethylene glycol) diacrylate (PEGDA) and large poly(ethylene glycol) (PEG) chains of 300,000 g·mol -1 in the presence of a photoinitiator. Atomic force microscopy and cryoscanning electron microscopy (cryoSEM) were employed to characterize the structures of the hydrogel membranes. We find that the 20 nm particle permeation depends on both the PEGDA/PEG composition and the pressure applied during filtration. In contrast, we do not measure a significant permeation of the 100 nm and 1 μm particles, despite the presence of large cavities of 1 μm evidenced by the cryoSEM images. We suggest that the PEG chains induce local nanoscale defects in the cross-linking of PEGDA-rich walls separating the micrometer-sized cavities, which control the permeation of particles and water. Moreover, we discuss the decline of the permeation flux observed in the presence of latex particles compared to that of pure water. We suggest that a thin layer of particles forms on the surface of the hydrogels.

Hypothesis: Aqueous foams are expected to constitute exquisite particularly suitable reactive medium for the oxidation of metals, since the reactant H + can be supplied through the continuous liquid phase, while the reactant O 2 can be transported through the gas bubbles., Experiments: To test this hypothesis, we investigated the oxidation of a metallic copper cylinder immersed in an aqueous foam. To study the relation between the transport of these reactants and the kinetics of the chemical reaction we use a forced drainage setup which enables us to control both the advection velocity of the H + ions through the foam and the foam liquid fraction., Findings: We find experimentally that the mass of dissolved copper presents a maximum with the drainage flow rate, and thus with the foam liquid fraction. Modeling analytically the transfer of H + and O 2 through the foams enables us to show that this non-monotonic behavior results from a competition between the advective flux of H + ions and the unsteady diffusion of O 2 through the thin liquid films which tends to be slower as the area of the thin liquid films decreases with the drainage flow rate and the liquid fraction. This study shows for the first time how to optimize the foam structure and drainage flow in reactive foams in which the reactants are present both in the liquid and gaseous phases., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Monteux reports was provided by French National Research Agency., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Foams can resist destabilizaton in ways that appear similar on a macroscopic scale, but the microscopic origins of the stability and the loss thereof can be quite diverse. Here, we compare both the macroscopic drainage and ultimate collapse of aqueous foams stabilized by either a partially hydrolyzed poly(vinyl alcohol) (PVA) or a nonionic low-molecular-weight surfactant (BrijO10) with the dynamics of individual thin films at the microscale. From this comparison, we gain significant insight regarding the effect of both surface stresses and intermolecular forces on macroscopic foam stability. Distinct regimes in the lifetime of the foams were observed. Drainage at early stages is controlled by the different stress-boundary conditions at the surfaces of the bubbles between the polymer and the surfactant. The stress-carrying capacity of PVA-stabilized interfaces is a result of the mutual contribution of Marangoni stresses and surface shear viscosity. In contrast, surface shear inviscidity and much weaker Marangoni stresses were observed for the nonionic surfactant surfaces, resulting in faster drainage times, both at the level of the single film and the macroscopic foam. At longer times, the PVA foams present a regime of homogeneous coalescence where isolated coalescence events are observed. This regime, which is observed only for PVA foams, occurs when the capillary pressure reaches the maximum disjoining pressure. A final regime is then observed for both systems where a fast coalescence front propagates from the top to the bottom of the foams. The critical liquid fractions and capillary pressures at which this regime is obtained are similar for both PVA and BrijO10 foams, which most likely indicates that collapse is related to a universal mechanism that seems unrelated to the stabilizer interfacial dynamics.

Soft or rigid particles, suspended in a liquid melt, interact with an advancing solidification front in various industrial and natural processes, such as fabrication of particle-reinforced-composites, growth of crystals, cryopreservation, frost heave, and growth of sea ice. The particle dynamics relative to the front determine the microstructure as well as the functional properties of the solidified material. Previous studies have extensively investigated the interaction of foreign objects with a moving solid-liquid interface in pure melts while in most real-life systems, solutes or surface active impurities are almost always present. Here we study experimentally the interaction of spherical oil droplets with a moving planar ice-water interface, while systematically increasing the surfactant concentration in the bulk liquid, using in situ cryo-confocal microscopy. We demonstrate that a small amount of surfactant in the bulk liquid can instigate long-range droplet repulsion, extending over a length scale of 40 to 100 μm, in contrast to the short-range predicted previously (<1 μm). We report on the droplet deformation, while they are in contact with the ice-water interface, as a function of the bulk surfactant concentration, the droplet size, and the crystal growth rate. We also depict the dynamic evolution of solute-enriched premelted films (≈5 μm). Our results demonstrate how an increasing concentration of surfactant in the bulk and its subsequent segregation during solidification can dramatically alter the solidification microstructures. We anticipate that our experimental study can aid in the development of theoretical models incorporating solute effects.

Cherry tree branches ( Prunus avium var burlat Rosaceae) are agricultural by-products that are often neglected, yet they are rich in phenolic compounds and highly appreciated for their numerous biological activities. Extracts of cherry tree branches were evaluated for their use in cosmetics, particularly for their antioxidant, anti-tyrosinase, and antimicrobial activities. Samples were obtained by accelerated solvent extraction (ASE) at different ethanol percentages and different temperatures. Fourteen phenolic compounds were identified in the extracts by mass spectrometry. Three major compounds were identified (catechin, genistin, and prunin) representing 84 wt% of the total phenolic compounds. Optimal operating conditions maximizing the content of phenolic compounds were determined using a one factor at a time (OFAT) approach (70% aqueous ethanol, 70 °C). The extract obtained under these conditions also showed the highest antioxidant and anti-tyrosinase activities, certainly due to a high catechin content. Although the antimicrobial activities of extracts are less versatile than those of synthetic molecules, they are nonetheless interesting. According to these results, the extracts of cherry tree branches could be used in cosmetics for their interesting properties.

Complexation of polymers at liquid interfaces is an emerging technique to produce all-liquid printable and self-healing devices and membranes. It is crucial to control the assembly process, but the mechanisms at play remain unclear. Using two different reflectometric methods, we investigate the spontaneous growth of H-bonded PPO-PMAA (polypropylene oxide-polymetacrylic acid) membranes at a flat liquid-liquid interface. We find that the membrane thickness h grows with time t as h ∼ t 1/2 , which is reminiscent of a diffusion-limited process. However, counterintuitively, we observe that this process is faster as the PPO molar mass increases. We are able to rationalize these results with a model which considers the diffusion of the PPO chains within the growing membrane. The architecture of the latter is described as a gel-like porous network, with a pore size much smaller than the radius of the diffusing PPO chains, thus inducing entropic barriers that hinder the diffusion process. From the comparison between the experimental data and the result of the model, we extract some key piece of information about the microscopic structure of the membrane. This study opens the route toward the rational design of self-assembled membranes and capsules with optimal properties.

The interaction of objects suspended in a liquid melt with an advancing solidification front is of special interest in nature and engineering sciences. The front can either engulf the object into the growing crystal or repel it. Therefore, the object-front confrontation can have a strong influence on the microstructure and mechanical or functional properties of the solidified material. The past theoretical models and experimental studies have mostly investigated the interaction of isolated, spherical, and hard objects in pure melts. However, the outcome of object-front interactions in complex (more realistic) systems, where multiple objects and solutes are present, is still poorly understood. Here we show the interaction of multiple oil droplets with an ice-water front in the absence and presence of solute effects using in situ cryo-confocal microscopy. We report on how the object size, number of objects, and bulk solute concentration influence the the object-front interaction and the front morphology, as well as the subsequent object spatial distribution. We suggest that the volume fraction of objects suspended in a liquid melt in conjunction with the amount of bulk solute concentration are two important parameters to be incorporated in the development of object-front interaction models.

Flexible dielectrics that harvest mechanical energy via electrostatic effects are excellent candidates as power sources for wearable electronics or autonomous sensors. The integration of a soft dielectric composite (polydimethylsiloxane PDMS-carbon black CB) into two mechanical energy harvesters is here presented. Both are based on a similar cantilever beam but work on different harvesting principles: variable capacitor and triboelectricity. We show that without an external bias the triboelectric beam harvests a net density power of 0.3 [Formula: see text] under a sinusoidal acceleration of 3.9g at 40 Hz. In a variable capacitor configuration, a bias of 0.15 [Formula: see text] is required to get the same energy harvesting performance under the same working conditions. As variable capacitors' harvesting performance are quadratically dependent on the applied bias, increasing the bias allows the system to harvest energy much more efficiently than the triboelectric one. The present results make CB/PDMS composites promising for autonomous portable multifunctional systems and intelligent sensors.

Encapsulation of chemicals using polymer membranes enables control of their transport and delivery for applications such as agrochemistry or detergency. To rationalize the design of polymer capsules, it is necessary to understand how the membranes' mechanical properties control the transport and release of the cargo. In this article, we use microfluidics to produce model polymer capsules and study in situ their behavior in controlled divergent flows. Our model capsules are obtained by assembling polymer mono and hydrogen-bonded bilayers at the surface of an oil droplet in water. We also use microfluidics to probe in situ the mechanical properties of the membranes in a controlled divergent flow generated by introducing the capsules through a constriction and then in a larger chamber. The deformation and relaxation of the capsules depend on their composition and especially on the molecular interactions between the polymer chains that form the membranes and the anchoring energy of the first layer. We develop a model and perform numerical simulations to extract the main interfacial properties of the capsules from the measurement of their deformations in the microchannels.

If confocal microscopy is an ubiquitous tool in life science, its applications in chemistry and materials science are still, in comparison, very limited. Of particular interest in these domains is the use of confocal microscopy to investigate temperature-dependent phenomena such as self-assembly, diffusio- or thermophoresis, or crystal growth. Several hurdles must be solved to develop a temperature-controlled stage for laser scanning confocal microscopy, in particular regarding the influence of an elevated temperature gradient close to the microscope objective, which most people try very hard to avoid. Here we report the design of a temperature-controlled stage able to generate stable temperature gradients in both positive and negative temperature range and does not require use of liquid nitrogen. Our setup provides an excellent control of the temperature gradient, which can be coupled with a controlled displacement of the sample, making it useful in particular for a variety of solidification, chemistry, and interfacial problems. We illustrate the benefits of our setup with several case studies of interest in chemistry and materials science: the 3D real-time imaging of ice growth, the segregation of hard particles by growing crystals, the freezing behaviour of single emulsions, the self-shaping of oil droplets upon cooling, and the self-assembly of amphiphile molecules into helical structures. These results show how confocal microscopy coupled with a temperature-controlled stage that provides a controlled temperature gradient is a welcome addition to the toolkit of chemists and materials scientists., (Copyright © 2018 Elsevier B.V. All rights reserved.)