Syneresis and delayed detachment in agar plates

Flow Characterization in a Partially Liquefied Vitreous Humor

The purpose of this study is to systematically examine the basic fluid dynamics associated with a fully liquid region within a porous material. This work has come about as a result of our investigation on the ocular fluid dynamics and transport process in a partially liquefied vitreous humor. The liquid is modeled as a sphere with Stokes flow while the surrounding infinite porous region is described by Brinkman flow. The development here provides basic three-dimensional axisymmetric results on flow characterization and also serves to evaluate the limits of validity of Darcy flow analysis for the same geometry. In the Darcy flow model, the liquid region is also treated as a porous region with a much higher permeability. Therefore, both liquid and porous regions are modeled by Darcy's law. Besides the analytical results from Brinkman-Stokes model, the simpler case of Darcy-Darcy flow for the same geometry has been provided. The results of both cases are compared and the differences between the two sets of results provide the range of validity of our computational model (Khoobyar et al. in J Heat Transf 144:031208, 2022). Some interesting fluid-dynamical aspects of the system are observed through the analysis. For the Darcy-Darcy system, the liquid region velocity is uniform throughout, as expected for potential flow. With the Brinkman-Stokes model, the liquid region has a paraboloidal profile with the maximum possible peak value of six times the far-field velocity in the porous medium. With the liquid region having a lower resistance, the flow tends to converge there for both models as it seeks the path of least resistance. As for the validation of the Darcy-Darcy model, it is a good approximation as far as the exterior flow is concerned. However, the liquid region flow profiles for the two models are different as noted. The current Brinkman-Stokes model has led to explicit analytical solutions for the flow field for both regions. This has permitted an asymptotic analysis giving deeper insight into the flow characterization.

Surface stress and shape relaxation of gelling droplets

Solidification is a heterogeneous transformation from liquid to solid, which usually combines transport, phase transition and mechanical strain. Predicting the shapes resulting from such a complex process is fascinating and has a wide range of implications from morphogenesis in biological tissues to industrial processes. For soft solids initially at equilibrium, elastic stresses, whether tensile or compressive, can be induced by heterogeneous volumetric deformations of the material. These stresses trigger surface instabilities leading to variations of curvature and shape of the solids. In this article, we study the shape evolution of elongated droplets of polymer and particle suspensions undergoing a solidification process caused by the inward diffusion of a gelling agent from the surface. We show experimentally and numerically that there appears a layer of gelled material growing at the surface. Due to volume contraction, this layer induces tensile stresses and drives a flow in the ungelled liquid core, resulting in the relaxation of the droplets toward spherical shapes. Over time, the thickness of this elastic membrane grows, hence the bending stiffness required to change its shape eventually balances the surface stresses, which arrests the relaxation process. These results provide general rules to understand the shape of solidifying materials combining both tension and bending driven deformations.

Syneresis of self-crowded calcium-alginate hydrogels as a self-driven athermal aging process

The assembly of biopolymers into a hydrated elastic network often goes along with syneresis, a spontaneous process during which the hydrogel slowly shrinks and releases solvent. The tendency to syneresis of calcium-alginate hydrogels, widely used biocompatible materials, is a hindrance to applications for which dimensional integrity is crucial. Although calcium-induced aggregation of specific block-sequences has been long known as the microscopic process at work in both primary cross-linking and syneresis, the nature of the coupling between these structural events and the global deswelling flow has remained so far elusive. We have tackled this issue within the regime of entangled pregels that yield highly cross-linked, self-crowded hydrogels with stiff networks. Using an original, stopped-flow extrusion experiment, we have unveiled a robust, stretched-exponential kinetics of shrinking, spanning more than six decades of time and quasi-independent of the alginate concentration. A careful analysis of the puzzling dynamical features of syneresis in these gels has led us to propose that due to the network rigidity, the calcium-fueled, random collapse events that drive solvent locally, are not thermally activated but rather controlled by the average poroelastic flow itself, according to a self-sustained mechanism described here for the first time.

Sulfate groups position determines the ionic selectivity and syneresis properties of carrageenan systems

The salt sensitivity and selectivity feature of α-carrageenan (α-Car) were investigated and compared with κ-carrageenan (κ-Car) and iota-carrageenan (ι-Car). These carrageenans are identified by one sulfate group on the 3,6-anhydro-D-galactose (DA) for α-Car, D-galactose (G) for κ-Car and on both carrabiose moieties (G and DA) for ι-Car. The viscosity and temperature, where order-disorder transition have been observed, were greater in presence of CaCl₂ for α-Car and ι-Car compared with KCl and NaCl. Conversely, the reactivity of κ-Car systems were greater in presence of KCl than CaCl₂. Unlike κ-Car systems, the gelation of α-Car in presence of KCl was observed without syneresis. Thus, the position of sulfate group on the carrabiose determines the importance of counterion valency too. The α-Car could be a good alternative to κ-Car to reduce the syneresis effects.

Characterization of an In Vitro/Ex Vivo Mucoadhesiveness Measurement Method of PVA Films

Transmucosal drug delivery systems can be an attractive alternative to conventional oral dosage forms such as tablets. There are numerous in vitro methods to estimate the behavior of mucoadhesive dosage forms in vivo. In this work, a tensile test system was used to measure the mucoadhesion of polyvinyl alcohol films. An in vitro screening of potential influencing variables was performed on biomimetic agar/mucin gels. Among the test device-specific factors, contact time and withdrawal speed were identified as influencing parameters. In addition, influencing factors such as the sample area, which showed a linear relationship in relation to the resulting work, and the liquid addition, which led to an abrupt decrease in adhesion, could be identified. The influence of tissue preparation was investigated in ex vivo experiments on porcine small intestinal tissue. It was found that lower values of F_max and W_ad were obtained on processed and fresh tissue than on processed and thawed tissue. Film adhesion on fresh, unprocessed tissue was lowest in most of the animals tested. Comparison of ex vivo measurements on porcine small intestinal tissue with in vitro measurements on agar/mucin gels illustrates the inter- and intra-individual variability of biological tissue.

The Rheological Properties and Texture of Agar Gels with Canola Oil-Effect of Mixing Rate and Addition of Lecithin

This study aimed to determine the effect of different mixing rates and the addition of lecithin on the rheological mechanical, and acoustic properties of agar gels with the addition of canola oil. The mixing rate of the agar-oil mixture was changed from 10,000 to 13,000 rpm. Additionally, agar gels with the addition of lecithin from 1 to 5% were prepared. The frequency sweep test was used (at 4 and 50 °C) within the linear viscoelastic region (LVR) in oscillatory measurement. The agar-oil mixture was cooled from 80 to 10 °C, enabling the obtainment of the gelling temperature. Texture profile analysis (TPA) and compression tests, as well as the acoustic emission method, were applied to analyse the texture of the gels. The syneresis and stability of gels during storage were also measure. The increase in mixing rate in the case of agar gel with canola oil causes an increase in the elastic component of materials as well hardness and gumminess. Also, samples prepared with the higher mixing rate have more uniform and stable structures, with small bubbles. The increase in the concentration of lecithin is ineffective due to the formation of gels with a weak matrix and low hardness, gumminess, and stability during storage.

Green Synthesis of Silver Nanoparticles Coated by Water Soluble Chitosan and Its Potency as Non-Alcoholic Hand Sanitizer Formulation

The synthesis of silver nanoparticles using plant extracts, widely known as a green synthesis method, has been extensively studied. Nanoparticles produced through this method have applications as antibacterial agents. Bacterial and viral infection can be prevented by use of antibacterial agents such as soap, disinfectants, and hand sanitizer. Silver nanoparticles represent promising hand sanitizer ingredients due to their antibacterial activity and can enable reduced use of alcohol and triclosan. This study employed silver nanoparticles synthesized using Kepok banana peel extract (Musa paradisiaca L.). Nanoparticle effectiveness as a hand sanitizer can be enhanced by coating with a biocompatible polymer such as chitosan. The characterization of silver nanoparticles was conducted using UV-Vis, with an obtained peak at 434.5 nm. SEM-EDX analysis indicated nanoparticles with a spherical morphology. Silver nanoparticles coated with chitosan were characterized through FTIR to verify the attached functional groups. Gel hand sanitizers were produced using silver nanoparticles coated with different chitosan concentrations. Several tests were undertaken to determine the gel characteristics, including pH, syneresis, and antibacterial activity. Syneresis leads to unstable gels, but was found to be inhibited by adding chitosan at a concentration of 2%. Antibacterial activity was found to increase with increase in chitosan concentration.

Lower critical concentration temperature as thermodynamic origin of syneresis: Case of kappa-carrageenan solution

Polysaccharide ubiquity is trimmed for applications of low syneresis impact. This syneresis may be crucial for specific applications that are very sensitive to gel dimension stability, namely, 3D scaffolds for cell culture for disease diagnosis and tissue engineering. We hypothesized that the syneresis origin results from the kappa-carrageenan (kC) polysaccharide thermodynamic instability, and we demonstrated this by measuring the critical (coil-to-coil contact) concentration as a function of temperature. The impact of 5 mM, 10 mM and 15 mM KCl salt on the critical concentration of the solution and the lower critical concentration temperature (LCCT) were particularly investigated. For the kC polysaccharide, the gelation temperature (T_g) falls at temperatures below the LCCT, which explains the shrinking or syneresis reaction of the polysaccharide gels. The gap between T_g and LCCT would be the thermomotive force of the syneresis of many colloidal gels.

A Morphable Ionic Electrode Based on Thermogel for Non-Invasive Hairy Plant Electrophysiology

Plant electrophysiology lays the foundation for smart plant interrogation and intervention. However, plant trichomes with hair-like morphologies present topographical features that challenge stable and high-fidelity non-invasive electrophysiology, due to the inadequate dynamic shape adaptability of conventional electrodes. Here, this issue is overcome using a morphable ionic electrode based on a thermogel, which gradually transforms from a viscous liquid to a viscoelastic gel. This transformation enables the morphable electrode to lock into the abrupt hairy surface irregularities and establish a conformal and adhesive interface. It achieves down to one tenth of the impedance and 4-5 times the adhesive strengths of conventional hydrogel electrodes on hairy leaves. As a result of the improved electrical and mechanical robustness, the morphable electrode can record more than one order of magnitude higher signal-to-noise ratio on hairy plants and maintains high-fidelity recording despite plant movements, achieving superior performance to conventional hydrogel electrodes. The reported morphable electrode is a promising tool for hairy plant electrophysiology and may be applied to diversely textured plants for advanced sensing and modulation.

Determination of available breaking stress of agar and gellan gum plate culture methods and the duration of bacterial culture under strong acidic conditions

AIMS

Several acidophilic bacteria have not been cultured, primarily owing to the lack of suitable culture methods under strong acidic conditions. This study aimed to quantitatively evaluate the strengths of the agar plates (AP) and gellan gum plates (GP), and optimal culture periods under strong acidic conditions.

METHODS AND RESULTS

To define the lower limit of plate strength for bacterial isolation culture, the diameter of Escherichia coli K12 colonies and the breaking stress of plates at different concentrations of gelling agents, medium composition and pH conditions were determined. The lower limit of available strength of AP and GP was 19·6 and 14·8 kPa, respectively. Medium composition slightly affected AP breaking stress, although GP with a high cationic concentration medium could not be prepared.

CONCLUSIONS

Assessment of the strength limits of AP and GP revealed that AP is not suitable for prolonged bacterial culture (≥72 h). Furthermore, GP was completely ineffective for bacterial culture under highly acidic conditions (≤pH 1·0).

SIGNIFICANCE AND IMPACT OF THE STUDY

Our quantitative evaluation method based on breaking stress is a potentially valuable tool to understand the state and the suitable limit of plate culture methods in more detail under various conditions.

Comparison of replica leaf surface materials for phyllosphere microbiology

Artificial surfaces are routinely used instead of leaves to enable a reductionist approach in phyllosphere microbiology, the study of microorganisms residing on plant leaf surfaces. Commonly used artificial surfaces include, flat surfaces, such as metal and nutrient agar, and microstructured surfaces, such as isolate leaf cuticles or reconstituted leaf waxes. However, interest in replica leaf surfaces as an artificial surface is growing, as replica surfaces provide an improved representation of the complex topography of leaf surfaces. To date, leaf surfaces have predominantly been replicated for their superhydrophobic properties. In contrast, in this paper we investigated the potential of agarose, the elastomer polydimethylsiloxane (PDMS), and gelatin as replica leaf surface materials for phyllosphere microbiology studies. Using a test pattern of pillars, we investigated the ability to replicate microstructures into the materials, as well as the degradation characteristics of the materials in environmental conditions. Pillars produced in PDMS were measured to be within 10% of the mold master and remained stable throughout the degradation experiments. In agarose and gelatin the pillars deviated by more than 10% and degraded considerably within 48 hours in environmental conditions. Furthermore, we investigated the surface energy of the materials, an important property of a leaf surface, which influences resource availability and microorganism attachment. We found that the surface energy and bacterial viability on PDMS was comparable to isolated Citrus × aurantium and Populus × canescens leaf cuticles. Hence indicating that PDMS is the most suitable material for replica leaf surfaces. In summary, our experiments highlight the importance of considering the inherent material properties when selecting a replica leaf surface for phyllosphere microbiology studies. As demonstrated, a PDMS replica leaf offers a control surface that can be used for investigating microbe-microbe and microbe-plant interactions in the phyllosphere, which will enable mitigation strategies against pathogens to be developed.

Force percolation of contractile active gels

Living systems provide a paradigmatic example of active soft matter. Cells and tissues comprise viscoelastic materials that exert forces and can actively change shape. This strikingly autonomous behavior is powered by the cytoskeleton, an active gel of semiflexible filaments, crosslinks, and molecular motors inside cells. Although individual motors are only a few nm in size and exert minute forces of a few pN, cells spatially integrate the activity of an ensemble of motors to produce larger contractile forces (∼nN and greater) on cellular, tissue, and organismal length scales. Here we review experimental and theoretical studies on contractile active gels composed of actin filaments and myosin motors. Unlike other active soft matter systems, which tend to form ordered patterns, actin-myosin systems exhibit a generic tendency to contract. Experimental studies of reconstituted actin-myosin model systems have long suggested that a mechanical interplay between motor activity and the network's connectivity governs this contractile behavior. Recent theoretical models indicate that this interplay can be understood in terms of percolation models, extended to include effects of motor activity on the network connectivity. Based on concepts from percolation theory, we propose a state diagram that unites a large body of experimental observations. This framework provides valuable insights into the mechanisms that drive cellular shape changes and also provides design principles for synthetic active materials.

Shear-induced slab-like domains in a directed percolated colloidal gel

We explore the structural changes of a gel-forming colloid polymer mixture under shear by employing Brownian dynamics simulations of a colloidal system with short-ranged attractive depletion interaction in a linear flow profile. While the structure of unpercolated systems changes only slightly under shearing, we discover the formation of slab-like clusters in sheared directed percolated gel networks that are confined between two walls. These gel-slabs are stable over a long time and seem to be related to the syneresis phenomena that can be observed in directed percolated colloidal gels. Only at large shear strength the slabs are destroyed and a homogeneous state with many unbounded particles can be observed. We also quantitatively analyze our results by determining void volumes.

Syneresis in Gels of Highly Ferulated Arabinoxylans: Characterization of Covalent Cross-Linking, Rheology, and Microstructure

Arabinoxylans (AXs) with high ferulic acid (FA) content (7.18 µg/mg AXs) were cross-linked using laccase. Storage (G') modulus of AX solutions at 1% (AX-1) and 2% (AX-2) (w/v) registered maximum values of 409 Pa and 889 Pa at 180 min and 83 min, respectively. Atomic force microscopy revealed the grained and irregular surface of the AX-1 gel and the smoother surface without significant depressions of the AX-2 gel. Cured AX gels exhibited a liquid phase surrounding the samples indicating syneresis. The syneresis ratio percentage (% Rs) of the gels was registered over time reaching stabilization at 20 h. The % Rs was not significantly different between AX-1 (60.0%) and AX-2 (62.8%) gels. After 20 h of syneresis development, the dimers of the FA in the AX-1 and AX-2 gels significantly increased by 9% and 78%, respectively; moreover, the trimers of the FA in the AX-1 and AX-2 gels, by 94% and 300%, respectively. Scanning electron microscopy showed that, after syneresis stabilization, AX gels presented a more compact microstructure. Syneresis development in the gels of highly ferulated AXs could be related to the polymer network contraction due to the additional formation of dimers and trimers of the FA (cross-linking structures), which may act like a "zipping" process, increasing the polymer chains' connectivity.

Impact of saccharides on the drying kinetics of agarose gels measured by in-situ interferometry

Agarose gels are viscoelastic soft solids that display a porous microstructure filled with water at 90% w/w or more. Despite an extensive use in food industry and microbiology, little is known about the drying kinetics of such squishy solids, which suffers from a lack of time-resolved local measurements. Moreover, only scattered empirical observations are available on the role of the gel composition on the drying kinetics. Here we study by in-situ interferometry the drying of agarose gels of various compositions cast in Petri dishes. The gel thinning is associated with the displacement of interference fringes that are analyzed using an efficient spatiotemporal filtering method, which allows us to assess local thinning rates as low as 10 nm/s with high accuracy. The gel thinning rate measured at the center of the dish appears as a robust observable to quantify the role of additives on the gel drying kinetics and compare the drying speed of agarose gels loaded with various non-gelling saccharides of increasing molecular weights. Our work shows that saccharides systematically decrease the agarose gel thinning rate up to a factor two, and exemplifies interferometry as a powerful tool to quantify the impact of additives on the drying kinetics of polymer gels.

Directed percolation identified as equilibrium pre-transition towards non-equilibrium arrested gel states

The macroscopic properties of gels arise from their slow dynamics and load-bearing network structure, which are exploited by nature and in numerous industrial products. However, a link between these structural and dynamical properties has remained elusive. Here we present confocal microscopy experiments and simulations of gel-forming colloid–polymer mixtures. They reveal that gel formation is preceded by continuous and directed percolation. Both transitions lead to system-spanning networks, but only directed percolation results in extremely slow dynamics, ageing and a shrinking of the gel that resembles synaeresis. Therefore, dynamical arrest in gels is found to be linked to a structural transition, namely directed percolation, which is quantitatively associated with the mean number of bonded neighbours. Directed percolation denotes a universality class of transitions. Our study hence connects gel formation to a well-developed theoretical framework, which now can be exploited to achieve a detailed understanding of arrested gels.

Gels exhibit very slow dynamics, for which a structural reason remains elusive. Here, Kohl et al. show the gel formation is accompanied by a succession of continuous and directed percolation, with only the latter found to lead to the arrested dynamics.