This work involves an experimental investigation of the spreading of liquids on gel layers in the presence of surfactants. Of primary interest is the instability that accompanies the cracking of gels through the deposition and subsequent spreading of a drop of surfactant solution on their surfaces. This instability manifests itself via the shaping of crack-like spreading "arms", in formations that resemble starbursts. The main aim of this study is to elucidate the complex interactions between spreading surfactants and underlying gels and to achieve a fundamental understanding of the mechanism behind the observed phenomenon of the cracking pattern formation. By spreading SDS and Silwet L-77 surfactant solutions on the surfaces of agar gels, the different ways that system parameters such as the surfactant chemistry and concentration and the gel strength can affect the morphology and dynamics of the starburst patterns are explored. The crack propagation dynamics is fitted to a power law by measuring the temporal evolution of the length of the spreading arms that form each one of the observed patterns. The values of the exponent of the power law are within the predicted limits for Marangoni-driven spreading on thick layers. Therefore, Marangoni stresses, induced by surface tension gradients between the spreading surfactant and the underlying gel layer, are identified to be the main driving force behind these phenomena, whereas gravitational forces were also found to play an important role. A mechanism that involves the "unzipping" of the gel in a manner perpendicular to the direction of the largest surface tension gradient is proposed. This mechanism highlights the important role of the width of the arms in the process; it is demonstrated that a cracking pattern is formed only within the experimental conditions that allow S/Δw to be greater than G', where S is the spreading coefficient, Δw is the change in the width of the crack, and G' is the storage modulus of the substrate.
Recording kinetics during a reaction is a challenging effort that provides significant insights into gelation. We recently published our work based on a novel custom-made rheometric setup for in situ crosslinking reaction [Besiri et al., Carbohydr. Polym., 2020, 246, 116615]. It facilitates the instant injection of CaCl2 solution into alginate via micro-holes of the lower plate configuration to initiate the process. Considering that the time evolution of the viscoelastic parameters is related to the developed structure, we can obtain the reaction kinetics. This study aims to improve the setup by increasing the number of micro-holes from two to four, investigating the mass ratio effects, and considering the proposed design as a batch reactor. As the volume and concentration of the reactants can be controlled during the initiation of the process, we investigate the molarity effect on the gelation. The long-term behavior of rheological oscillatory shear experiments indicates that the reaction is based on the mass of cations. The stoichiometry of reactants affects the diffusion of ions to alginate since, at high concentration and low volume of CaCl2, the mechanical properties are increased compared to lower concentration and higher volume of the cationic solution. Systematic time sweep experiments prove that at low angular frequencies, ω, the driving force of the reaction is the distribution of ions to the polymer. For higher values of ω, the force acting on the oscillating geometry of the rheometer is possibly the factor causing an enhanced mixing of the reactants, with a corresponding increase in moduli.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.