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Soft Matter

Alexandre Darmon, Michael Benzaquen, Simon Čopar, Olivier Dauchot, Teresa Lopez-Leon
We investigate experimentally and numerically the defect configurations emerging when a cholesteric liquid crystal is confined to a spherical shell. We uncover a rich scenario of defect configurations, some of them non-existent in nematic shells, where new types of defects are stabilized by the helical ordering of the liquid crystal. In contrast to nematic shells, here defects are not simple singular points or lines, but have a large structured core. Specifically, we observe five different types of cholesteric shells...
October 21, 2016: Soft Matter
Kelly Muijlwijk, Wenqian Huang, Jan-Eise Vuist, Claire Berton-Carabin, Karin Schroën
Surfactant adsorption during emulsification can be quantified by measuring the acting interfacial tension using a Y-junction microfluidic device. To obtain insight into the surfactant transport mechanism to the interface, the effect of shear force on the acting interfacial tension was assessed by systematically varying the continuous phase viscosity and velocity. Varying the continuous phase viscosity did not affect the acting interfacial tension, indicating that surfactant adsorption during Y-junction emulsification is not diffusion-limited...
October 21, 2016: Soft Matter
Etienne Jambon-Puillet, Dominic Vella, Suzie Protière
We study the effect of film density on the uniaxial compression of thin elastic films at a liquid-fluid interface. Using a combination of experiments and theory, we show that dense films first wrinkle and then fold as the compression is increased, similarly to what has been reported when the film density is neglected. However, we highlight the changes in the shape of the fold induced by the film's own weight and extend the model of Diamant and Witten [Phys. Rev. Lett., 2011, 107, 164302] to understand these changes...
October 21, 2016: Soft Matter
Awaneesh Singh, Olga Kuksenok, Anna C Balazs
Using computational modeling, we design a composite that encompasses a thermo-responsive gel and photo-responsive fibers that extend from the surface of the gel. By simulating the effect of light and heat on the sample, we isolate scenarios where cooperative interactions within the system allow the gel to actuate the "finger-like" motion of the embedded fibers. To achieve this distinctive behavior, we consider a gel formed from poly(N-isopropylacrylamide) (PNIPAAm), which shrinks when heated above the lower critical solution temperature (LCST)...
October 19, 2016: Soft Matter
Yue Zhang, Richard Potter, William Zhang, Zahra Fakhraai
Recent studies have shown that diffusion on the surface of organic glasses can be many orders of magnitude faster than bulk diffusion. Developing new probes that can readily measure surface diffusion can help study the effect of parameters such as chemical structure, intermolecular interaction, molecules' shape and size on the enhanced surface diffusion. In this study, we develop a novel probe that significantly simplifies these types of studies. Tobacco mosaic virus (TMV) is used as probe particle to measure surface diffusion coefficient of molecular glass N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD)...
October 19, 2016: Soft Matter
John M Frostad, Daniele Tammaro, Luciano Santollani, Simone Bochner de Araujo, Gerald G Fuller
Understanding and enabling the control of the properties of foams is important for a variety of commercial processes and consumer products. In these systems, the role of surface active compounds has been the subject of many investigations using a wide range of techniques. The study of their influence on simplified geometries such as two bubbles in a liquid or a thin film of solution (such as in the well-known Scheludko cell), has yielded important fundamental understanding. Similarly, in this work an interferometric technique is used to study the dynamic evolution of the film formed by a single bubble being pressed against a planar air-liquid interface...
October 18, 2016: Soft Matter
Rabea Seyboldt, Dimitri Merger, Fabian Coupette, Miriam Siebenbürger, Matthias Ballauff, Manfred Wilhelm, Matthias Fuchs
The leading nonlinear stress response in a periodically strained concentrated colloidal dispersion is studied experimentally and by theory. A thermosensitive microgel dispersion serves as well-characterized glass-forming model, where the stress response at the first higher harmonic frequency (3ω for strain at frequency ω) is investigated in the limit of small amplitude. The intrinsic nonlinearity at the third harmonic exhibits a scaling behavior which has a maximum in an intermediate frequency window and diverges when approaching the glass transition...
October 18, 2016: Soft Matter
Cristina Munoz-Menendez, Ivan Conde-Leboran, David Serantes, Roy Chantrell, Oksana Chubykalo-Fesenko, Daniel Baldomir
In the magnetic fluid hyperthermia (MFH) research field, it is usually assumed that achieving a uniform temperature enhancement (ΔT) of the entire tumour is a key-point for treatment. However, various experimental works reported successful cell apoptosis via MFH without a noticeable ΔT of the system. A possible explanation of the success of these negligible-ΔT experiments is that a local ΔT restricted to the particle nanoenvironment (i.e. with no significant effect on the global temperature T) could be enough to trigger cell death...
October 18, 2016: Soft Matter
Xiaoqing Liu, Marie Haddou, Isabelle Grillo, Zohra Mana, Jean-Paul Chapel, Christophe Schatz
Polyelectrolyte complexes (PECs) between poly(acrylic acid) (PAA) and poly(diallyldimethylammonium chloride) (PDADMAC), a model system forming coacervate particles via electrostatic interaction at pH 10, were prepared by a stopped-flow (SF) fast mixing technique at different mixing charge ratios (z) and ionic strengths. Both PEC final morphologies prepared by either SF or manual one-shot mixing are similar at bench time. In situ light scattering combined with the SF technique pointed-out, however, the presence of three distinct early stage kinetic behaviors in the formation of PECs...
October 17, 2016: Soft Matter
Naiyin Yu, Abhijit Ghosh, Michael F Hagan
We use computer simulations and simple theoretical models to analyze the morphologies that result when rod-like particles end-attach onto a curved surface, creating a finite-thickness monolayer aligned with the surface normal. This geometry leads to two forms of frustration, one associated with the incompatibility of hexagonal order on surfaces with Gaussian curvature, and the second reflecting the deformation of a layer with finite thickness on a surface with non-zero mean curvature. We show that the latter effect leads to a faceting mechanism...
October 17, 2016: Soft Matter
Alberto Giacomello, Lothar Schimmele, Siegfried Dietrich, Mykola Tasinkevych
A liquid droplet placed on a geometrically textured surface may take on a "suspended" state, in which the liquid wets only the top of the surface structure, while the remaining geometrical features are occupied by vapor. This superhydrophobic Cassie-Baxter state is characterized by its composite interface which is intrinsically fragile and, if subjected to certain external perturbations, may collapse into the fully wet, so-called Wenzel state. Restoring the superhydrophobic Cassie-Baxter state requires a supply of free energy to the system in order to again nucleate the vapor...
October 17, 2016: Soft Matter
Maolin Sha, Yusheng Liu, Huaze Dong, Fabao Luo, Fangling Jiang, Zhongfeng Tang, Guanglai Zhu, Guozhong Wu
Room-temperature ionic liquids (ILs) are generally considered as structurally heterogeneous with inherent polar/apolar phase separation. However, even after a decade of research, local dynamics in the heterogeneous structures of ILs remain neglected. Such local dynamics may influence the ion transport of electrolytes, as well as the reaction rate of solvents. In this study, we performed molecular dynamics simulation to analyze the local dynamics for the structural heterogeneity of ILs. Calculations of the diffusion, reorientation, and association dynamics showed a distinct heterogeneous dynamics between the polar and apolar regions of ILs...
October 17, 2016: Soft Matter
Koushik Viswanathan, Narayan K Sundaram, Srinivasan Chandrasekar
Stick-slip in sliding of soft adhesive surfaces has long been associated with the propagation of Schallamach waves, a type of slow surface wave. Recently it was demonstrated using in situ experiments that two other kinds of slow waves-separation pulses and slip pulses-also mediate stick-slip (Viswanathan et al., Soft Matter, 2016, 12, 5265-5275). While separation pulses, like Schallamach waves, involve local interface detachment, slip pulses are moving stress fronts with no detachment. Here, we present a theoretical analysis of the propagation of these three waves in a linear elastodynamics framework...
October 17, 2016: Soft Matter
Xuejuan Liu, Falin Tian, Tongtao Yue, Xianren Zhang, Chongli Zhong
The shape deformation of membrane nanotubes is studied by a combination of theoretical analysis and molecular simulation. First we perform free energy analysis to demonstrate the effects of various factors on two ideal states for the pearling transition, and then we carry out dissipative particle dynamics simulations, through which various types of membrane tube deformation are found, including membrane pearling, buckling, and bulging. Different models for inducing tube deformation, including the osmotic pressure, area difference and spontaneous curvature models, are considered to investigate tubular instabilities...
October 17, 2016: Soft Matter
Alpha A Lee
Recent molecular dynamics simulations show that thermal gradients can induce electric fields in water that are comparable in magnitude to electric fields seen in ionic thin films and biomembranes. This surprising non-equilibrium phenomenon of thermomolecular orientation is also observed more generally in simulations of polar and non-polar size-asymmetric dumbbell fluids. However, a microscopic theory linking thermomolecular orientation and polarization to molecular properties is yet unknown. Here, we formulate an analytically solvable microscopic model of size-asymmetric dumbbell molecules in a temperature gradient using a mean-field, local equilibrium approach...
October 14, 2016: Soft Matter
B Rossi, V Venuti, F D'Amico, A Gessini, A Mele, C Punta, L Melone, V Crupi, D Majolino, C Masciovecchio
The focus of the present work is to shed light on possible modifications of the molecular properties of polysaccharide hydrogels induced by the establishment of specific non-covalent interactions during the loading of a guest compound inside the gel phase. With this aim, a case study of the encapsulation of caffeine (Caf) inside cyclodextrin-based hydrogels, namely, cyclodextrin nanosponges (NS), is systematically investigated here by using UV Raman scattering experiments. The UV Raman spectra of the hydrogels, analysed as a function of temperature, concentration of the guest molecule loaded in the gel phase and pH, prove particularly informative both on the structural rearrangements of the hydrophobic/hydrophilic groups of the polymeric network and on the breaking/formation of specific guest-matrix interactions...
October 13, 2016: Soft Matter
Anna Wang, Ryan McGorty, David M Kaz, Vinothan N Manoharan
Previous experiments have shown that spherical colloidal particles relax to equilibrium slowly after they adsorb to a liquid-liquid interface, despite the large interfacial energy gradient driving the adsorption. The slow relaxation has been explained in terms of transient pinning and depinning of the contact line on the surface of the particles. However, the nature of the pinning sites has not been investigated in detail. We use digital holographic microscopy to track a variety of colloidal spheres-inorganic and organic, charge-stabilized and sterically stabilized, aqueous and non-aqueous-as they breach liquid interfaces...
October 13, 2016: Soft Matter
XiaoHao Sun, HengAn Wu, Rong Long
We study a covalent adaptable polymer that can rearrange its network topology through thermally activated bond exchange reactions. When the polymer is deformed, such a network rearrangement leads to macroscopic stress relaxation, which allows the polymer to be thermoformed without a mold. Based on a previously developed constitutive model, we investigate thermal-mechanical behaviors of this material under a non-uniform and evolving temperature field through numerical simulations. Our focus is on the complex coupling between mechanical deformation, heat conduction and bond exchange reactions...
October 12, 2016: Soft Matter
Shaoting Lin, Tal Cohen, Teng Zhang, Hyunwoo Yuk, Rohan Abeyaratne, Xuanhe Zhao
Soft elastic layers with top and bottom surfaces adhered to rigid bodies are abundant in biological organisms and engineering applications. As the rigid bodies are pulled apart, the stressed layer can exhibit various modes of mechanical instabilities. In cases where the layer's thickness is much smaller than its length and width, the dominant modes that have been studied are the cavitation, interfacial and fingering instabilities. Here we report a new mode of instability which emerges if the thickness of the constrained elastic layer is comparable to or smaller than its width...
October 12, 2016: Soft Matter
E Beauvier, S Bodea, A Pocheau
We experimentally address the propagation of reaction-diffusion fronts in vortex lattices by combining, in a Hele-Shaw cell and at low Reynolds number, forced electroconvective flows and an autocatalytic reaction in solution. We consider both vortex chains and vortex arrays, the former referring to mixed free/rigid boundary conditions for vortices and the latter to free boundary conditions. Varying the depth of the fluid layer, we observe no variation of the mean front velocities for vortex arrays and a noticeable variation for vortex chains...
October 12, 2016: Soft Matter
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