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Showing papers in "Soft Matter in 2017"


Journal ArticleDOI
TL;DR: The consequences of the cholesteric geometry in living matter are described, which are far from being fully defined and understood, and discusses various perspectives.
Abstract: Liquid crystals play an important role in biology because the combination of order and mobility is a basic requirement for self-organisation and structure formation in living systems. Cholesteric liquid crystals are omnipresent in living matter under both in vivo and in vitro conditions and address the major types of molecules essential to life. In the animal and plant kingdoms, the cholesteric structure is a recurring design, suggesting a convergent evolution to an optimised left-handed helix. Herein, we review the recent advances in the cholesteric organisation of DNA, chromatin, chitin, cellulose, collagen, viruses, silk and cholesterol ester deposition in atherosclerosis. Cholesteric structures can be found in bacteriophages, archaea, eukaryotes, bacterial nucleoids, chromosomes of unicellular algae, sperm nuclei of many vertebrates, cuticles of crustaceans and insects, bone, tendon, cornea, fish scales and scutes, cuttlebone and squid pens, plant cell walls, virus suspensions, silk produced by spiders and silkworms, and arterial wall lesions. This article specifically aims at describing the consequences of the cholesteric geometry in living matter, which are far from being fully defined and understood, and discusses various perspectives. The roles and functions of biological cholesteric liquid crystals include maximisation of packing efficiency, morphogenesis, mechanical stability, optical information, radiation protection and evolution pressure.

218 citations


Journal ArticleDOI
TL;DR: This work combines 3D printing, printed electronics, and liquid crystal elastomers (LCEs) to achieve soft actuators with free-standing two-way shape changing behaviors to achieve controllable shape change structures for a variety of potential practical applications.
Abstract: In this work, we advance printed active composites by combining 3D printing, printed electronics, and liquid crystal elastomers (LCEs) to achieve soft actuators with free-standing two-way shape changing behaviors. Incorporated LCE strips are activated by Joule heating produced by printed conductive wires, while uniaxial deformation of the LCE strip is utilized as a driving force to achieve bending in the printed composite. The bending behavior of laminated hinges is first characterized in order to obtain a precise control of actuation, which is then exploited to actuate four demonstrative designs: a morphing airplane, a miura-ori structure, a cubic box, and a soft crawler. The soft morphing airplane and miura-ori structure are designed and fabricated with multiple laminated hinges to demonstrate the synergistic actions during actuation. The cubic box is constructed to show the capability of sequential folding by implementing multiple groups of conductive wires to achieve accurately addressable heating with temporal control. Finally, the two-way transformation is utilized as a driving force for the locomotion of a soft crawler stimulated by a periodic rectangular wave current. These examples show the great potential of using the hybrid 3D printing and pick-and-place method and using LCEs to achieve controllable shape change structures for a variety of potential practical applications.

209 citations


Journal ArticleDOI
TL;DR: The concept of an equivalent isothermal weld time is developed and its relationship to fracture energy is tested to understand the development of inter-layer weld strength from the perspective of polymer interdiffusion under conditions of rapidly changing mobility.
Abstract: Material extrusion (ME) is a layer-by-layer additive manufacturing process that is now used in personal and commercial production where prototyping and customization are required. However, parts produced from ME frequently exhibit poor mechanical performance relative to those from traditional means; moreover, fundamental knowledge of the factors leading to development of inter-layer strength in this highly non-isothermal process is limited. In this work, we seek to understand the development of inter-layer weld strength from the perspective of polymer interdiffusion under conditions of rapidly changing mobility. Our framework centers around three interrelated components: in situ thermal measurements (via infrared imaging), temperature dependent molecular processes (via rheology), and mechanical testing (via mode III fracture). We develop the concept of an equivalent isothermal weld time and test its relationship to fracture energy. For the printing conditions studied the equivalent isothermal weld time for Tref = 230 °C ranged from 0.1 ms to 100 ms. The results of these analysis provide a basis for optimizing inter-layer strength, the limitations of the ME process, and guide development of new materials.

191 citations


Journal ArticleDOI
TL;DR: This review presents the clogged mechanisms recently reported for suspension flows of colloidal particles and for biofluids in microfluidic channels, including sieving, bridging and aggregation of particles, and discusses the technological implications of the clogging of microchannels and the schemes that leverage the formation of clogs.
Abstract: The transport of suspensions of microparticles in confined environments is associated with complex phenomena at the interface of fluid mechanics and soft matter. Indeed, the deposition and assembly of particles under flow involve hydrodynamic, steric and colloidal forces, and can lead to the clogging of microchannels. The formation of clogs dramatically alters the performance of both natural and engineered systems, effectively limiting the use of microfluidic technology. While the fouling of porous filters has been studied at the macroscopic level, it is only recently that the formation of clogs has been considered at the pore-scale, using microfluidic devices. In this review, we present the clogging mechanisms recently reported for suspension flows of colloidal particles and for biofluids in microfluidic channels, including sieving, bridging and aggregation of particles. We discuss the technological implications of the clogging of microchannels and the schemes that leverage the formation of clogs. We finally consider some of the outstanding challenges involving clogging in human health, which could be tackled with microfluidic methods.

183 citations


Journal ArticleDOI
TL;DR: The theoretical framework of stochastic thermodynamics is revisited, and some guidance on how the work extracted from colloidal heat engines can be used to generate net particle or energy currents is provided, proposing a new generation of experiments with colloidal systems.
Abstract: Stochastic heat engines can be built using colloidal particles trapped using optical tweezers. Here we review recent experimental realizations of microscopic heat engines. We first revisit the theoretical framework of stochastic thermodynamics that allows to describe the fluctuating behavior of the energy fluxes that occur at mesoscopic scales, and then discuss recent implementations of the colloidal equivalents to the macroscopic Stirling, Carnot and steam engines. These small-scale motors exhibit unique features in terms of power and efficiency fluctuations that have no equivalent in the macroscopic world. We also consider a second pathway for work extraction from colloidal engines operating between active bacterial reservoirs at different temperatures, which could significantly boost the performance of passive heat engines at the mesoscale. Finally, we provide some guidance on how the work extracted from colloidal heat engines can be used to generate net particle or energy currents, proposing a new generation of experiments with colloidal systems.

176 citations


Journal ArticleDOI
TL;DR: This review covers recent advances in the field of ice-binding proteins and their synthetic analogues, focusing on fundamental insights of biological and technological relevance.
Abstract: Ice-binding proteins (IBP) facilitate survival under extreme conditions in diverse life forms. IBPs in polar fishes block further growth of internalized environmental ice and inhibit ice recrystallization of accumulated internal crystals. Algae use IBPs to structure ice, while ice adhesion is critical for the Antarctic bacterium Marinomonas primoryensis. Successful translation of this natural cryoprotective ability into man-made materials holds great promise but is still in its infancy. This review covers recent advances in the field of ice-binding proteins and their synthetic analogues, highlighting fundamental insights into IBP functioning as a foundation for the knowledge-based development of cheap, bio-inspired mimics through scalable production routes. Recent advances in the utilisation of IBPs and their analogues to e.g. improve cryopreservation, ice-templating strategies, gas hydrate inhibition and other technologies are presented.

153 citations


Journal ArticleDOI
TL;DR: Analytical expressions for both the apparent contact angle and contact angle hysteresis can be interpreted as 'weighted sums' between the contact angles of the infusing liquid relative to the droplet and surrounding gas phases, where the weighting coefficients are given by ratios of the fluid surface tensions.
Abstract: We theoretically investigate the apparent contact angle and contact angle hysteresis of a droplet placed on a liquid infused surface. We show that the apparent contact angle is not uniquely defined by material parameters, but also has a dependence on the relative size between the droplet and its surrounding wetting ridge formed by the infusing liquid. We derive a closed form expression for the contact angle in the limit of vanishing wetting ridge, and compute the correction for small but finite ridge, which corresponds to an effective line tension term. We also predict contact angle hysteresis on liquid infused surfaces generated by the pinning of the contact lines by the surface corrugations. Our analytical expressions for both the apparent contact angle and contact angle hysteresis can be interpreted as ‘weighted sums’ between the contact angles of the infusing liquid relative to the droplet and surrounding gas phases, where the weighting coefficients are given by ratios of the fluid surface tensions.

139 citations


Journal ArticleDOI
Jiangjiang Duan1, Xichao Liang1, Kunkun Zhu1, Jinhua Guo1, Lina Zhang1 
TL;DR: This work has constructed bio-hydrogel actuators inspired by the bilayer structures of plant organs from chitosan and cellulose/carboxymethylcellulose (CMC) solution in an alkali/urea aqueous system containing epichlorohydrin (ECH) as a crosslinker, and demonstrated tight adhesion between two layers through strong electrostatic attraction and chemical crosslinking.
Abstract: Smart hydrogel actuators with excellent biocompatibility and biodegradation are extremely desired for biomedical applications. Herein, we have constructed bio-hydrogel actuators inspired by the bilayer structures of plant organs from chitosan and cellulose/carboxymethylcellulose (CMC) solution in an alkali/urea aqueous system containing epichlorohydrin (ECH) as a crosslinker, and demonstrated tight adhesion between two layers through strong electrostatic attraction and chemical crosslinking. The bilayer hydrogels with excellent mechanical properties could carry out rapid, reversible, and repeated self-rolling deformation actuated by pH-triggered swelling/deswelling, and transformed into rings, tubules, and flower-, helix-, bamboo-, and wave-like shapes by effectively designing the geometric shape and size. The significant difference in the swelling behavior between the positively charged chitosan and the negatively charged cellulose/CMC layers generated enough force to actuate the performance of the hydrogels as soft grippers, smart encapsulators, and bioinspired lenses, showing potential applications in a wide range of fields including biomedicine, biomimetic machines, etc.

135 citations


Journal ArticleDOI
TL;DR: The design promotes the initiation of macro-cracks and enables the reduction of ice adhesion by at least ∼50% regardless of the curing temperature, weight ratio and size of internal holes, reaching a lowest iceAdhesion of 5.7 kPa.
Abstract: Preventing icing on exposed surfaces is important for life and technology. While suppressing ice nucleation by surface structuring and local confinement is highly desirable and yet to be achieved, a realistic roadmap of icephobicity is to live with ice, but with lowest possible ice adhesion. According to fracture mechanics, the key to lower ice adhesion is to maximize crack driving forces at the ice–substrate interface. Herein, we present a novel integrated macro-crack initiator mechanism combining nano-crack and micro-crack initiators, and demonstrate a new approach to designing super-low ice adhesion surfaces by introducing sub-structures into smooth polydimethylsiloxane coatings. Our design promotes the initiation of macro-cracks and enables the reduction of ice adhesion by at least ∼50% regardless of the curing temperature, weight ratio and size of internal holes, reaching a lowest ice adhesion of 5.7 kPa. The multiscale crack initiator mechanisms provide an unprecedented and versatile strategy towards designing super-low ice adhesion surfaces.

134 citations


Journal ArticleDOI
TL;DR: It is proposed that the starting point for understanding conjugated polymers includes understanding chain conformations and phase behavior and the factors that influence these properties, thereby providing opportunities for the development of novel optoelectronic materials based on conjugation polymers.
Abstract: Conjugated polymers may play an important role in various emerging optoelectronic applications because they combine the chemical versatility of organic molecules and the flexibility, stretchability and toughness of polymers with semiconducting properties. Nevertheless, in order to achieve the full potential of conjugated polymers, a clear description of how their structure, morphology, and macroscopic properties are interrelated is needed. We propose that the starting point for understanding conjugated polymers includes understanding chain conformations and phase behavior. Efforts to predict and measure the persistence length have significantly refined our intuition of the chain stiffness, and have led to predictions of nematic-to-isotropic transitions. Exploring mixing between conjugated polymers and small molecules or other polymers has demonstrated tremendous advancements in attaining the needed properties for various optoelectronic devices. Current efforts continue to refine our knowledge of chain conformations and phase behavior and the factors that influence these properties, thereby providing opportunities for the development of novel optoelectronic materials based on conjugated polymers.

129 citations


Journal ArticleDOI
TL;DR: The nature of the friction generated as a drop glides on a textured material infused by another liquid is discussed, and this specific feature is proposed to be responsible for the special frictions observed on both adhesive and non-adhesive substrates.
Abstract: We discuss in this paper the nature of the friction generated as a drop glides on a textured material infused by another liquid. Different regimes are found, depending on the viscosities of both liquids. While a viscous drop simply obeys a Stokes-type friction, the force opposing a drop moving on a viscous substrate becomes non-linear in velocity. A liquid on an infused material is surrounded by a meniscus, and this specific feature is proposed to be responsible for the special frictions observed on both adhesive and non-adhesive substrates.

Journal ArticleDOI
TL;DR: It is found that association-dissociation reactions should have a strong influence on swimmers' behaviour, and therefore should be included in future modelling, and the effect of the Debye screening length itself is investigated, going beyond the infinitely-thin-screening-length approximation used by previous analytical theories.
Abstract: Polar solvents like water support the bulk dissociation of themselves and their solutes into ions, and the re-association of these ions into neutral molecules in a dynamic equilibrium, e.g., H2O2 ⇌ H+ + HO2−. Using continuum theory, we study the influence of these association–dissociation reactions on the self-propulsion of colloids driven by surface chemical reactions (chemical swimmers). We find that association–dissociation reactions should have a strong influence on swimmers' behaviour, and therefore should be included in future modelling. In particular, such bulk reactions should permit charged swimmers to propel electrophoretically even if all species involved in the surface reactions are neutral. The bulk reactions also significantly modify the predicted speed of chemical swimmers propelled by ionic currents, by up to an order of magnitude. For swimmers whose surface reactions produce both anions and cations (ionic self-diffusiophoresis), the bulk reactions produce an additional reactive screening length, analogous to the Debye length in electrostatics. This in turn leads to an inverse relationship between swimmer radius and swimming speed, which could provide an alternative explanation for recent experimental observations on Pt-polystyrene Janus swimmers [S. Ebbens et al., Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2012, 85, 020401]. We also use our continuum theory to investigate the effect of the Debye screening length itself, going beyond the infinitely-thin-screening-length approximation used by previous analytical theories. We identify significant departures from this limiting behaviour for micron-sized swimmers under typical experimental conditions and find that the approximation fails entirely for nanoscale swimmers.

Journal ArticleDOI
TL;DR: This study hypothesized that tuning the LC phases in main-chain LCE systems can be achieved by varying the spacer length while maintaining the same mesogen (RM257), and presented several possibilities of a thiol-acrylate reaction in modulation of the thermomechanical and liquid-crystalline properties of LCEs.
Abstract: Control of the mesophase in liquid crystalline elastomers (LCEs) is a critical aspect in harnessing their unique stimuli-responsive properties Few studies have compared nematic and smectic main-chain LCEs in a direct way Traditionally, it is believed that the mesogen core and synthetic route determines the phase behavior In this study, we hypothesized that tuning the LC phases in main-chain LCE systems can be achieved by varying the spacer length while maintaining the same mesogen (RM257) By increasing the length of dithiol alkyl spacers containing two to eleven carbons along the spacer backbone (C2 to C11), we can modulate the mesophase from nematic to smectic, tailor the nematic to isotropic transition temperature between 90 and 140 °C, and increase the average work capacity from 128 to 262 kJ m−3 Phase nano-segregation resulting in the smectic C phase is achieved at room temperature for the C6, C9, and C11 spacers In a shape switching system, this manifests in impressive actuation stroke of 700% Upon heating from room temperature, these samples transition into the nematic and later, the isotropic phase Furthermore, this segregation occurs along with polymer chain crystallinity, which increases the modulus of the networks by an order of magnitude; however, the crystallization rate is highly time dependent on the spacer length and can vary between 5 minutes for the C11 spacer and 24 hours for shorter spacers This study presents several possibilities of a thiol–acrylate reaction in modulation of the thermomechanical and liquid-crystalline properties of LCEs and discusses their potential use for biomedical applications

Journal ArticleDOI
TL;DR: This work uses nonlinear manifold learning to infer structural relationships between particles according to the topology of their local environment, which yields unbiased structural information which allows them to quantify the crystalline character of particles near defects, grain boundaries, and interfaces.
Abstract: We present a machine learning technique to discover and distinguish relevant ordered structures from molecular simulation snapshots or particle tracking data. Unlike other popular methods for structural identification, our technique requires no a priori description of the target structures. Instead, we use nonlinear manifold learning to infer structural relationships between particles according to the topology of their local environment. This graph-based approach yields unbiased structural information which allows us to quantify the crystalline character of particles near defects, grain boundaries, and interfaces. We demonstrate the method by classifying particles in a simulation of colloidal crystallization, and show that our method identifies structural features that are missed by standard techniques.

Journal ArticleDOI
TL;DR: By confining an active nematic fluid within a channel, this work finds a regular motion of disclinations, in conjunction with a well defined and dynamic vortex lattice, producing a dynamic ordered state, reminiscent of Ceilidh dancing.
Abstract: The spontaneous emergence of collective flows is a generic property of active fluids and often leads to chaotic flow patterns characterised by swirls, jets, and topological disclinations in their orientation field. However, the ability to achieve structured flows and ordered disclinations is of particular importance in the design and control of active systems. By confining an active nematic fluid within a channel, we find a regular motion of disclinations, in conjunction with a well defined and dynamic vortex lattice. As pairs of moving disclinations travel through the channel, they continually exchange partners producing a dynamic ordered state, reminiscent of Ceilidh dancing. We anticipate that this biomimetic ability to self-assemble organised topological disclinations and dynamically structured flow fields in engineered geometries will pave the road towards establishing new active topological microfluidic devices.

Journal ArticleDOI
TL;DR: The mechanism of this elastic instability is presented and characterize it based on the pressure drop fluctuation spectral density, and the size of stationary dead zones correlates with the level of shear-thinning, by varying the amount of salt in HPAM solution.
Abstract: We experimentally investigate the flow of hydrolyzed polyacrylamide (HPAM) solution with and without salt in model porous media at high Weissenberg numbers (Wi > 1.0). The effect of pore shapes on the flow pattern and pressure drop is explored by using periodic arrays of circular and square pillars in aligned and staggered layouts. In the apparent shear-thinning regime, we observe stationary dead zones upstream of the pillars. In addition, we confirm that the size of stationary dead zones correlates with the level of shear-thinning, by varying the amount of salt in HPAM solution. At higher shear rates (or Wi), these dead zones are periodically washed away. We present the mechanism of this elastic instability and characterize it based on the pressure drop fluctuation spectral density.

Journal ArticleDOI
TL;DR: A theoretical approach to coacervation based on a transfer matrix formalism that is an alternative to traditional field-based approaches is outlined and provides a starting point for understanding coacervate-driven self-assembly or biophysics.
Abstract: Oppositely charged polyelectrolytes can undergo a macroscopic, associative phase separation in solution, via a process known as complex coacervation. Significant recent effort has gone into providing a clear, physical picture of coacervation; most work has focused on improving the field theory picture that emerged from the classical Voorn–Overbeek theory. These methods have persistent issues, however, resolving the molecular features that have been shown to play a major role in coacervate thermodynamics. In this paper, we outline a theoretical approach to coacervation based on a transfer matrix formalism that is an alternative to traditional field-based approaches. We develop theoretical arguments informed by experimental observation and simulation, which serve to establish an analytical expression for polymeric complex coacervation that is consistent with the molecular features of coacervate phases. The analytical expression provided by this theory is in a form that can be incorporated into more complicated theoretical or simulation formalisms, and thus provides a starting point for understanding coacervate-driven self-assembly or biophysics.

Journal ArticleDOI
TL;DR: The use of a time-salt superposition allows for a detailed analysis of changes in the linear viscoelastic response for both liquid complex coacervates and solid polyelectrolyte complexes as a function of salt concentration, and facilitates unambiguous determination of the mechanism for this phase transition.
Abstract: Polyelectrolyte complexation has long been known to result in both liquid and solid complexes. However, the exact nature of the liquid-to-solid transition remains an open question. We have used rheology to explain this phenomenon for the model system of poly(4-styrenesulfonic acid, sodium salt) (PSS) and poly(diallyldimethyl ammonium chloride) (PDADMAC) in the presence of potassium bromide (KBr). The use of a time-salt superposition allows for a detailed analysis of changes in the linear viscoelastic response for both liquid complex coacervates and solid polyelectrolyte complexes as a function of salt concentration, and facilitates unambiguous determination of the mechanism for this phase transition. Decreasing salt concentration, and the commensurate decrease in the water content of PSS/PDADMAC/KBr complexes is shown to lead to the formation of a physical gel due to the development of a network with trapped electrostatic crosslinks that percolates the sample at a critical salt concentration.

Journal ArticleDOI
TL;DR: This work designs a polymeric bilayer that changes shape in response to both heat and water by the incorporation of a water-responsive hydrophilic polymer with a heat-responsive liquid crystal elastomer and demonstrates bilayers that fold and bend based on the pattern within the LCE.
Abstract: Materials that change shape are attractive candidates to replace traditional actuators for applications with power or size restrictions. In this work, we design a polymeric bilayer that changes shape in response to both heat and water by the incorporation of a water-responsive hydrophilic polymer with a heat-responsive liquid crystal elastomer. The distinct shape changes based on stimulus are controlled by the molecular order, and consequently the anisotropic modulus, of a liquid crystal elastomer. In response to water, the hydrophilic polymer layer expands, bending the bilayer along the path dictated by the anisotropic modulus of the liquid crystal elastomer layer, which is approximately 5 times higher along the molecular orientation than in perpendicular directions. We demonstrate that by varying the direction of this stiffer axis in LCE films, helical pitch of the swollen bilayer can be controlled from 0.1 to 20 mm. By spatially patterning the stiffer axis with a resolution of 900 μm2, we demonstrate bilayers that fold and bend based on the pattern within the LCE. In response to heat, the liquid crystal elastomer contracts along the direction of molecular order, and when this actuation is constrained by the hydrophilic polymer, this contraction results in a 3D shape that is distinct from the shape seen in water. Furthermore, by using the vitrification of the dry hydrophilic polymer this 3D shape can be retained in the bilayer after cooling. By utilizing sequential exposure to heat and water, we can drive the initially flat bilayer to reversibly shift between 3D shapes.

Journal ArticleDOI
TL;DR: All of the data indicate that both the hydrogen bonds in the first network and the ionic coordination interactions in the second network act as reversible sacrificial bonds to dissipate energy, thus conferring high mechanical and recovery properties to the prepared Agar/PAAc-Fe3+ DN gels.
Abstract: Toughness and self-healing properties are desirable characteristics in engineered hydrogels used for many practical applications. However, it is still challenging to develop hydrogels exhibiting both of these attractive properties in a single material. In this work, we present the fabrication of fully physically-linked Agar/PAAc-Fe3+ DN gels. These hydrogels exhibited dual physical crosslinking through a hydrogen bonded crosslinked agar network firstly, and a physically linked PAAc-Fe3+ network via Fe3+ coordination interactions secondly. Due to this dual physical crosslinking, the fabricated Agar/PAAc-Fe3+ DN gels exhibited very favorable mechanical properties (tensile strength 320.7 kPa, work of extension 1520.2 kJ m-3, elongation at break 1130%), fast self-recovery properties in Fe3+ solution (100% recovery within 30 min), in 50 °C conditions (100% recovery within 15 min), and under ambient conditions (100% recovery of the initial properties within 60 min), as well as impressive self-healing properties under ambient conditions. All of the data indicate that both the hydrogen bonds in the first network and the ionic coordination interactions in the second network act as reversible sacrificial bonds to dissipate energy, thus conferring high mechanical and recovery properties to the prepared Agar/PAAc-Fe3+ DN gels.

Journal ArticleDOI
TL;DR: This work outlines a materials design paradigm that includes morphological organization based on jammed versus networked microstructures, collected scaling laws for predictive design, and low-dimensional descriptions of function-valued flow data to create an ontology and database for the engineering of yield-stress fluids.
Abstract: We present a paradigm for the design of yield-stress fluids, using six archetypal materials for demonstration. By applying concepts of engineering design, we outline a materials design paradigm that includes (i) morphological organization based on jammed versus networked microstructures, (ii) collected scaling laws for predictive design, (iii) low-dimensional descriptions of function-valued flow data, (iv) consideration of secondary properties including viscous behavior, and (v) a strategy for material concept synthesis based on the juxtaposition of microstructures. By explicitly specifying these design strategies, we seek to create an ontology and database for the engineering of yield-stress fluids. Our proposed design strategy increases the likelihood of finding an optimal material and prevents design fixation by considering multiple material classes to achieve a desired rheological performance. This flips the typical structure-to-rheology analysis to become the inverse: rheology-to-structure with multiple possible materials as solutions.

Journal ArticleDOI
TL;DR: A comparison of the χ parameter obtained from FR theory with values for the linear polymer reveals that the agreement between experiment and theory is somewhat fortuitous, and although the FR model can accurately describe experimental data, the accuracy of the obtained fit parameters is significantly poorer.
Abstract: The swelling of thermoresponsive microgels is widely modelled through Flory–Rehner theory, which combines Flory–Huggins solution thermodynamics with the affine network model of elasticity. While it has been shown that FR theory closely follows experimental results for a range of systems, the large number of free parameters required to fit size vs. temperature data make a proper evaluation of the theory difficult. In order to test the applicability of FR theory to microgel particles, we analyse viscosity and light scattering data for PNIPAM microgels as a function of temperature, cross-linking degree (f) and molar mass. In the collapsed state, the polymer volume fraction is estimated to be ϕC ≃ 0.44, independent of cross linking degree and molar mass. Fixing ϕC, f and the θ temperature to independent estimates, the FR model appears to describe microgel swelling well, particularly for high cross-linking densities. Estimates for the various fit parameters differ from earlier reports by an order of magnitude. A comparison of the χ parameter obtained from FR theory with values for the linear polymer reveals that the agreement between experiment and theory is somewhat fortuitous. Although the FR model can accurately describe experimental data, the accuracy of the obtained fit parameters is significantly poorer.

Journal ArticleDOI
TL;DR: A novel supramolecular polymer P5BD-DPHB has been constructed by the collaboration of a naphthalimide functionalized-pillar[5]arene host (P5BD) and a bis-bromohexane functionalization-pillar-arene guest (DPHB) and shows selective fluorescent "turn-on" detection for Hg2+via cation-π interactions with high selectivity and sensitivity.
Abstract: The development of novel materials for the detection and removal of Hg2+ is a very important issue due to the acute toxicity of Hg2+. Herein, a novel supramolecular polymer P5BD-DPHB has been constructed by the collaboration of a naphthalimide functionalized-pillar[5]arene host (P5BD) and a bis-bromohexane functionalized-pillar[5]arene guest (DPHB). P5BD-DPHB could form a stable supramolecular gel (P5BD-DPHB-G). Interestingly, P5BD-DPHB-G shows selective fluorescent "turn-on" detection for Hg2+via cation-π interactions with high selectivity and sensitivity. Furthermore, the Hg2+ coordinated supramolecular gel P5BD-DPHB-HgG can detect I- successively. The detection limits for Hg2+ and I- are 1.65 × 10-9 and 1.84 × 10-8 mol L-1, respectively. Even more significantly, the xerogel of P5BD-DPHB-G could remove Hg2+ from aqueous solution with excellent recyclability and ingestion capacity, and with a Hg2+ removal rate of 98%.

Journal ArticleDOI
TL;DR: Extracting the free chains (oligomers) from the silicone elastomers is shown to make the work of adhesion independent of the number of contacts, and carbon black filled hydrogenated nitrile butadiene rubber compounds reveal that strain softening at the crack tip may play an important role in determining the workof adhesion.
Abstract: Adhesion between silica glass or acrylic balls and silicone elastomers and various industrial rubbers is investigated. The work of adhesion during pull-off is found to strongly vary depending on the system, which we attribute to the two opposite effects: (1) viscoelastic energy dissipation close to an opening crack tip and (2) surface roughness. Introducing surface roughness on the glass ball is found to increase the work of adhesion for soft elastomers, while for the stiffer elastomers it results in a strong reduction in the work of adhesion. For the soft silicone elastomers a strong increase in the work of adhesion with increasing pull-off velocity is observed, which may result from the non-adiabatic processes associated with molecular chain pull-out. In general, the work of adhesion is decreased after repeated contacts due to the transfer of molecules from the elastomers to the glass ball. Thus, extracting the free chains (oligomers) from the silicone elastomers is shown to make the work of adhesion independent of the number of contacts. The viscoelastic properties (linear and nonlinear) of all of the rubber compounds are measured, and the velocity dependent crack opening propagation energy at the interface is calculated. Silicone elastomers show a good agreement between the measured work of adhesion and the predicted results, but carbon black filled hydrogenated nitrile butadiene rubber compounds reveal that strain softening at the crack tip may play an important role in determining the work of adhesion. Additionally, adhesion measurement under submerged conditions in distilled water and water + soap solutions are also performed: a strong reduction in the work of adhesion is measured for the silicone elastomers submerged in water, and a complete elimination of adhesion is found for the water + soap solution attributed to an osmotic repulsion between the negatively charged surface of the glass and the elastomer.

Journal ArticleDOI
TL;DR: Several essential differences are discovered compared to purely soft hydrogel particles, which shed light on the role played by the hard cores in the assembly and compression of these composite monolayers.
Abstract: Soft hydrogel particles show a rich structural and mechanical behaviour compared to hard particles, both in bulk and when confined in two dimensions at a fluid interface. Moreover, encapsulation into hydrogel shells makes it possible to transfer the tunability of soft steric interactions to hard nanoparticle cores, which bear interest for applications, e.g. in terms of optical, magnetic and reinforcement properties. In this work, we investigate the microstructures formed by hard core–soft shell particles at liquid–liquid interfaces upon compression. We produced model particles with the same silica core and systematically varied the shell-to-core ratio by synthesising shells with three different thicknesses. These particles were spread at an oil–water interface in a Langmuir–Blodgett trough and continuously transferred onto a solid support during compression. The transferred microstructures were analysed by atomic force and scanning electron microscopy. Quantitative image analysis provided information on the particle packing density, the inter-particle distance, and the degree of order of the monolayers. We discovered several essential differences compared to purely soft hydrogel particles, which shed light on the role played by the hard cores in the assembly and compression of these composite monolayers.

Journal ArticleDOI
TL;DR: The proposed emulsion-based ink system offers great flexibility in terms of 3D shaping and local compositional control, and can potentially help address current challenges involving the delivery of incompatible compounds in biomedical applications.
Abstract: 3D printing via direct ink writing (DIW) is a versatile additive manufacturing approach applicable to a variety of materials ranging from ceramics over composites to hydrogels. Due to the mild processing conditions compared to other additive manufacturing methods, DIW enables the incorporation of sensitive compounds such as proteins or drugs into the printed structure. Although emulsified oil-in-water systems are commonly used vehicles for such compounds in biomedical, pharmaceutical, and cosmetic applications, printing of such emulsions into architectured soft materials has not been fully exploited and would open new possibilities for the controlled delivery of sensitive compounds. Here, we 3D print concentrated emulsions into soft materials, whose multiphase architecture allows for site-specific incorporation of both hydrophobic and hydrophilic compounds into the same structure. As a model ink, concentrated emulsions stabilized by chitosan-modified silica nanoparticles are studied, because they are sufficiently stable against coalescence during the centrifugation step needed to create a bridging network of droplets. The resulting ink is ideal for 3D printing as it displays high yield stress, storage modulus and elastic recovery, through the formation of networks of droplets as well as of gelled silica nanoparticles in the presence of chitosan. To demonstrate possible architectures, we print biocompatible soft materials with tunable hierarchical porosity containing an encapsulated hydrophobic compound positioned in specific locations of the structure. The proposed emulsion-based ink system offers great flexibility in terms of 3D shaping and local compositional control, and can potentially help address current challenges involving the delivery of incompatible compounds in biomedical applications.

Journal ArticleDOI
TL;DR: In this paper, the microstructural and mechanical changes which occur during oscillatory shear flow and reformation after flow cessation of an intermediate volume fraction colloidal gel using rheometry and Brownian Dynamics (BD) simulations are examined.
Abstract: We examine the microstructural and mechanical changes which occur during oscillatory shear flow and reformation after flow cessation of an intermediate volume fraction colloidal gel using rheometry and Brownian Dynamics (BD) simulations. A model depletion colloid–polymer mixture is used, comprising a hard sphere colloidal suspension with the addition of non-adsorbing linear polymer chains. The results reveal three distinct regimes depending on the strain amplitude of oscillatory shear. Large shear strain amplitudes fully break the structure which results in a more homogenous and stronger gel after flow cessation. Intermediate strain amplitudes densify the clusters and lead to highly heterogeneous and weak gels. Shearing the gel to even lower strain amplitudes creates a less heterogonous stronger solid. These three regimes of shearing are connected to the microscopic shear-induced structural heterogeneity. A comparison with steady shear flow reveals that the latter does not produce structural heterogeneities as large as oscillatory shear. Therefore oscillatory shear is a much more efficient way of tuning the mechanical properties of colloidal gels. Moreover, colloidal gels presheared at large strain amplitudes exhibit a distinct nonlinear response characterized largely by a single yielding process while in those presheared at lower rates a two-step yielding process is promoted due to the creation of highly heterogeneous structures.

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TL;DR: In this paper, the authors used soft X-ray scattering (RSoXS) at the carbon K-edge to determine the helical pitch in the twist-bend nematic phase.
Abstract: Periodic structures of phases with orientational order of molecules but homogenous electron density distribution: a short pitch cholesteric phase, blue phase and twist-bend nematic phase, were probed by resonant soft X-ray scattering (RSoXS) at the carbon K-edge. The theoretical model shows that in the case of a simple heliconical nematic structure, two resonant signals corresponding to the full and half pitch band should be present, while only the full pitch band is observed experimentally. This suggests that the twist-bend nematic phase has a complex structure with a double-helix built of two interlocked, shifted helices. We confirm that the helical pitch in the twist-bend nematic phase is in a 10 nm range for both the chiral and achiral materials. We also show that the symmetry of the blue phase can be unambiguously determined through a resonant enhancement of the X-ray diffraction signals, by including polarization effects, which are found to be an important indicator in phase structure determination.

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TL;DR: In this article, the effect of humidity on the charge accumulation of polymer granulates shaken vertically in a stainless steel container was studied, and it was shown that the transition between these two regimes does depend on the material, as does the sign of the charge.
Abstract: We study the effect of humidity on the charge accumulation of polymer granulates shaken vertically in a stainless steel container. This setup allows us to control the humidity level from 5% to 100%RH while performing automated charge measurements in a Faraday cup directly connected to the shaking container. We find that samples of approximately 2000 polymer spheres become highly charged at low humidity levels (<30%RH), but acquire almost no charge for humidity levels above 80%RH. The transition between these two regimes does depend on the material, as does the sign of the charge. For the latter we find a correlation with the contact angle of the polymer with only very hydrophilic particles attaining positive charges. We show that this humidity dependence of tribo-charging can be used to control segregation in shaken binary mixtures.

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TL;DR: The connection between the results and recent experimental studies of dynamics of labeled DNA telomeres in living cells is discussed, and new chromatin tracking experiments are proposed that will allow one to determine the statistical properties of the active forces associated with chromatin remodeling processes.
Abstract: We consider how active forces modeled as non-thermal random noise affect the average dynamical properties of a Rouse polymer. As the power spectrum of the noise is not known we keep the analytical treatment as generic as possible and then present results for a few examples of active noise. We discuss the connection between our results and recent experimental studies of dynamics of labeled DNA telomeres in living cells, and propose new chromatin tracking experiments that will allow one to determine the statistical properties of the active forces associated with chromatin remodeling processes.