Shear-induced Transition Research Articles

Tracking Mechanical Stress and Cell Migration with Inexpensive Polymer Thin-Film Sensors.

Polydiacetylene (PDA) Langmuir films are well known for their blue to red chromatic transitions in response to a variety of stimuli, including UV light, heat, bio-molecule bindings and mechanical stress. In this work, we detail the ability to tune PDA Langmuir films to exhibit discrete chromatic transitions in response to applied mechanical stress. Normal and shear-induced transitions were quantified using the Surface Forces Apparatus and established to be binary and tunable as a function of film formation conditions. Both monomer alkyl tail length and metal cations were used to manipulate the chromatic transition force threshold to enable discrete force sensing from ~50 to ~500 nN μm-2 for normal loading and ~2 to ~40 nN μm-2 for shear-induced transitions, which are appropriate for biological cells. The utility of PDA thin-film sensors was demonstrated with the slime mold Physarum polycephalum. The fluorescence readout of the films enabled: the area explored by Physarum to be visualized, the forces involved in locomotion to be quantified, and revealed novel puncta formation potentially associated with Physarum sampling its environment.

Stress Overshoots in Simple Yield Stress Fluids.

Soft glassy materials such as mayonnaise, wet clays, or dense microgels display a solid-to-liquid transition under external shear. Such a shear-induced transition is often associated with a nonmonotonic stress response in the form of a stress maximum referred to as "stress overshoot." This ubiquitous phenomenon is characterized by the coordinates of the maximum in terms of stress σ_{M} and strain γ_{M} that both increase as weak power laws of the applied shear rate. Here we rationalize such power-law scalings using a continuum model that predicts two different regimes in the limit of low and high applied shear rates. The corresponding exponents are directly linked to the steady-state rheology and are both associated with the nucleation and growth dynamics of a fluidized region. Our work offers a consistent framework for predicting the transient response of soft glassy materials upon startup of shear from the local flow behavior to the global rheological observables.

Fine-Tuning Crystallization-Induced Gelation in Amphiphilic Double-Brush Polymers.

A series of amphiphilic double-brush polymers based on itaconate diesters were synthesized with the objective of tailoring the thermal and mechanical properties of hydrogels formed by them; the amphiphilic itaconate diesters carried an MPEG350 segment and an alkyl chain, whose length was varied from C12 to C18. As was reported by us earlier (Macromolecules 2017, 50, 5004), the formation of the hydrogel was due to the crystallization of alkyl segments, as confirmed by the match of the rheological gel-to-sol transition with that of differential scanning calorimetry melting transition of the gel. In an effort to fine-tune the hydrogel-melting temperature and its strength, we varied the length of the alkyl chain length while keeping the hydrophilic segment length constant at MPEG350; apart from varying the alkyl chain length, an oxyethylene spacer was incorporated to examine the effect of decoupling the alkyl side-chain crystallization from the backbone. With these modifications, the melting temperature of the hydrogel was varied from 30 to 56 °C. Likewise, the strength of the hydrogel, as reflected by its storage modulus, varied from around 220 to 970 Pa; the softer gels typically exhibited a slightly larger critical shear strain beyond which the gel transformed into a sol. The thermal and shear-induced gel-to-sol transitions were reversible; however, the modulus after the shear-induced transition did not fully recover instantly (∼80%), suggesting that the formation of the extended gel network is slow. Further fine-tuning could be achieved by copolymerization of two different amphiphilic itaconate monomers, namely, those with C16 and C18, which provided an intermediate gel-melting temperature; however, co-gelation of the two preformed homopolymer gels yielded two distinct gel-melting transitions. Thus, this class of tuneable stimuli-responsive polymeric hydrogels prepared from biobenign components, namely, itaconic acid, 1-alkanols, and MPEGs, could serve as potential candidates for biomedical applications.

Unexpected rheological behavior of solutions of aromatic polyamide in transient physical states

The subject of this study was an aromatic polyamide in dimethylacetamide/LiCl solutions in a concentration range from 0.5 vol. % – 5 vol. %. Dilute and semi-dilute solutions of this polymer demonstrate a complex of unexpected time- and temperature-dependent rheological effects under shearing in a heating–cooling cycle. In a static state, all systems under study are transparent solutions and no temperature-dependent thermal or visual effects are observed. However, superposition of shearing radically changes the situation. Heating up to 100 °C–140 °C leads to the phase separation with the coexistence of the amorphous and LC phases. On cooling of low-concentrated solutions, a decrease in the temperature leads to a several-times increase in the viscosity, and the subsequent viscosity decrease takes place at further temperature decrease. Both changes are kinetic effects. The first one is treated as an order-to-disorder transition. The decrease in viscosity is accompanied by a heat release, which reflects the reverse process of the disorder-to-order state transition. The isothermal viscosity decline in time is described by the Maxwell relaxation law with temperature-independent relaxation time. Hence, this is a non-temperature-activated process. At higher concentrations, strong temperature thixotropic behavior with much lower viscosity values on cooling, in comparison with the heating, is characteristic of these solutions due to their tendency for undercooling. The shear-induced transition, conjugated with the heat excess, was observed in semi-dilute solutions at the same temperature similar to that observed for dilute solutions. So during cooling under shearing, the solutions under study display a rather unusual phenomenon of a first-order transition. This type of phenomenon has not been described before. The phase transitions become quite evident in the polarized light. The observed kinetic effects in transient physical states are discussed on the basis of the concept of delayed and latent structure and phase transitions, including the formation of the LC state, which are initiated by the shearing.

Diagenetic and shear-induced transitions of frictional strength of carbon-bearing faults and their implications for earthquake rupture dynamics in subduction zones

Subduction-related diagenetic reactions affect fault strength and are thus important for understanding earthquake rupture dynamics in subduction zones. Carbonaceous material (CM) is found worldwide in active plate-boundary and intracontinental faults, yet the effect of its transformation on frictional strength and rupture dynamics remains unknown. We conducted high-velocity friction experiments together with organochemical analyses on CM in the form of lignite, bituminous coal, anthracite and graphite. Results clearly show that an increase in CM maturity and crystallinity leads to a decrease in the peak friction coefficient (from 0.5 to 0.2). We also infer that friction applied to low-grade CM increases its maturity, but friction applied to high-grade CM reduces its maturity. These findings suggest that both diagenetic and shear-induced transformations of CM strongly affect the frictional strength of CM-bearing faults, potentially affecting the depth-dependences of frictional strength and rupture dynamics on plate-subduction faults.

Shear-induced assembly of a transient yet highly stretchable hydrogel based on pseudopolyrotaxanes.

Dissipative self-assembly is common in biological systems, where it serves to maintain a far-from-equilibrium functional state through fuel consumption. Synthetic dissipative systems have been prepared that can mimic some of the properties of biological systems, but they often show poor mechanical performance. Here, we report a shear-induced transient hydrogel that is highly stretchable. The system is constructed by adding Cu(II) into the aqueous solution of a pseudopolyrotaxane, which is itself formed by threading molecular tubes on polyethylene glycol chains. Vigorous shaking transforms the solution into a gel, which gradually relaxes back to the sol state over time. This cycle can be repeated at least five times. A mechanism is proposed that relies on a shear-induced transition from intrachain to interchain coordination and subsequent thermal relaxation. The far-from-equilibrium hydrogel is highly stretchable, which is probably due to 'frictional' sliding of the molecular tubes on the polyethylene glycol chains. On shaking, the hydrogel undergoes fast self-healing.

A theoretical prediction on the shear-induced phase transformation of TKX-50.

Dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50) is a new and attractive energetic material that outperforms numerous common explosives because of its excellent properties and performance, and is thus a promising candidate to replace some of them. Nevertheless, knowledge of its physico-chemical properties, in particular, the underlying mechanism for it undergoing external stimuli for complete decay still remains poor. In the present study, we ascertain a preferred slip system of (010)/[101] and a shear-induced phase transition of TKX-50 with the aid of theoretical calculations. In other words, a new phase of TKX-50, γ-TKX-50, is observed to be formed by shearing TKX-50 along a slip system of (010)/[101] or (010)/[101[combining macron]] with a space group of P21/a, elevated energy of 9.4 kcal mol-1 and a unit cell expanded 4%, relative to the original TKX-50. Moreover, γ-TKX-50 can most readily be formed by shearing TKX-50 along (010)/[101] with a lowest energy barrier of 18.6 kcal mol-1, which is much below that for TKX-50 decay. The predicted elastic constants of γ-TKX-50 verify its mechanical stability with decreased mechanical anisotropy relative to the original TKX-50. In addition, we find that, after phase transition, the hydrogen bonding is weakened, while the electrostatic repulsion of Hδ+Hδ+ increases, which disfavors the proton transfer from NH3OH+ to C2O2N82- to facilitate the thermal decay of TKX-50. This suggests that the shear-induced transition from TKX-50 to γ-TKX-50 can enhance thermal stability by elevating the energy barrier for proton transfer, potentially contributing to the low mechanical sensitivity of TKX-50. Hopefully, this study would enrich the insight into the underlying mechanism of TKX-50 against external thermal-mechanical stimuli. Moreover, in combination with the newly found heat-induced phase, the shear-induced phase observed in the present study and the original one, there are at least three phases for TKX-50.

On the use of shear rheology to formulate stable foams. Example of a lyotropic lamellar phase

In order to formulate stable foams with gels, it is important to know their shear rheology properties. We illustrate this fact using a weak gel made of a lyotropic lamellar phase (SDS/hexanol/brine system). The effect of shear is investigated using rheology experiments coupled with Small Angle X-ray Scattering (rheo-SAXS) using synchrotron radiation. According to oscillatory tests, the samples exhibit a weak gel behavior. Steady state experiments evidence a shear-induced transition from lamellar to onion (multilamellar vesicle – MLV) textures at moderate shear rates. After the shear is stopped, there is a recovery from the onion texture to the initial lamellar texture within 30min, as directly demonstrated by the rheo-SAXS data. The lamellar to onion transition is therefore reversible. This excludes the possibility for the foams to be stabilized by onions adsorbed at the air–water interface as in foams made with other types of lamellar phases. In the example presented, the foam stability originates from the bulk rheological properties of the lamellar phase.

The cation-controlled and hydrogen bond-mediated shear-thickening behaviour of a tree-fern isolated polysaccharide

The shear-thickening rheological behaviour (between 5 and 20s−1) of a 5% (w/w) viscoelastic gum extracted from the fronds of the native New Zealand black tree fern or mamaku in Māori was further explored by manipulating the salt content. The freeze-dried mamaku gum contained a high mineral content and sugars which upon removal via dialysis, resulted in the loss of shear thickening. However, this loss was reversible by the addition of salts to the dialysed dispersion. The mechanism of shear-thickening behaviour was therefore hypothesised to be due to shear-induced transition of intra- to intermolecular hydrogen bonding, promoted by the screening effect of cations. Mono-, di- and trivalent salts, i.e. Na+, K+, N(CH3)4+, Ca2+, Mg2+, Al3+ and La3+ at concentrations between 0.001 and 1.0M were tested to support the hypothesis as well as to demonstrate the sensitivity of the biopolymer to cation valency and concentrations. The cation valency and concentration were crucial factors in determining: (i) zero-shear viscosity, (ii) critical shear rate, γ˙c (or shear rate at the onset of shear-thickening) and (iii) the extent of shear-thickening of the solution. For mono- and divalent cations these parameters were similar at equivalent ionic strengths and fairly independent of the cation type. Trivalent cations (La3+) however caused precipitation of the gum in the concentration range of 0.005–0.05M but clear dispersions were obtained above 0.05M.

Suspensions of carbon nanofibers in organic medium: rheo-electrical properties.

The nonaqueous suspensions of carbon nanofibers (CNFs) in 1 M lithium bis(trifluoromethanesulfonaimide) in propylene carbonate electrolyte reveal unique structural evolution and shear-induced transition due to the high aspect ratio. The rheo-electrical behavior elucidates a microstructural transition from entangled-to-aggregated networks above a distinct percolation threshold. Under shear flow, both networks show a three-regime flow curve and an inverted-bell-like conductivity curve as a consequence of shear-induced alignment (entangled network) and shear-induced breaking up (aggregated network). The different particle morphology of carbon nanofibers (anisometric) and carbon black (CB; isometric) causes different aggregation mechanisms (aggregate vs. particulate) and then varied microstructure for their suspensions in the same electrolyte. This fact explains the higher rigidity and lower electric conductivity of CNFs than CB suspensions. Interestingly, the suspension of hybrid carbons at the optimum mixing ratio merges the advantages of both carbons to operate efficiently as precursors in the formulation of electrodes for energy storage systems.

Nonequilibrium dynamics of a confined colloidal bilayer in a planar shear flow

Using Brownian dynamics (BD) simulations we investigate the impact of shear flow on structural and dynamical properties of a system of charged colloids confined to a narrow slit pore. Our model consists of spherical microions interacting through a Derjaguin-Landau-Verwey-Overbeek (DLVO) and a soft-sphere potential. The DLVO parameters were chosen according to a system of moderately charged silica particles (with valence Z~35) in a solvent of low ionic strength. At the confinement conditions considered, the colloids form two well-pronounced layers. In the present study we investigate shear-induced transitions of the translational order and dynamics in the layers, including a discussion of the translational diffusion. In particular, we show that diffusion in the shear-melted state can be described by an analytical model involving a single shear-driven particle in a harmonic trap. We also explore the emergence of zigzag motion characterized by spatiotemporal oscillations of the particles in the layers in the vorticity direction. Similar behavior has been recently observed in experiments of much thicker colloidal films.

Minimal model for zero-inertia instabilities in shear-dominated non-Newtonian flows

The emergence of fluid instabilities in the relevant limit of vanishing fluid inertia (i.e., arbitrarily close to zero Reynolds number) has been investigated for the well-known Kolmogorov flow. The finite-time shear-induced order-disorder transition of the non-Newtonian microstructure and the corresponding viscosity change from lower to higher values are the crucial ingredients for the instabilities to emerge. The finite-time low-to-high viscosity change for increasing shear characterizes the rheopectic fluids. The instability does not emerge in shear-thinning or -thickening fluids where viscosity adjustment to local shear occurs instantaneously. The lack of instabilities arbitrarily close to zero Reynolds number is also observed for thixotropic fluids, in spite of the fact that the viscosity adjustment time to shear is finite as in rheopectic fluids. Renormalized perturbative expansions (multiple-scale expansions), energy-based arguments (on the linearized equations of motion), and numerical results (of suitable eigenvalue problems from the linear stability analysis) are the main tools leading to our conclusions. Our findings may have important consequences in all situations where purely hydrodynamic fluid instabilities or mixing are inhibited due to negligible inertia, as in microfluidic applications. To trigger mixing in these situations, suitable (not necessarily viscoelastic) non-Newtonian fluid solutions appear as a valid answer. Our results open interesting questions and challenges in the field of smart (fluid) materials.

Rheological and Microstructural Behavior of a Model Concentrated Fabric Softener

Shear-induced microstructural transitions in aqueous systems of a cationic commercial esterquat-type surfactant used in concentrated fabric softeners were rheologically and microscopically characterized. Binary (surfactant/water) and ternary (surfactant/CaCl2/water) systems were studied. The obtained flow curves demonstrated the shear-thickening behavior in ternary systems at high shear rates, provoked by shear-induced transitions from surfactant bilayers to multilamellar vesicles. This fact was confirmed by hysteresis-loop experiments which proved that a critical shear rate was required to observe apparent antithixotropic behavior in the ternary systems. Start-up flow tests indicated that the increase of shear stress with shear time was more marked as the shear rate was increased. Finally, a comparative aging study of ternary systems with a surfactant used in concentrated fabric softeners and the ternary systems with a surfactant utilized in diluted and semiconcentrated fabric softeners was performed.

Diblock copolymer sphere morphology in ultra thin films under shear

We demonstrate that the shear alignment and the shear-induced transitions in sphere-forming diblock copolymer single layer and bilayer films observed experimentally [Y.-R Hong, D. H. Adamson, P. M. Chaikin and R. A. Register, Soft Matter, 2009, 5, 1687] can be explained by cell dynamics simulation, a simple model with a Ginzburg–Landau Hamiltonian. In two layer films the spheres align in various arrangements, like (100) or (110) bcc planes, or transform to cylinders depending on the shear rate and the temperature. For the first time, we present a nontrivial alignment mechanism of a single layer of spherical domains in shear via slug-like movement of transient cylindrical micelles. In addition, we clarify the formation of the perpendicular cylinders, found in the recent particle based simulation [A. Chremos, K. Margaritis, A. Z. Panagiotopoulos, Soft Matter, 2010, 6, 3588].

Rheological behaviour of polyoxometalate-doped lyotropic lamellar phases

We study the influence of nanoparticle doping on the lyotropic liquid crystalline phase of the industrial surfactant Brij®30 (C₁₂E₄) and water, doped with spherical polyoxometalate nanoparticles smaller than the characteristic dimensions of the host lamellar phase. We present viscometry and in situ rheology coupled with small-angle X-ray scattering data that show that, with increasing doping concentration, the nanoparticles act to decrease the shear viscosity of the lamellar phase, and that a shear-induced transition to a multilamellar vesicle "onion" phase is pushed to higher shear rates, and in some cases completely suppressed. X-ray data reveal that the nanoparticles remain encapsulated within the membranes of the vesicles, thus indicating a viable method for the fabrication of nanoparticle incorporating organic vesicles.

Microphase transitions of block copolymer/nanorod composites under shear flow

The shear-induced reorientations and phase transitions of symmetric diblock copolymer/nanorod nanocomposites subjected to steady shear flow, are systematically investigated via dissipative particle dynamics (DPD) simulation method. Both selective and nonselective nanorods (NRs) are considered, with low and high NRs concentrations. To preserve lamellar morphology in the nanocomposites, the NRs concentration is controlled below 15%. The final morphologies of nanocomposites depend on the interplay between NRs and diblock copolymers (DBCPs) under shear flow. The cooperatively induced microphase structures include lamellar (parallel, diagonal and perpendicular alignment), ribbon and cylindrical phases. The presence of anisotropic NRs not only delays or advances the shear-induced transition in lamellae orientation, but also induces the phase transitions by changing the effective composition of the copolymers (feff) in nanocomposites. Meanwhile, the low viscosity in the perpendicular lamellae of DBCPs provides an energetic reason for the lamellae changing into perpendicular orientation, with the shear thinning. The shear thinning corresponds to the reorientation of the lamellae, while the shear thickening corresponds to the phase transition from lamellar to ribbon, and to cylindrical structure.

Heterogeneous yielding dynamics in a colloidal gel

Attractive colloidal gels display a solid-to-fluid transition as shear stresses above the yield stress are applied. This shear-induced transition is involved in virtually any application of colloidal gels. It is also crucial for controlling material properties. Still, the yielding transition is far from understood, mainly because rheological measurements are spatially averaged over the whole sample. We use high-frequency ultrasound during creep and oscillatory shear experiments to observe the local dynamics of opaque attractive colloidal gels. The transition proceeds from the cell walls and heterogeneously fluidizes the whole sample with a characteristic time that exponentially decreases with the applied stress. The present results reveal the importance of activated processes for the gel dynamics and raise a number of open questions in the attempt to better understand the yielding transition.

Phase behavior and microstructure evolution in aqueous mixtures of cetyltrimethylammonium bromide and sodium dodecyl tri-oxyethylene sulfate

Abstract The phase behavior and microstructural changes in aqueous mixtures of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl tri-oxyethylene sulfate (C12E3S) are reported. The mixing behavior of the two surfactants is investigated via surface tension measurements and the regular solution approximation yielded a β value of −13, indicating attractive interaction. The difference in the growth behavior of the micelles on the CTAB-rich and C12E3S-rich sides of the phase diagram is studied using viscosity, quasielastic light scattering, small angle neutron scattering, and cryogenic transmission electron microscopy (cryoTEM). Branched micelles are formed close to the phase boundary and are more prevalent in CTAB-rich micelles. A shear-induced transition of cylindrical to ribbon-like micelles is observed by cryoTEM in CTAB-rich micelles near the phase boundary. A temperature-induced phase transition is observed for C12E3S-rich micelles due to the dehydration and/or conformational changes of the hydrophilic oxyethylene part of C12E3S in the mixed micelles.

Shear-Induced Transitions between a Planar Lamellar Phase and Multilamellar Vesicles: Continuous versus Discontinuous Transformation

The shear-induced transitions between an oriented lamellar phase and shear-induced multilamellar vesicles (MLVs) in a nonionic surfactant system were studied by deuterium rheo-NMR spectroscopy as a function of time in start-up experiments at several temperatures and shear rates. By starting from an initial state of oriented lamellae and observing the transformation to the final steady state of MLVs and vice-versa, two different mechanisms were found, depending on the direction of the transition and the initial state. The transition is continuous when MLVs are formed, starting from the oriented lamellar phase. On the other hand, a discontinuous nucleation-and-growth process with a coexistence region is observed when transforming MLVs into an oriented lamellar phase.

Giant Micelles of Organoplatinum(II) Gemini Amphiphiles

Organoplatinum(II) gemini amphiphiles with two different chain lengths are synthesized and characterized. Self-assembly at the air-water interface is investigated as a function of chain length and reduction in surface area by using Langmuir-trough techniques. The Langmuir-trough experiments lead to a conjecture that surface aggregates may be the adsorbing units. Atomic force microscopy on the transferred Langmuir-Schaefer films reveals spontaneous formation of wormlike micellar aggregates. A shear-induced transition and alignment are proposed for the observed effects.