Reversible shear-induced crystallization above equilibrium freezing temperature in a lyotropic surfactant system

Abstract

We demonstrate a unique shear-induced crystallization phenomenon above the equilibrium freezing temperature ðT o K Þ in weakly swollen isotropic ðL i Þ and lamellar ðL α Þ mesophases with bilayers formed in a cationic-anionic mixed surfactant system. Synchrotron rheological X-ray diffraction study reveals the crystallization transition to be reversible under shear (i.e., on stopping the shear, the nonequilibrium crystalline phase L c melts back to the equilibrium mesophase). This is different from the shear-driven crystallization below T o K , which is irreversible. Rheological optical observations show that the growth of the crystalline phase occurs through a preordering of the L i phase to an L α phase induced by shear flow, before the nucleation of the L c phase. Shear diagram of the L i phase constructed in the parameter space of shear rate ð_ γÞ vs. temperature exhibits L i → L c and L i → L α transitions above the equilibrium crystallization temperature ðT o K Þ, in addition to the irreversible shear-driven nucleation of L c in the L i phase below T o K . In addition to revealing a unique class of nonequilibrium phase transition, the present study urges a unique approach toward understanding shear-induced phenomena in concentrated mesophases of mixed amphiphilic systems.shear-induced phase separation | strongly binding counterions | coagels S hear is known to assist crystallization below the equilibrium freezing temperature in complex fluids like colloidal glasses (1, 2), dense granular suspensions (3), polymer melts (4, 5), micellar solutions of block copolymers (6), and multicomponent surfactant systems (7,8). Shear-driven crystallization is equally relevant for simple fluids like bulk metallic glasses (9), molecular liquids (10), and atomic systems (11). The general understanding is that shear lowers the energy barrier for nucleation and accelerates the growth of a stable crystalline phase from a metastable, amorphous/ isotropic solution at Peclet number Pe = σa 3 kBT > 1, where σ is the shear stress, a is the characteristic length scale, and k B T is the thermal energy (12). The crystalline phase primarily induced by the effect of flow on the internal structure and ordering of the constituents does not revert to the starting fluid state when the imposed shear is removed, indicating that the phenomenon is not a dynamic phase transition. In the present study, we report a unique phenomenon, where under steady shear, crystallization occurs above the equilibrium crystallization temperature ðT o K Þ in an isotropic mesophase ðL i Þ consisting of bilayers formed in a lyotropic surfactant system. Notably, above T o K , when the imposed shear is removed, the crystalline phase melts back to the starting L i phase.The studies were carried out in a cationic-anionic mixed surfactant system formed by SDS and the strong binding counter-ion paratoluene hydrochloride (PTHC) in water. At equilibrium, the organic counter-ion PTHC has the tendency to remain at the micelle-water interface, decreasing the spontaneous curvat...

The non-equilibrium phase diagrams of flow-induced crystallization and melting of polyethylene

“…Reports on FIC at temperatures well above the equilibrium melting point are rather rare. Nevertheless, sufficiently strong flow may reverse the relative stabilities of different phases and induce the occurrence of crystallization as a dynamic phase transition, where crystal melts after flow is removed 13 14 . Because of non-equilibrium dissipative nature, here appearing and surviving of crystal depend critically on the flow strength.…”

Formation of crystal-like structures and branched networks from nonionic spherical micelles

“…2(c) ). We speculate that due to the coupling between low temperature and high strain ( T < 15 °C and γ ~ 10 4 –10 6 ), the spherical micelles in the precursor are initially aligned in the flow direction, subsequently stacked in both neutral and velocity gradient directions over time, inducing micellar growth, creating possible nucleation sites for crystallization 29 30 33 34 35 36 37 38 39 40 41 42 43 44 46 47 . This observation is consistent with existing reports for both surfactants and colloidal solutions 29 30 33 34 35 36 37 38 39 40 41 42 43 44 46 47 , where isotropic phases could grow in the neutral direction of the flow velocity.…”

“…Shear flow is also known to induce ordering, crystallization, and phase transitions in surfactant solutions, polymer solutions, and colloidal systems 3 14 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 . The transition from nonionic surfactant micelles to higher order structures (e.g., lamellar, nematic, or onion phases) has been accomplished by coupling shear flow with temperature variations 29 30 33 34 35 36 37 38 39 40 41 42 . For example, Linemann et al 34 studied the orientation of C m E n nonionic surfactants in isotropic, cubic and hexagonal phases by using rheo-SALS while varying the temperature from 5 to 45 °C.…”

“…For example, Berret 39 reported the formation of shear-induced nematic phases from ionic wormlike micelles under transient shear flows. Rathee et al 29 studied the crystallization of a lyotropic ionic surfactant solution, proposing that shear-induced modification of spontaneous curvature in amphiphilic molecules could drive phase transition in concentrated bilayers. Similarly, Koppi et al 33 showed that isotropic diblock copolymers could be ordered under shear, forming lamellar structures perpendicular to the flow-velocity gradient.…”

Extensional Flow‐Induced Dynamic Phase Transitions in Isotactic Polypropylene

“…Above the melting point of β or α crystals ( T mβ or T mα ), FIC may be induced by strong flow due to reversed thermodynamic stabilities of melt and crystals, which can also be regarded as the increase of the crystallization temperature under flow . Once flow or stress is withdrawn, crystals melt back, where FIC is a dynamic phase transition depending more on external flow rather than temperature . In this case, dynamic competitions among melting, crystallizations of α and β as well as β–α transition may take place during and after flow.…”