Patrick Baroni

Two-dimensional neutron scattering in a floating heavy water bridge

When a high voltage is applied to pure water in two filled beakers kept close to each other, a connection forms spontaneously, giving the impression of a floating water bridge. This phenomenon is of special interest, since it comprises a number of phenomena currently tackled in modern water science. In this work, the first two-dimensional structural study of a floating heavy water bridge is presented as a function of the azimuthal angle. A small anisotropy in the angular distribution of the intensity of the first structural peak was observed, indicating a preferred orientation of a part of the D2O molecules along the electric field lines without breaking the local tetrahedral symmetry. The experiment is carried out by neutron scattering on a D2O bridge.

Identification of a low-frequency elastic behaviour in liquid water.

This article deals with the identification of solid-like properties measured at room temperature at a sub-millimetre length scale in liquid water. At a macroscopic scale, normal liquids (i.e. above melting temperature), and in particular water, are typically and empirically defined by the absence of shear elasticity, in contrast to solids or plastic fluids that require a stress threshold for flowing. A novel method optimizing the transmission of the shear stress to the sample enables a more complete probe of the mechanical response of liquids. It reveals that glass formers and viscous alkanes actually exhibit finite macroscopic shear elasticity away from any phase transition. This protocol is here applied for the first time to liquid water at room temperature, revealing, at the sub-millimetre scale, a low-frequency solid-like property.

Identification of finite shear-elasticity in the liquid state of molecular and polymeric glass-formers

Finite shear elasticity has been identified in the liquid state. The study is expanded to a van der Waals molecular glass-former, o-terphenyl, and to an ordinary polymer melt, polybutylacrylate, as a function of their molecular weight. These fluids exhibit shear elasticity at the sub-millimetre scale and far above any phase transition. This macroscopic property challenges the conventional terminal relaxation modes (α-process or terminal viscoelastic times (reptation)) in terms of individual molecular process.

New Light on Old Wisdoms on Molten Polymers: Conformation, Slippage and Shear Banding in Sheared Entangled and Unentangled Melts

The flow of viscoelastic materials is usually interpreted as resulting from intramolecular properties. Typically, the non-linear flow behaviour and sluggish relaxation dynamics in entangled polymers are interpreted by a disentanglement process. This molecular interpretation has never been validated by direct observation. We report here on in situ observations of polymer melts under steady-state shear flow using neutron scattering and particle tracking velocimetry. It is shown that the chains remain largely undeformed under steady-state shear flow whereas wall slippage and shear-banding are identified in both entangled and unentangled polymer melts. These observations are of prime importance; they reveal that the flow mechanism and its viscoelastic signature reflect a collective effect and not properties of individual chains.

Highlighting a Cooling Regime in Liquids under Submillimeter Flows.

The shear flow of ordinary liquids is for the first time observed at the submillimeter scale by thermal imaging. We report on microinfrared experiments, showing that liquids as important as water flowing on wetting surfaces produce cooling, while the academic view would foresee heating production. This apparent counterintuitive cooling effect shows that the increase of the internal energy due to the flow can result in different shapes, including a cooling process, before reaching the conventional heating regime at higher shear rates. This unknown property might be interpreted as a transient stretching state of the liquid. Shearing liquids might be a promising alternative compared to conventional endothermic processes (gas expansion or vaporization of a liquid, the Peltier effect, and so forth).

Shear-Induced Isotropic to Nematic Transition of Liquid-Crystal Polymers: Identification of Gap Thickness and Slipping Effects

The shear-induced isotropic-nematic transition is a generic property of liquid-crystalline polymer melts which is identified by the emergence in the isotropic phase of a strong birefringence above a critical shear rate. Although spectacular, this transition cannot be explained on the basis of a conventional approach (coupling with pretransitional fluctuations or with viscoelastic relaxation times). We investigate the asymptotic rheo-optical behavior of the shear-induced phase. The sample is an unentangled cyanobiphenyl side-chain polyacrylate. We show that the birefringence increases almost linearly and then saturates above a given shear rate depending on the gap thickness. Similarly, the shear stress versus shear rate curve exhibits for the same thickness and using the same substrate (quartz), an overshoot occurring at about the same shear rates followed by the stress plateau (indicating a stress-optical equivalence). In contrast, when the substrate is different, the stress-optical equivalence is no more valid. We interpret the overshoot and the plateau observed in birefringence and in shear stress curves as the entrance in a so far unidentified macroscopic sliding regime. The slipping mechanism is corroborated by the recent identification of macroscopic long-range correlations in unentangled polymer melt and contrasts with a description in terms of a nucleation-growth process as in wormlike micellar solutions.

Bringing to Light Hidden Elasticity in the Liquid State Using In-Situ Pretransitional Liquid Crystal Swarms.

The present work reveals that at the sub-millimeter length-scale, molecules in the liquid state are not dynamically free but elastically correlated. It is possible to “visualize” these hidden elastic correlations by using the birefringent properties of pretransitional swarms persistent in liquids presenting a weak first order transition. The strategy consists in observing the optical response of the isotropic phase of mesogenic fluids to a weak (low energy) mechanical excitation. We show that a synchronized optical response is observable at frequencies as low as 0.01Hz and at temperatures far away from any phase transition (up to at least 15°C above the transition). The observation of a synchronized optical signal at very low frequencies points out a collective response and supports the existence of long-range elastic (solid-like) correlations existing at the sub-millimeter length-scale in agreement to weak solid-like responses already identified in various liquids including liquid water. This concept of elastically linked molecules differs deeply with the academic view of molecules moving freely in the liquid state and has profound consequences on the mechanisms governing collective effects as glass formation, gelation and transport, or synchronized processes in physiological media.

Structural investigation of the pressure induced effects in liquid crystals

Liquid crystal phases are characterized by a long range orientational order. Numerous studies on liquid crystals under hydrostatic pressure display interesting pressure induced phenomena which seem to indicate that this long range order is disturbed. It is shown for example that re-entrant phases appear and that phase transition temperature shifts are commonly observed. These pressure induced effects result from the packing properties exacerbated by the pressure induction of the molar volume. Despite numerous developments, the structural modifications which accompany these pressure induced effects, as the phase transition temperature shifts, have hardly been investigated. We provide, using neutron diffraction, the physical reasons for these temperature shifts. We report here on the very few structural studies of the influence of hydrostatic pressure on the stability of a liquid crystal phase. This study is carried out using two specifically designed neutron pressure cells reaching pressure values up to 120 MPa. The liquid crystal system is described in terms of pressure-induced correlation lengths and layer spacing, which are the relevant parameters to account for the phase structure. It will be shown that the structural investigation is particularly noteworthy in the lamella phase since the characteristic lengths can be tremendously modified under pressure, underlining a correlated change of dynamics. In the case of high molecular weight liquid crystals (side-liquid crystal polymers), it will be shown that the re-entrant nematic-smectic transition is unchanged with respect to the pressure, indicating that the pressure induced reduction of the specific volume concerns the polymer component only.

Harmonic strain-optical response revealed in the isotropic (liquid) phase of liquid crystals

A strong optical birefringence is observed when applying a small amplitude oscillatory strain to the liquid phase of a liquid crystal. This unpredicted birefringence is found to oscillate at the same frequency as the driving frequency, with frequencies down to 0.01 Hz. This birefringence is visible up to 15 °C above the liquid crystal transition. This opto-dynamic property is interpreted as a result of a coupling of the orientational pretransitional fluctuations existing in the isotropic phase and long range elastic interactions recently identified in liquids. The conversion of the mechanical wave in an optical response is shapeable. Two examples of synchronized periodic signals are shown: the sine and the square waves. The optimization of the signal is analyzed using a Heaviside-step shear test. This optical property is immediately exploitable to design low energy on/off switching materials.

Pressure effects revealed by small angle neutron scattering on block copolymer gels.

We study the effects of hydrostatic pressure (P) on aqueous solutions and gels of the block copolymer B(20)E(610) (E, oxyethylene; B, oxybutylene; subscripts, number of repeats), by performing simultaneous small angle neutron scattering/pressure experiments. Micellar cubic gels were studied for 9.5 and 4.5 wt % B(20)E(610) at T = 20-80 and 35-55 degrees C, respectively, while micellar isotropic solutions where studied for 4.5 wt % B(20)E(610) at T > 55 degrees C. We observed that the interplanar distance d 110 (cubic unit cell parameter a = [see text for formula]) decreases while the correlation length of the cubic order (delta) increases, upon increasing P at a fixed T for 9.5 wt % B(20)E(610). The construction of master curves for d(110) and delta corresponding to 9.5 wt % B(20)E(610) proved the correlation between changes in T and P. Neither d(110) and delta nor the cubic-isotropic phase transition temperature was affected by the applied pressure for 4.5 wt % B(20)E(610). The dramatic contrast between the pressure-induced behavior observed for 9.5 and 4.5 wt % B(20)E(610) suggests that pressure induced effects might be more effectively transmitted through samples that present wider domains of cubic structure order (9.5 wt % compared to 4.5 wt % B(20)E(610)).