Phase transition oscillations induced by a strongly focused laser beam

We report here the observation of a surprising phenomenon consisting in a oscillating phase transition which appears in a binary mixture, PMMA/3-octanone, when this is enlightened by a strongly focused infrared laser beam. PMMA/3-octanone has a UCST (Upper Critical Solution Temperature) which presents a critical point at temperature Tc = 306.6 K and volume fraction $\phi$c = 12.8 % [Crauste et al., ArXiv 1310.6720, 2012]. This oscillatory phenomenon appears because of thermophoretic and electrostriction effects and non-linear diffusion. We analyze these oscillations and we propose a simple model which includes the minimal ingredients to produce the oscillatory behavior. Phase transitions in binary mixtures are still a widely studied subject, specifically near the critical point where several interesting and not completely understood phenomena may appear, among them we recall the critical Casimir forces [2],[3], confinement effects [4], [5] and out-of-equilibrium dynamics after a quench. The perturbation of the binary mixtures by mean of external fields is also an important and recent field of investigation [6]. For example, a laser can induce interesting phenomena in demixing binary mixtures because the radiation pressure can deform the interface between the two phases and it can be used to measure the interface tension [7]. Depending on the nature of the binary mixtures, laser illumination can also lead to a mixing or demixing transition. In ref.[8], focused infrared laser light heats the medium initially in the homogeneous phase and causes a separation in the LCST (Low Critical Solution Temperature) system. The radiation pressure gradients in a laser beam also contribute in the aggregation of polymers , thus producing a phase transition. The local heating may induce thermophoretic forces which attract towards the laser beam one of the binary-mixture components [9]. Other forces like electrostriction can also be involved [10]. In this letter, we report a new phenomenon, which consists in an oscillating phase transition induced by a constant illumination from an infrared laser beam in the heterogeneous region of an UCST (Upper Critical Solution Temperature) binary mixture. Oscillation phenomena in phase transition have already been reported in slow cooling UCST [11],[12] but as far as we know, never induced by a stationary laser illumination. After describing our experimental setup , we will present the results. Then we will use a very simplified model which contains the main necessary physical ingredients to induce this oscillation phenomenon

Thermal noise calibration of functionalized cantilevers for force microscopy: effects of the colloidal probe position

Colloidal probes are often used in force microscopy when the geometry of the tip-sample interaction should be well controlled. Their calibration requires the understanding of their mechanical response, which is very sensitive to the details of the force sensor consisting of a cantilever and the attached colloid. We present analytical models to describe the dynamics of the cantilever and its load positioned anywhere along its length. The thermal noise calibration of such probes is then studied from a practical point of view, leading to correction coefficients that can be applied in standard force microscope calibration routines. Experimental measurements of resonance frequencies and thermal noise profiles of raw and loaded cantilevers demonstrate the validity of the approach

Simultaneous and accurate measurement of the dielectric constant at many frequencies spanning a wide range

We present an innovative technique which allows the simultaneous measurement of the dielectric constant of a material at many frequencies, spanning a four orders of magnitude range chosen between 10 --2 Hz and 10 4 Hz. The sensitivity and accuracy are comparable to those obtained using standard single frequency techniques. The technique is based on three new and simple features: a) the precise real time correction of the amplication of a current amplier; b) the specic shape of the excitation signal and its frequency spectrum; and c) the precise synchronization between the generation of the excitation signal and the acquisition of the dielectric response signal. This technique is useful in the case of relatively fast dynamical measurements when the knowledge of the time evolution of the dielectric constant is needed

Dynamique moléculaire et plasticité d'un polymère amorphe sous traction

L'amélioration des performances des matériaux polymères passent notamment par une meilleure compression du lien entre dynamiques moléculaires à l'échelle microscopique et propriétés mécaniques macroscopiques. Dans ce but, la mobilité des chaînes polymères a été sondée par une mesure de spectroscopie diélectrique lors de la déformation plastique d'un polymère vitreux

Etude expérimentale d'une longueur de corrélation dynamique à la transition vitreuse

Many liquids do not crystallize when temperature is lowered. Their viscosity goes up very fast and they freeze in a disordered solid, called glass. This phenomenon is still poorely understood. The glass transition scenario of dynamical heterogeneities supposes that molecules movements are correlated in clusters growing in size when temperature is lowered, as in a phase transition. This had never been measured in supercooled liquids. In order to measure these correlations, two methods were proposed, based on linear and non linear susceptibility measurements. We developed an experiment to measure the dielectric susceptibilies. Since the non linear signal is very small, an original experimental setup was specially developed. Our measurements show that the number of correlated molecules grows when temperature is lowered. The frequency dependence of non-linear response presents a nice scaling that can be compared to different models. These first of the kind experimental results give an interpretation of glass transition in terms of critical phenomenon.Le ralentissement de la dynamique des liquides à la transition vitreuse reste un phénomène encore mal compris. Beaucoup de liquides peuvent, sous certaines conditions, passer la température de cristallisation, rester liquides (surfondus) et avoir une viscosité qui augmente énormément jusqu'à ce qu'ils se figent en un verre à la température de transition vitreuse Tg. De nombreuses données expérimentales ont été obtenues par des techniques différentes et beaucoup de modèles théoriques ont été explorés pour rendre compte de ces faits et mieux les comprendre. Le scénario qui nous a plus particulièrement guidés ici est celui des hétérogénéités dynamiques : des zones se développent dans le liquide où les mouvements des molécules sont corrélés, nécessitant un apport en énergie pour relaxer qui augmente lorsque la température diminue et que la taille des zones augmente. Cette image est liée à l'existence possible d'une transition de phase sous-jacente. Mais ce nombre de molécules corrélées, et surtout son évolution en température n'avait encore jamais été mesurée de manière directe dans les liquides surfondus. Deux méthodes basées sur les mesures de susceptibilité linéaire pour la première et non-linéaire pour la seconde ont été proposées, qui permettent de sonder directement ce nombre de molécules corrélées. Nous avons développé une expérience originale qui permet de mesurer la susceptibilité diélectrique linéaire mais aussi la réponse non-linéaire. La mesure de la susceptibilité non-linéaire est délicate puisque le signal recherché est 106 fois plus faible que le signal linéaire usuellement mesuré. Nous avons donc particulièrement travaillé le choix des appareils électroniques (source de tension et détection synchrone), des circuits de mesure, et le développement des condensateurs à liquide nécessaires. Nous avons mesuré alors un nombre de molécules corrélées qui croît lorsque la température diminue, par les deux techniques. La mesure non-linéaire donne un nombre qui croît lentement, d'environ 80 % entre Tg + 8 K et Tg + 38 K. Nous avons comparé cette croissance à celle obtenue par les mesures linéaire et constaté que les deux évolutions sont globalement similaires avec toutefois des différences qu'il serait utile de mieux comprendre. Nous avons aussi étudié l'évolution en fréquence de la réponse non-linéaire et montré l'existence d'une courbe maîtresse où toutes les courbes à différentes températures se recalent. Cette courbe a pu être comparée à un modèle phénoménologique d'échauffement des hétérogénéités dynamiques et à des prédictions théoriques basées en partie sur une des grandes théories des verres, la théorie de couplage de modes. Ces résultats expérimentaux originaux apportent un éclairage nouveau sur la transition vitreuse et notamment en terme de phénomènes critiques, rapprochant la transition vitreuse de la transition des verres de spins, qui elle est une transition de phase

Characterization of PMMA–3-octanone binary by turbidity and light scattering measurements.

We measure the coexistence curve and the critical point properties of a binary mixture composed by polymethylmethacrylate (PMMA) at Mw = 55900 g/mol with 3- octanone. This binary mixture which has a demixing transition with an upper critical solution temperature Tc has indeed interesting properties which may be useful for several application: a) its correlation length is larger than that of a liquidliquid binary mixture, b) it is less viscous than a polymer blend; c) 3- octanone has an evaporation rate much smaller than other solvents. The mixture is first characterized by turbidity, to get the demixing temperature for different volumic fraction of PMMA. The coordinates of the critical point are obtained: φc = 12.8 ± 0.2 % and Tc = 306.5 ± 0.1 K. The correlation length ξ near the critical point is then measured in a solution with a 12.8 % volumic fraction of PMMA using static light scattering. Using the fact that PMMA-octanone mixture has scaling exponents compatible with Ising 3D, we determine more precisely the critical temperature Tc = 306.58±0.04 K and we find that ξ ≃ ξ0[(T −Tc)/Tc] −0.63 with ξ0 = 0.97±0.02 nm. The discrepancy between this value and that extrapolated from other measurements based on turbidity is discussed.

Calibrated force measurement in Atomic Force Microscopy using the Transient Fluctuation Theorem

International audienceThe transient fluctuation theorem is used to calibrate an atomic force microscope by measuring the fluctuations of the work performed by a time-dependent force applied between a colloidal probe and the surface. From this measure one can easily extract the value of the interaction force and the relevant parameters of the cantilever. The results of this analysis are compared with those obtained by standard calibration methods

Microscopic Dynamics in the Strain Hardening Regime of Glassy Polymers

International audienceWe study by Dielectric Spectroscopy the molecular dynamics of relaxation processes during plastic ow of glassy polymers up to the strain hardening regime, for 3 dierent protocols of deformation. The measured dielectric spectra cover 4 decades in frequencies and allow us to measure the evolution as a function of the applied strain of the dominant relaxation time τα and of the width wτ of the distribution of relaxation times.The rst protocol is performed at constant strain rate λ. We conrm that for increasing stress both τα and wτ rst decrease reaching a minimum in the stress softening regime before increasing in the strain hardening regime. In the second protocol we stop the deformation at some point λw in the strain hardening regime and we let the sample age for a waiting time tw, during which the applied stress remains high. Upon resuming the deformation at constant λ, strain-stress displays a yield stress and a stress softening regime comparable in magnitude to that of the reference protocol before rejoining the reference curve. In contrast the dielectric spectrum measured during the second protocol recovers that measured during the reference curve much later than strain-stress. In the third protocol the stress is canceled during tw. In this case after recovering the constant λ the dielectric spectrum and the stress-strain curve rejoin almost immediately the reference curve. We interpret these dierent behaviors as the consequence of changes in the free energy barriers for α-relaxation induced by the stress applied to the sample. These changes are the sum of two contributions: a) the rst one, which allows for plastic ow, is due to the applied stress σ and, according to a recently published theory, scales as −σ 2 ; b) The second contribution κ(λ), which is a function of the chain orientation at the monomer level, is positive and is responsible for the stress hardening regime. The rst one evolves immediately upon varying the stress whereas the second relaxes very slowly upon cessation of the applied stress. Our interpretation for the results of the third protocol is that aging dynamics is frozen when the stress is removed, as it is known for polycarbonate at room temperature. Our experiments set precise conditions for a theory of strain hardening