laboratoire de physique statistique
 
 
laboratoire de physique statistique

Publications

Rechercher
 
2013
1
Thin Films in Complete Wetting and the Specific Case of Nematic Liquid Crystals - Cazabat, A. M. and Delabre, U. and Richard, C. and Sang, Y. Yip Cheung
in NANOSCALE LIQUID INTERFACES: WETTING, PATTERNING, AND FORCE MICROSCOPY AT THE MOLECULAR SCALE edited by Ondarcuhu, T and Aime, JP (2013) 
LPS
 
2012
Precursor films in wetting phenomena - Popescu, M. N. and Oshanin, G. and Dietrich, S. and Cazabat, A-M
JOURNAL OF PHYSICS-CONDENSED MATTER 24 (2012) 
LPS


Abstract : The spontaneous spreading of non-volatile liquid droplets on solid substrates poses a classic problem in the context of wetting phenomena. It is well known that the spreading of a macroscopic droplet is in many cases accompanied by a thin film of macroscopic lateral extent, the so-called precursor film, which emanates from the three-phase contact line region and spreads ahead of the latter with a much higher speed. Such films have been usually associated with liquid-on-solid systems, but in the last decade similar films have been reported to occur in solid-on-solid systems. While the situations in which the thickness of such films is of mesoscopic size are fairly well understood, an intriguing and yet to be fully understood aspect is the spreading of microscopic, i.e. molecularly thin, films. Here we review the available experimental observations of such films in various liquid-on-solid and solid-on-solid systems, as well as the corresponding theoretical models and studies aimed at understanding their formation and spreading dynamics. Recent developments and perspectives for future research are discussed.
 
2011
Experimental study of hybrid nematic wetting films - Cazabat, A. M. and Delabre, U. and Richard, C. and Sang, Y. Yip Cheung
ADVANCES IN COLLOID AND INTERFACE SCIENCE 16829-39 (2011) 
LPS


Abstract : Liquid crystal layers, with thickness less than 1 mu m, are deposited on isotropic - solid or liquid - substrates and investigated in the bulk nematic range of temperatures. The boundary conditions at interfaces are antagonist ones, therefore the layers are distorted due to nematic elasticity. These films are referred to as ``hybrid nematics''. The consequences are complex. First, a forbidden range of film thickness is observed, depending only on temperature. Second, the anisotropy of the elastic response gives rise to striking stripe patterns in the thicker films. This behavior is common to several members of the series of n-cyanobiphenyls deposited on oxidized silicon wafers, water and glycerol. The aim of the study is to collect data, and determine which ones find a place within a common theoretical framework. (C) 2011 Elsevier B.V. All rights reserved.
 
2010
Evaporation of macroscopic sessile droplets - Cazabat, Anne-Marie and Guena, Geoffroy
SOFT MATTER 62591-2612 (2010) 
LPS


Abstract : This review is aimed at presenting the evaporation of macroscopic sessile droplets on inert substrates in normal atmosphere in simple cases, as a basis for more complex analyses.
Coalescence Driven by Line Tension in Thin Nematic Films - Delabre, U. and Cazabat, A-M.
PHYSICAL REVIEW LETTERS 104 (2010) 
LPS


Abstract : Thin nematic films deposited on liquid substrates provide a unique situation to investigate coalescence: the whole process can be followed under microscope over a wide range of times, and temperature allows us to monitor the surface viscosity of the surrounding fluid. For the first time, the complete scenario of 2D coalescence has been recorded for a given system in both inviscid limit and viscous environment, enabling us to identify the successive routes of dissipation. In particular, 2D ``viscous bubbles'' of the surrounding viscous fluid with a bulbous shape formed in the gap between coalescing films are observed. Available models are adapted to our specific case and account satisfactorily for the whole process.
Thin Liquid Crystal Films on Liquids in the Nematic Range of Temperatures - Delabre, Ulysse and Richard, Celine and Sang, Yann Yip Cheung and Cazabat, Anne-Marie
LANGMUIR 2613368-13376 (2010) 
LPS


Abstract : Hybrid nematic films deposited on liquid substrates reveal a complex behavior, which is not fully understood. Here, the behavior of the n-cyanobiphenyl series on water and glycerol has been studied in a wide temperature range, including the vicinity of the nematic isotropic (NI) transition. Wettability, allowed film thicknesses, and line tension of nematic domains have been investigated. The study provides a coherent picture of hybrid nematic films, allowing us to account for lower thickness threshold, structure of the Film edge, and line tension of domains in the whole nematic range of temperatures.
Instability patterns in ultrathin nematic films: Comparison between theory and experiment - Manyuhina, O. V. and Cazabat, A. -M. and Ben Amar, M.
EPL 92 (2010) 
LPS


Abstract : Motivated by recent experimental observations (Delabre U. et al., Langmuir, 24 (2008) 3998), we reconsider an instability of ultrathin nematic films, spread on liquid substrates. Within a continuum elastic theory of liquid crystals, in the harmonic approximation, we find an analytical expressions for the critical thickness as well as for the critical wave number, characterizing the onset of instability towards the stripe phase. Comparing theoretical predictions with experimental observations, we establish the utility of surface-like term such as an azimuthal anchoring. Copyright (C) EPLA, 2010
 
2009
Thin Nematic Films on Liquid Substrates - Deabre, U. and Richard, C. and Cazabat, A. M.
JOURNAL OF PHYSICAL CHEMISTRY B 1133647-3652 (2009) 
LPS


Abstract : Thin films of cyanobiphenyl liquid crystals (nCB) deposited on water or glycerol have been studied in the nematic temperature range. A common property of the systems is the hybrid anchoring conditions at the film interfaces. The preferred orientation of the nematic director is planar at the liquid interface, and it is homeotropic and somewhat weaker at the air interface. The resulting structure of the film depends on its thickness. Films thicker than 0.5 mu m show the usual defects of nematics. Between 0.5 mu m and 20-30 nm, complex instability patterns such as stripes, ``chevrons'', or squares are observed in extended films. Then there is a forbidden range of thickness below in which much thinner structures (usually monolayers and trilayers) are present. The present paper investigates this common behavior in various systems and gives arguments for its analysis.
Post-Tanner stages of droplet spreading: the energy balance approach revisited - Mechkov, S. and Cazabat, A. M. and Oshanin, G.
JOURNAL OF PHYSICS-CONDENSED MATTER 21 (2009) 
LPS


Abstract : The spreading of a circular liquid drop on a solid substrate can be described in terms of the time evolution of its base radius R(t). In complete wetting, the quasistationary regime (far away from initial and final transients) typically obeys the so-called Tanner law, with R similar to t(alpha T), alpha(T) = 1/10. Late-time spreading may differ significantly from the Tanner law: in some cases the drop does not thin down to a molecular film and instead reaches an equilibrium pancake-like shape; in other situations, as revealed by recent experiments with spontaneously spreading nematic crystals, the growth of the base radius accelerates after the Tanner stage. Here we demonstrate that these two seemingly conflicting trends can be reconciled within a suitably revisited energy balance approach, by taking into account the line tension contribution to the driving force of spreading: a positive line tension is responsible for the formation of pancake-like structures, whereas a negative line tension tends to lengthen the contact line and induces an accelerated spreading (a transition to a faster power law for R(t) than in the Tanner stage).
DOI
10
Post-Tanner spreading of nematic droplets - Mechkov, S. and Cazabat, A. M. and Oshanin, G.
JOURNAL OF PHYSICS-CONDENSED MATTER 21 (2009) 
LPS


Abstract : The quasistationary spreading of a circular liquid drop on a solid substrate typically obeys the so-called Tanner law, with the instantaneous base radius R(t) growing with time as R similar to t(1/10)-an effect of the dominant role of capillary forces for a small-sized droplet. However, for droplets of nematic liquid crystals, a faster spreading law sets in at long times, so that R similar to t(alpha) with a significantly larger than the Tanner exponent 1/10. In the framework of the thin film model (or lubrication approximation), we describe this `acceleration' as a transition to a qualitatively different spreading regime driven by a strong substrate-liquid interaction specific to nematics (antagonistic anchoring at the interfaces). The numerical solution of the thin film equation agrees well with the available experimental data for nematics, even though the non-Newtonian rheology has yet to be taken into account. Thus we complement the theory of spreading with a post-Tanner stage, noting that the spreading process can be expected to cross over from the usual capillarity-dominated stage to a regime where the whole reservoir becomes a diffusive film in the sense of Derjaguin.
DOI
11
Some specificities of wetting by cyanobiphenyl liquid crystals - Delabre, U. and Richard, C. and Cazabat, A. M.
JOURNAL OF PHYSICS-CONDENSED MATTER 21 (2009) 
LPS


Abstract : The present paper provides an up to date restatement of the wetting behaviour of the series of cyanobiphenyl liquid crystals (LCs) on usual substrates, i.e. oxidized silicon wafers, water and glycerol, at both the macroscopic and microscopic scale, in the nematic range of temperature. We show that on water the systems are close to a wetting transition, especially 5CB and 7CB. In that case, the wetting behaviour is controlled by the presence of impurities. On a mesoscopic scale, we observe for all our (thin LC film-substrate) systems an identical, complex, but well defined general scenario, not accounted for by the available models. In the last part, we present a study on line tension which results from the specific organization of LCs at the edge of the nematic film. We report preliminary results on two-dimensional film coalescence where this line tension plays a major role.
 
2008
DOI
12
Nematic pancakes revisited - Delabre, Ulysse and Richard, Celine and Guena, Geoffroy and Meunier, Jacques and Cazabat, Anne-Marie
LANGMUIR 243998-4006 (2008) 
LPS


Abstract : The spontaneous spreading of the 5CB nematic liquid crystal on solid substrates has been extensively studied in the last years both at the microscopic(1-4) and macroscopic(5-6) scales. The remarkable feature at the microscopic scale is the presence of a discontinuity in the thickness profile of the films. On the other hand, the spreading dynamics of macroscopic drops is quite specific. The drop first spreads like a simple liquid, and then progressively faster, while a remarkable bell-shaped profile develops at the bottom.(5-6) How the behaviors at the various scales are linked is an open question. Any answer requires reconsidering these wetting experiments deeper into the context of nematic films. More specifically, the anchoring of molecules at the interfaces(7-8) and the competition between nematic elasticity(9) and anchoring 10 must be discussed quantitatively. For the thinnest films, the problem proves to be more complex than expected and contradictory data are found in the literature. Therefore, we decided to complete our previous studies with further experiments using another compound of the cyanobiphenyls series, the 6CB in the nematic phase, and also on liquid substrates, water and glycerol. These new data confirm that the description of the thinnest nematic films is not yet fully understood.
DOI
13
Line tension of nematic pancakes at the air-water interface: Static and dynamic measurements - Delabre, U. and Richard, C. and Meunier, J. and Cazabat, A. -M.
EPL 83 (2008) 
LPS


Abstract : The line tension of nematic Langmuir films on water is investigated using the 6CB member of the cyanobiphenyl liquid-crystals series. Both static and dynamic measurements are performed independently. The line tension increases linearly with film thickness up to 0.2 mu m. The increment of line tension is significantly smaller than for the smectic 8CB compound, due to the difference in the allowed elastic deformations of smectic and nematic films. A simple model of the nematic edge accounts both for the linear dependence and the order of magnitude of the line tension. Moreover, it sheds new light on the structure of thin nematic films on liquid substrates. Copyright (c) EPLA, 2008
 
2007
DOI
14
Contact line stability of ridges and drops - Mechkov, S. and Oshanin, G. and Rauscher, M. and Brinkmann, M. and Cazabat, A. M. and Dietrich, S.
EPL 80 (2007) 
LPS


Abstract : Within the framework of a semi-microscopic interface displacement model in the small slope approximation we analyze the linear stability of sessile ridges and drops of a non-volatile liquid on a homogeneous, partially wet substrate, for both signs and arbitrary amplitudes of the three-phase contact line tension. Focusing on perturbations which correspond to deformations of the three-phase contact line, we find that drops are generally stable while ridges are subject only to the long-wavelength Rayleigh-Plateau instability leading to a breakup into droplets, in contrast to the predictions of capillary models which take line tension into account. We argue that the short-wavelength instabilities predicted within the framework of the latter macroscopic capillary theory occur outside its range of validity and thus are spurious. Copyright (c) EPLA, 2007.
DOI
15
The dynamics of evaporating sessile droplets - Guena, G. and Poulard, C. and Cazabat, A. -M.
COLLOID JOURNAL 691-8 (2007) 
LPS


Abstract : Experiments on sessile drops evaporating in a normal atmosphere without an applied thermal gradient are reported and compared with an available theoretical model. The liquids used are alkanes; water; and, more recently, polydimethylsiloxane oligomers. The substrates are silicon wafers, completely wetted by the liquid. Experiments with hanging drops allow us first to discard any influence of convection in the gas phase on the drop dynamics. The model assumes the process to be controlled by the stationary diffusion of the evaporating molecules in the gas phase. For alkanes and water, and in a limited range of drop sizes where gravity can be ignored, the model accounts very well for the dynamics of the drop radius, and rather well for the contact angle. This is no longer the case with the polydimethylsiloxane oligomers, where the very small contact angles require a more elaborated analysis of the drop edge.
DOI
16
Receding contact angle in the situation of complete wetting: Experimental check of a model used for evaporating droplets - Guena, Geoffroy and Allancon, Pascal and Cazabat, Anne-Marie
COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS 300307-314 (2007) 
LPS


Abstract : This paper provides experimental checks of assumptions needed in a model describing the moving contact line of a wetting droplet evaporating in an inert atmosphere. Special interest is given to the maximum extension of the droplet, when the contact line starts receding. The predicted power law dependence between the values of the radius R-0 and the contact angle Theta(0) at the maximum extension is checked against experiments. We find fairly good agreement with theory for very small droplets. However, discrepancies show Lip rapidly when the droplet size increases. (C) 2007 Published by Elsevier B.V.
DOI
17
The leading edge of evaporating droplets - Guena, G. and Poulard, C. and Cazabat, A. M.
JOURNAL OF COLLOID AND INTERFACE SCIENCE 312164-171 (2007) 
LPS


Abstract : New experiments on drops evaporating in normal atmosphere from smooth substrates in the situation of complete wetting are reported and compared with the available theoretical model. They are the continuation of previous work with alkane or water sessile drops, which is first briefly summarized. The model accounts very well for the dynamics of the drop radius, but the predictions are only qualitative for the contact angle, especially for small angles. Experiments with hanging drops allow us first to discard any influence of convection in the gas phase on the drops dynamics. Then the main part of the paper concerns new experiments with polydimethylsiloxane oligomers. These silicone oils are similar to alkanes as far as evaporation rate is concerned, but have lower surface tensions, and therefore smaller dynamic contact angles. The purity of the oils appears to be critical for the experiments, and requires a preliminary investigation. Then a systematic study of the drops dynamics is presented, as a basis for forthcoming theoretical work. (c) 2006 Elsevier Inc. All rights reserved.