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8 2 Probing elastic anisotropy from defect dynamics in Langmuir Monolayers Nonirradiated 8Az3COOH trans isomer monolayers exhibit complicated stripe like patterns (Crusats et al., 2004), Fig. 2.3a. If the azobenzene solution is exposed to room light, a mixture of cis and trans isomers is obtained which, after spreading a monolayer, is organized in circular droplets of a birefringent trans phase with a central topological defect embedded in an isotropic cis phase (Ignés-Mullol et al., 2005), Fig. 2.3b. We will use spread monolayers of trans and cis isomers to study annihilation of topological defects that arise from the fusion of a pair of droplets (see below). (a) (b) Fig. 2.3. (a) Nonirradiated textures of 8Az3COOH trans isomer monolayers, image modified from (Crusats et al., 2004). (b) Circular droplet of a trans isomer embedded in a cis phase. Both images are obtained using Brewster angle microscopy (BAM). Imaging of thin films at a gas/liquid interface can be achieved by several techniques (for instance fluorescence microscopy and AFM). Many of these techniques perturb the system, either by probe compounds (such as fluorescent dye) or by direct interaction (AFM). A non invasive technique that allows to study such systems without perturbing its original state is based on the Brewster angle, Fig. 2.4. 2.2.2 Brewster angle microscopy For a beam of a p-polarized light (parallel to the plane of incidence), there is an angle of incidence θ (Brewster angle) at which no reflection occurs, Fig. 2.4 (left). When a thin film is placed between the two phases, the optical properties of the system change, and a small amount of light is reflected, Fig. 2.4 (right). In general, the polarization of the reflected light differs from the incident light. A Brewster angle microscope (BAM), is built using a source of p-polarized light (a laser and a polarizer) and a receptor (a CCD camera and a polarizer), Fig. 2.1a. We use a calibration of the gray-scale distribution in the BAM im-
θ θ air thin film subphase Fig. 2.4. Scheme of the Brewster angle. 2.3 Results 9 ages and the orientational parameter as reported in (Ignés-Mullol et al., 2004), Fig. 2.5. Grey level φ(deg) Fig. 2.5. Gray level as a fucntion of the polar angle for a droplet of trans isomer in a matrix of cis isomer, image modified from (Crusats et al., 2004). 2.3 Results For a wide range of experimental conditions, the organization of the amphiphilic molecules inside the circular domains is characterized by a uniform tilt (around 45 ◦ (Crusats et al., 2004)) with respect to the air-water interface normal, Fig. 2.6. Molecular ordering can thus be described by a twodimensional vector field n. Constant-angle anchoring at the droplet boundary (Crusats et al., 2004; Reigada et al., 2004) results in the inclusion of inner point defects of total charge +1 (Mermin, 1979; Tabe et al., 1999). Small enough droplets feature a single s =+1 defect near the center (in the core
Page 1: Studies of dynamical phenomena in s
Page 5 and 6: Acknowledgements This thesis has be
Page 7 and 8: Contents 1 General introduction . .
Page 9: Contents v 4.6 Conclusions . . . .
Page 12 and 13: 2 1 General introduction gies. In t
Page 14 and 15: 4 1 General introduction Finally, i
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Page 35 and 36: 3.1 Introduction 25 Fluctuating for
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Page 59: 3.4 Conclusions 49 motor itself but
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58 4 Membrane-cortex interactions:
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5 General conclusions In this work
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5 General conclusions 115 number of
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A Resum en català El present treba
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A.1 Auto-organització i cooperativ
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A.2 Mesura de l’anisotropia elàs
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A.3 Interaccions de membrana i còr
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A.3 Interaccions de membrana i còr
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B Résumé en Français Ce travail
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B.1 Auto-organisation et coopérati
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B.2 Mesure de anisotropie élastiqu
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B.3 Interactions de membrane et cor
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B.4 Conclusions 135 une perturbatio
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References Alberts, B., Johnson, A.
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REFERENCES 139 Evans, E. (2001). Pr
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REFERENCES 141 Kruse, K., Joanny, J
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REFERENCES 143 Sit, P., Spector, A.
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