Experiments on perturbed Saffman-Taylor flows

[eng] We have performed pattern formation experiments where a relatively well understood system (flow in a Hele-Shaw cell) is perturbed either by means of a lattice of grooves or by the use of viscoelastic fluids. We have extended the qualitative analysis found in the literature for anisotropic fingering patterns, presenting a more quantitative approach that may prove useful as a tool to attack more complex problems. We have analyzed the different morphological regimes and looked for signatures of the transition between phases, with partial success when we try to characterize a given morphology quantitatively. In our studies of viscoelastic Hele-Shaw flow with associative polymer solutions, we have observed a transition from viscous fingering patterns into a regime where the growing patterns resemble the fracture in brittle solids. We have been able to rescale the threshold for these transitions, and we have observed interesting properties in a regime of fracturelike patterns where, under sorne circumstances, we have measured a characteristic oscillation frequency which shows interesting regularities. We have also studied the pressure in the viscoelastic flow, and found consistent results that may be used to implement a better theoretical model to fully understand the dynamics

Experiments on perturbed Saffman-Taylor flows

We have performed pattern formation experiments where a relatively well understood system (flow in a Hele-Shaw cell) is perturbed either by means of a lattice of grooves or by the use of viscoelastic fluids. We have extended the qualitative analysis found in the literature for anisotropic fingering patterns, presenting a more quantitative approach that may prove useful as a tool to attack more complex problems. We have analyzed the different morphological regimes and looked for signatures of the transition between phases, with partial success when we try to characterize a given morphology quantitatively. In our studies of viscoelastic Hele-Shaw flow with associative polymer solutions, we have observed a transition from viscous fingering patterns into a regime where the growing patterns resemble the fracture in brittle solids. We have been able to rescale the threshold for these transitions, and we have observed interesting properties in a regime of fracturelike patterns where, under sorne circumstances, we have measured a characteristic oscillation frequency which shows interesting regularities. We have also studied the pressure in the viscoelastic flow, and found consistent results that may be used to implement a better theoretical model to fully understand the dynamics

Self-organizing propagation patterns from dynamic self-assembly in monolayers

Propagation of localized orientational waves, as imaged by Brewster angle microscopy, is induced by low intensity linearly polarized light inside axisymmetric smectic-C confined domains in a photosensitive molecular thin film at the air/water interface (Langmuir monolayer). Results from numerical simulations of a model that couples photoreorientational effects and long-range elastic forces are presented. Differences are stressed between our scenario and the paradigmatic wave phenomena in excitable chemical media

Reconfigurable Flows and Defect Landscape of Confined Active Nematics

Using novel micro-printing techniques, we develop a versatile experimental setup that allows us to study how lateral confinement tames the active flows and defect properties of the microtubule/kinesin active nematic system. We demonstrate that the active length scale that determines the self-organization of this system in unconstrained geometries loses its relevance under strong lateral confinement. Dramatic transitions are observed from chaotic to vortex lattices and defect-free unidirectional flows. Defects, which determine the active flow behavior, are created and annihilated on the channel walls rather than in the bulk, and acquire a strong orientational order in narrow channels. Their nucleation is governed by an instability whose wavelength is effectively screened by the channel width. All these results are recovered in simulations, and the comparison highlights the role of boundary conditions

Optimizing gold nanoparticle size and shape for the fabrication of sers substrates by means of the langmuir–blodgett technique

The Langmuir–Blodgett technique, in which a layer of nanoparticles is spread at the water/air interface and further transferred onto a solid support, is a versatile approach for the preparation of SERS substrates with a controllable arrangement of hotspots. In a previous work, we demonstrated that fine-tuning the lateral packing and subsequent seed growth of 10 nm gold nanoparticles led to a quasi-resonant enhanced in the SERS signal of a test analyte. Here, we explore further enhancements by modifying the size and shape of the spread gold nanoparticles in order to take advantage of the inherent interparticle repulsion mechanisms present at the interface. We show that the size of the used nanoparticles is also a determinant factor, which cannot be compensated by the subsequent electroless growth. We also show that, although the seeded growth leads to rough hotspots, the sensitivity can be optimized by self-assembling urchin-shaped nanoparticles, with a roughness that is fine-tuned a priori. Our results suggest an intriguing correlation between surface homogeneity and SERS signal enhancement, indicating that regular substrates will have the optimal performance

Thermodynamics and mesoscopic organisation in Langmuir monolayers of an azobenzene derivative

We have carried out the analysis of liquid crystalline Langmuir monolayers at the air-water interface composed of the amphiphilic azobenzene derivative 8Az5COOH. By varying the temperature and the isomeric (trans-cis) composition, the monolayer behaviour has been studied in comparison with a shorter homologue, 8Az3COOH, by measuring the surface pressure-area isotherms along with Brewster angle microscopy (BAM). Our data with the pure trans isomer enable a posterior thermodynamic analysis, which was not feasible with the shorter homologue. For the mixed trans-cis monolayers, BAM observations reveal a phase segregation with trans enriched domains surrounded by a cis enriched matrix. Line tension between the two phases is lower than in the shorter homologue. The organisation of the rodlike molecules inside the trans domains results in highly symmetric textures that make the quantitative analysis of the BAM images possible, and a better understanding of the microscopic structure of the monolayer can be achieved