Rotating Micro-Spheres for adsorption monitoring at a fluid interface.

HYPOTHESIS: A broad range of phenomena, such as emulsification and emulsion stability, foam formation or liquid evaporation, are closely related to the dynamics of adsorbing colloidal particles. Elucidation of the mechanisms implied is key to a correct design of many different types of materials. EXPERIMENTS: Microspheres forced to rotate near a fluid interface exhibit a roto-translational hydrodynamic mechanism that is hindered by capillary torques as soon as the particles protrude the interface. Under these conditions, the time evolution in the ratio of moving spheres provides a direct description of the adsorption kinetics, while microscopy monitoring of particle acceleration\deceleration informs about the adsorption\desorption dynamics. In this work, the proposed strategy is applied at an air/water interface loaded with spherical magnetic particles negatively charged, forced to rotate by the action of a rotating magnetic field. FINDINGS: The proposed method enables the adsorption/desorption dynamics to be followed during the earliest phase of the process, when desorption of a small fraction of particles is detected, as well as to estimate approximated values of the adsorption/desorption constants. The results obtained show that the addition of a monovalent salt or a cationic (anionic) surfactant promotes (inhibits) both adsorption and formation of permanent bonds between particles.

Influence of temperature on dynamic surface properties of spread DPPC monolayers in a broad range of surface pressures.

This work is focused on the study of the dynamic surface properties of spread monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), which is the main component of the pulmonary surfactant (PS), in the region of high surface pressures and at different temperatures. The increase of temperature from 25 to 35 °C led to a decrease of surface elasticity in the high surface pressure range corresponding to physiological conditions inside alveoli during breathing. Furthermore, the obtained results evidenced that the relaxation processes in spread DPPC monolayer were accelerated with the increase of temperature, which resulted in two different effects. On one hand, it led to the increase of hysteresis of surface pressure isotherms, which was an important condition for maximizing air penetration into alveoli; whereas on the other hand, it prevented reaching extremely high surface pressure, which could result in a premature alveolar collapse.

Magnetic Biohybrid Vesicles Transported by an Internal Propulsion Mechanism.

Some biological microorganisms can crawl or swim due to coordinated motions of their cytoskeleton or the flagella located inside their bodies, which push the cells forward through intracellular forces. To date, there is no demonstration of synthetic systems propelling at low Reynolds number via the precise actuation of the material confined within an enclosing lipid membrane. Here, we report lipid vesicles and other more complex self-assembled biohybrid structures able to propel due to the advection flows generated by the actuated rotation of the superparamagnetic particles they contain. The proposed swimming and release strategies, based on cooperative hydrodynamic mechanisms and near-infrared laser pulse-triggered destabilization of the phospholipid membranes, open new possibilities for the on-command transport of minute quantities of drugs, fluid or nano-objects. The lipid membranes protect the confined substances from the outside environment during transportation, thus enabling them to work in physiological conditions.

Linear shear rheology of aging ß-casein films adsorbing at the air/water interface.

In this work, the viscoelasticity of fragile ß-casein films has been followed using different macro- and microrheological techniques. The modulus of the complex surface viscosity |η∗| varies with time, allowing for the monitoring of the protein adsorption and annealing. ß-casein adsorption creates a soft glassy gel at the interface that experiences an aging process. Macrorheological experiments with multiple probe sizes in addition to microrheological experiments demonstrated the consistency of the surface rheological properties over a broad range of viscosities. Surface pressure measurements were performed to complement the characterization of the processes.

Phase Diagram of Fatty Acid Langmuir Monolayers from Rheological Measurements.

Langmuir monolayers of fatty acids and alcohols are two-dimensional systems with a rich equilibrium phase diagram. We have explored the temperature and surface-pressure-dependent shear response of monolayers formed by fatty acids of different chain lengths and a fatty alcohol. This has been accomplished with an interfacial shear rheometer utilizing magnetic tweezers and equipped with a refined temperature control and acquisition system. Our rheological results have allowed us to draw a phase diagram from the viscoelastic properties of these 2-D systems and show new phenomena that strongly depend on temperature: the existence of a maximum in viscosity at the L2' phase, the behavior of the elastic modulus to the storage modulus ratio at the L2 phase, and the increase or decrease in viscosity at the L2-LS phase transition. In addition, we unambiguously show that the LS phase displays a counterintuitive behavior in which the loss modulus increases with temperature. We demonstrate, through isothermal surface pressure sweeps and isobaric temperature sweeps, that the exponential dependence of the loss modulus on temperature at the LS phase appears for all hydrophobic tail lengths studied and for both acid and alcohol head groups.

Magnetic microwire probes for the magnetic rod interfacial stress rheometer.

The magnetic needle interfacial shear rheometer is a valuable tool for the study of the mechanical properties of thin fluid films or monolayers. However, it is difficult to differentiate the interfacial and subphase contributions to the drag on the needle. In principle, the problem can be addressed by decreasing the needle diameter, which decreases the bulk contribution while the interfacial contribution remains essentially the same. Here we show the results obtained when using a new type of needle, that of magnetic microwires with diameter approximately 10 times thinner than for commercial needles. We show that the lower inertia of the microwires calls for a new calibration procedure. We propose such a new calibration procedure based on the flow field solution around the needle introduced in refs 1 and 2. By measuring thin silicone oil films with well-controlled interfacial viscosities as well as eicosanol (C20) and pentadecanoic acid (PDA, C15) Langmuir monolayers, we show that the new calibration method works well for standard needles as well as for the microwire probes. Moreover, we show that the analysis of the force terms contributing to the force on the needle helps to ascertain whether the measurements obtained are reliable for given surface shear viscosity values. We also show that the microwire probes have at least a 10-fold-lower resolution limit, allowing one to measure interfacial viscosities as low as 10(-7) N·m/s.

Field-induced sublimation in perfect two-dimensional colloidal crystals.

Phase transitions in two-dimensional (2D) systems are of considerable fundamental and practical importance. However, the kinetics of these processes are difficult to predict and understand, even in simple systems for which equilibrium states are properly described, owing to the difficulty of studying crystallites with single-particle resolution and free of defects. Here we introduce an alternative method for the sublimation of 2D colloidal crystallites by a sudden induction of repulsive forces between the particles. The sublimation kinetics, studied in real space by microscopy and by computer simulations, shows a scaling behavior that suggests a universal mechanism fundamentally different from the one usually accepted for thermal sublimation. The universal behavior found for the early stages of the process may be useful for understanding the dynamic features of particle systems at liquid interfaces and for designing technological applications without the need of performing extensive experimental studies.

Phase behavior of dense colloidal binary monolayers.

In this work, we study how structures develop on 2D dense binary colloidal monolayers as a function of the relative concentration of small/large particles. Translational and orientational distribution functions have been used to monitor the continuous phase transition through a detailed characterization of the global and local order. We have observed how a gradual enhancement in the number of particles of different sizes leads to a continuous vitrification process and how homogeneous binary glasses form in equimolar mixtures. Also, we have performed a simple calculation that relates the structures found to the pair dipolar potential, allowing the forecast of local structures in other arbitrary binary mixtures. Finally, we have corroborated the goodness of the binary systems as a glass-forming model by comparing the established scenario with the structural features found in partially aggregated monolayers.

Dielectric and molecular dynamics study of the secondary relaxations of poly(styrene-co-methylmethacrylate) copolymers: Influence of the molecular architecture.

The effect of the structure of copolymers (random, alternate or diblock) on their dynamics has been studied by dielectric spectroscopy. Six copolymers of styrene and methyl methacrylate (three diblocks, one alternate and two random) have been studied. The results show that the sub- T (g) transitions of the diblock samples can be described by one asymmetric Havriliak-Negami (HN) function, while two are necessary for the rest of the copolymers (ß and γ relaxations). The characteristic times of the sub- T (g) relaxations show an Arrhenius temperature dependence and there is a strong coupling of the α and ß relaxations at high temperatures. The deconvolution of the merging relaxations has been made in the framework of the Williams Ansatz set out in terms of Havriliak-Negami distributions. Because the 2D (2)H-NMR results excluded any significant contribution from the rotation of the methoxy group of the methacrylate group around the C-OCH(3) bond, the γ relaxation may be assigned to the rotation of the methyl methacrylate group in a styrene-rich environment. The Molecular Dynamics simulations of a poly(methyl methacrylate) homopolymer and of the alternate copolymer are in qualitative agreement with the experimental results, although they predict smaller values for the activation energy of the sub- T (g) relaxations.

Rheology of poly(methyl methacrylate) Langmuir monolayers: percolation transition to a soft glasslike system.

An experimental study of the equilibrium properties and of the surface rheology of Langmuir monolayers of poly(methyl methacrylate) (PMMA) at the air/water interface has been carried out as a function of polymer concentration (Γ) and molecular weight (M(w)). Dilational and shear complex elasticity moduli covering a frequency range from 10(-3) to 0.2 Hz have been discussed. It was found that the air∕water interface behaves as a poor solvent for PMMA monolayers, thus suggesting that the polymer coils take collapsed soft-disks (pancakes) shape at the interface. The equilibrium and dynamic results suggest a fluid-to-soft-glass transition as the polymer concentration increases above a critical packing fraction at constant temperature. This two-dimensional transition is in agreement with results previously discussed for the dilational rheology of poly(4-hydroxystyrene) [F. Monroy, F. Ortega, R. G. Rubio, H. Ritacco, and D. Langevin, J. Chem. Phys. 95, 056103 (2005)]. Furthermore, the Γ-dependence of the relaxation dynamics of the monolayers suggests that the gel state may be considered as a fragile soft glass.

Freezing transition and interaction potential in monolayers of microparticles at fluid interfaces.

The structure and the interaction potential of monolayers of charged polystyrene microparticles at fluid interfaces have been studied by optical microscopy. Microparticles of different sizes have been studied over a broad range of surface particle densities. The structural characterization is based on the analysis of images obtained by digital optical microscopy. From the experimental images, radial distribution functions, hexagonal bond order correlation functions, and temporal orientational correlation functions have been calculated for different monolayer states at both the air/water and oil/water interfaces. The interaction potential has been calculated from the structure factor using integral equations within the hypernetted chain closure relationship. For particles trapped at the oil-water interface, it was found that, upon increasing the surface coverage, a freezing transition occurs, that leads to the formation of a 2D crystalline structure. We have studied the freezing densities of particle monolayers at the oil/water interface and compared them with Monte Carlo simulation results reported by H. Löwen. In contrast, at the air-water interface, freezing is inhibited due to the formation of particle aggregates.

Effect of the spreading solvent on the three-phase contact angle of microparticles attached at fluid interfaces.

We address a systematic study of the three-phase contact angle, θ, of microparticles at flat fluid-liquid interfaces by using different experimental methods. We measured the dependence of θ not only on the particle chemical composition and size, but also on the solvent used to spread the microparticles onto the fluid interface. We found a non-expected and non-regular dependence of θ with size, chemical nature and spreading solvent used for the different particles studied. We propose that these dependences are due to porosity/roughness of the particles that allows the adsorption of the spreading solvent onto the solid particle surface. This conclusion is supported by the values of the line tensions estimated for the different systems.

Dilational viscoelasticity of PEO-PPO-PEO triblock copolymer films at the air-water interface in the range of high surface pressures.

The dynamic dilational elasticity of adsorbed and spread films of PEO-PPO-PEO triblock copolymers at the air-water interface was measured as a function of surface pressure, surface age, and frequency. At low surface pressures (<10 mN/m), the surface viscoelasticity is identical to that of PEO homopolymer films. The results at higher surface pressures can be explained by the desorption of PPO segments from the interface and then mixing with PEO segments in water. Unlike some recent results, the spread and adsorbed films are not identical. Spread films exhibit a maximum real part of the dynamic surface elasticity of about 20 mN/m and probably begin to dissolve in water at surface pressures above 19 mN/m. However, the surface elasticity of the adsorbed films decreases beyond the maximum, indicating the formation of a loose surface structure.

Dilational rheology of monolayers of a miscible polymer blend: from good- to poor-solvent conditions.

The viscoelastic moduli (elasticity and dilational viscosity) of monolayers of PVAc + P4HS has been studied over a broad frequency range (0.1 mHz-200 kHz) using a combination of relaxation and capillary-waves techniques. The analysis of the surface pressure, the elasticity and the viscosity on the semidilute regime show that the air-water interface is a good solvent for the monolayers of PVAc-rich blends, and a poor (near-Theta) solvent for the monolayers of P4HS-rich blends. The solvent quality changes continuously over a broad concentration range. The results of viscoelastic moduli show that there is a broad relaxation process in the low-frequency range (omega < 1 Hz). While for PVAc-rich monolayers this relaxation process follows the reptation-like behavior described by Noskov, for P4HS-rich monolayers the model does not describe the amplitudes of the different relaxation modes. For PVAc-rich monolayers two processes are clearly distinguished at higher frequencies: one centered at around 500 Hz and another one at around 40 kHz. However, for P4HS-rich monolayers only one broad relaxation mode is found below 1 kHz. The crossover from one type of behavior to the other one takes place in a very narrow blend-composition range, and is not clearly related to the crossover from good- to poor-solvent condition.