Patrick Degen

Elastometry of Deflated Capsules: Elastic Moduli from Shape and Wrinkle Analysis

Elastic capsules, prepared from droplets or bubbles attached to a capillary (as in a pendant drop tensiometer), can be deflated by suction through the capillary. We study this deflation and show that a combined analysis of the shape and wrinkling characteristics enables us to determine the elastic properties in situ. Shape contours are analyzed and fitted using shape equations derived from nonlinear membrane-shell theory to give the elastic modulus, Poisson ratio and stress distribution of the membrane. We include wrinkles, which generically form upon deflation, within the shape analysis. Measuring the wavelength of wrinkles and using the calculated stress distribution gives the bending stiffness of the membrane. We compare this method with previous approaches using the Laplace-Young equation and illustrate the method on two very different capsule materials: polymerized octadecyltrichlorosilane (OTS) capsules and hydrophobin (HFBII) coated bubbles. Our results are in agreement with the available rheological data. For hydrophobin coated bubbles, the method reveals an interesting nonlinear behavior consistent with the hydrophobin molecules having a rigid core surrounded by a softer shell.

In Situ Observation of γ-Fe2O3 Nanoparticle Adsorption under Different Monolayers at the Air/Water Interface

We studied the adsorption of gamma-Fe 2O 3 nanoparticles from an aqueous solution under different charged Langmuir monolayers (stearic acid, stearyl alcohol, and stearyl amine). The aqueous subphase was composed of a colloidal suspension of gamma-Fe 2O 3 nanoparticles. The average hydrodynamic diameter of the nanoparticles measured by dynamic light scattering measurements was 16 nm. The observed zeta potential of +40 mV (at pH 4) results in a long-term stability of the colloidal dispersion. The behavior of the different monolayer/nanoparticle composites were studied with surface pressure/area (pi/ A) isotherms. The adsorption of the nanoparticles under the different monolayers induced an expansion of the monolayers. These phenomena depended on the charge of the monolayers. After the Langmuir/Blodgett transfer on glass substrates, the nanoparticle/monolayer composite films were studied by means of UV-vis spectroscopy. The spectra pointed to increasing adsorption of the nanoparticles with increasing electronegativity of the monolayers. On the basis of these results, we studied the in situ adsorption of nanoparticles under the different monolayers by X-ray reflectivity measurements. Electron density profiles of the liquid/gas interfaces were obtained from the X-ray reflectivity data. The results gave clear evidence for the presence of electrostatic interaction between the differently charged monolayers and the positively charged nanoparticles. While the adsorption process was favored by the negatively charged stearic acid monolayer, the positively charged layer of stearyl amine prevented the formation of ultrathin nanoparticle layers.

Mechanical Stability of Ionotropic Alginate Beads

Abstract In a series of experiments we measured the mechanical properties of single alginate beads by means of squeezing experiments between two parallel plates. We used multivalent counter-ions as cross-linking molecules for the formation of three dimensional alginate gels. In this article we examined pure Fe(II) (ferric), Fe(III) (ferrous) and Ca(II) (calcium) ions as cross-linking agents and different mixtures between these charged compounds. The results of squeezing experiments showed that capsules formed with pure ferrous ions were less stable than particles which were cross-linked with calcium or ferric ions. It turned out that at equal molar concentrations calcium and ferrous ions formed stronger gels than ferric ions. In addition to squeezing capsule experiments we also investigated different particles by optical microscopy and scanning electron microscopy. Experiments of Energy-dispersive X-ray spectroscopy (EDAX) show different compositions of these beads.

Magnetic switchable alginate beads

Calcium alginate beads are enclosed in a wide range of products including food, pharmaceuticals, and cosmetic formulations. The biopolymer matrix is often used to stabilize active ingredients and to provide a controlled release under well-defined conditions. In this context, it is of high interest to study the magnetic-induced attraction, elongation, and rupture of capsules or beads. In this work, we synthesized new types of magnetic switchable alginate beads. The magnetic sensitivity was achieved by incorporation of magnetic nanoparticles (MNPs) within the alginate gel. We measured the mechanical properties of single alginate beads in squeezing experiments, the evaporation of water and the magnetic sensitivity by stimulation of these beads in external fields. In all these measurements, the alginate and the nanoparticle concentration were systematically varied. We could show that the incorporation of MNPs generates a magnetic response of the beads and reduces the evaporation of water but has no influence on the mechanical stability of the beads during compression. Calculations of the shear modulus by means of the squeezing data result in good agreement in comparison to the shear moduli measured by rheological frequency sweep tests. With scanning electron microscopy, we could analyze the molecular structure of such composite systems, and we observed a homogeneous distribution of the MNPs within the gel matrix.

Effect of magnetic nanoparticles on the surface rheology of surfactant films at the water surface

The stability of fluid interfaces is important in many technical fields, e.g. suspensions, emulsions and foams. In this publication we investigated the influence of maghemite nanoparticles (γ-Fe2O3) on the surface stability of different surfactant films (SDS, CTAB, Brij 35). We investigated the interactions between nanoparticles and surfactant films by means of surface dilatation and surface shear rheological experiments. For further characterizations we used X-ray reflectivity (XRR) measurements, dynamic light scattering (DLS) and zeta (ζ)-potential measurements. For CTAB and more obvious for SDS it was found that at low to moderate surfactant concentrations, the viscoelasticity of the interface was increased drastically in the presence of the iron oxide nanoparticles. For films of Brij 35, however, the nanoparticles did not have any influence on the surface rheology.

Magneto-responsive alginate capsules

Upon incorporation of magnetic nanoparticles (mNPs) into gels, composite materials called ferrogels are obtained. These magneto-responsive systems have a wide range of potential applications including switches and sensors as well as drug delivery systems. In this article, we focus on the properties of calcium alginate capsules, which are widely used as carrier systems in medicine and technology. We studied the incorporation of different kinds of mNPs in matrix capsules and in the core and the shell of hollow particles. We found out that not all particle-alginate or particle-CaCl2 solution combinations were suitable for a successful capsule preparation on grounds of a destabilization of the nanoparticles or the polymer. For those systems allowing the preparation of switchable beads or capsules, we systematically studied the size and microscopic structure of the capsules, their magnetic behavior and mechanical resistance.

Structure of DPPC–hyaluronan interfacial layers – effects of molecular weight and ion composition

Hyaluronan and phospholipids play an important role in lubrication in articular joints and provide in combination with glycoproteins exceptionally low friction coefficients. We have investigated the structural organization of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) Langmuir layers at the solution-air interface at different length scales with respect to the adsorption of hyaluronan (HA). This allows us to assemble a comprehensive picture of the adsorption and the resulting structures, and how they are affected by the molecular weight of HA and the presence of calcium ions. Brewster angle microscopy and grazing incident diffraction were used to determine the lateral structure at the micro- and macro scale. The data reveals an influence of HA on both the macro and micro structure of the DPPC Langmuir layer, and that the strength of this effect increases with decreasing molecular weight of HA and in presence of calcium ions. Furthermore, from X-ray reflectivity measurements we conclude that HA adsorbs to the hydrophilic part of DPPC, but data also suggest that two types of interfacial structures are formed at the interface. We argue that hydrophobic forces and electrostatic interactions play important rules for the association between DPPC and HA. Surface pressure area isotherms were used to determine the influence of HA on the phase behavior of DPPC while electrophoretic mobility measurements were used to gain insight into the binding of calcium ions to DPPC vesicles and hyaluronan.

Sticking polydisperse hydrophobic magnetite nanoparticles to lipid membranes.

The formation of a layer of hydrophobic magnetite (Fe(3)O(4)) nanoparticles stabilized by lauric acid is analyzed by in situ X-ray reflectivity measurements. The data analysis shows that the nanoparticles partially disperse their hydrophobic coating. Consequently, a Langmuir layer was formed by lauric acid molecules that can be compressed into an untilted condensed phase. A majority of the nanoparticles are attached to the Langmuir film integrating lauric acid residue on their surface into the Langmuir film. Hence, the particles at the liquid-gas interface can be identified as so-called Janus beads, which are amphiphilic solids having two sides with different functionality.

Prebiotic Cell Membranes that Survive Extreme Environmental Pressure Conditions

Attractive candidates for compartmentalizing prebiotic cells are membranes comprised of single-chain fatty acids. It is generally believed that life may have originated in the depth of the protoocean, that is, under high hydrostatic pressure conditions, but the structure and physical-chemical properties of prebiotic membranes under such conditions have not yet been explored. We report the temperature- and pressure-dependent properties of membranes composed of prebiotically highly-plausible lipids and demonstrate that prebiotic membranes could not only withstand extreme temperatures, but also serve as robust models of protocells operating in extreme pressure environments. We show that pressure not only increases the stability of vesicular systems but also limits their flexibility and permeability to solutes, while still keeping the membrane in an overall fluid-like and thus functional state.

Salt induced reduction of lysozyme adsorption at charged interfaces

A study of lysozyme adsorption below a behenic acid membrane and at the solid-liquid interface between aqueous lysozyme solution and a silicon wafer in the presence of sodium chloride is presented. The salt concentration was varied between 1 mmol L(-1) and 1000 mmol L(-1). X-ray reflectivity data show a clear dependence of the protein adsorption on the salt concentration. Increasing salt concentrations result in a decreased protein adsorption at the interface until a complete suppression at high concentrations is reached. This effect can be attributed to a reduced attractive electrostatic interaction between the positively charged proteins and negatively charged surfaces by charge screening. The measurements at the solid-liquid interfaces show a transition from unoriented order of lysozyme in the adsorbed film to an oriented order with the short protein axis perpendicular to the solid-liquid interface with rising salt concentration.