Hans Riegler

Structural changes in lipid monolayers during the Langmuir-Blodgett transfer due to substrate/monolayer interactions

Langmuir monolayers have been studied on-line during their hydrophilic transfer onto solid substrates. Changes in the monolayer structure and composition were visualized with the transfer fluorescence microscope by doping the lipid monolayers with 2 mol% of fluorescence dye. With the substrate stationary, upon increasing the surface pressure, substrate mediated condensation of probe-depleted stripes is observed. During the slow Langmuir-Blodgett transfer (i.e. upstroke of the substrate under constant surface pressure), a domain-free gap is observed adjacent to the three-phase line. Both phenomena are caused by the substrate mediated condensation of the monolayer onto the solid surface in the moment of its deposition. The results are described by relating the meniscus height to the interfacial energies, or by fractional dye segregation and diffusional dye transport, respectively. It is proposed that monolayer/substrate interactions can lower the free energy of the monolayer and thus facilitate its condensation onto the substrate.

Physisorption Instabilities during Dynamic Langmuir Wetting

Continuous dynamic Langmuir wetting of lipid monolayers can create regular stripe patterns on the solid substrate surface. Monolayers doped with minor amounts of fluorescence dye may be deposited in stripes of alternating high and low dye concentration. In case of pure monolayers stripes are observed with AFM. The striations run normal to the dipping direction and have widths of typically several micrometres and distances varying from 1 to ≈ 100 μm. The stripe widths, distances, and fluorescence contrast depend on deposition speed, surface pressure, substrate surface preparation, and dye concentration. The patterns probably result from physisorption (meniscus) instabilities caused by a feedback between the meniscus height (contact angle) and changes of the work of adhesion at the substrate/monolayer/air interface due to variations of the monolayer molecular packing density.

Molecular adhesion interactions between Langmuir monolayers and solid substrates

Abstract Substrate-monolayer adhesion interactions between differently prepared SiO2 surfaces and Langmuir monolayers were investigated. From the relation between the contact angle and the surface tension of a Langmuir monolayer in the configuration of Langmuir wetting the work of adhesion between the substrate and the monolayer as a function of the molecular packing and the transfer ratios was determined. Thus the molecular work of adhesion of different SiO2-dimyristoylphosphatidylethanolamine surfaces was quantified. The relation between the local adhesion interactions and the molecular packing and structure is presented and substrate-induced phase transitions are discussed.

Quantitative experimental study on the transition between fast and delayed coalescence of sessile droplets with different but completely miscible liquids

Quantitative experimental data on the coalescence behavior of sessile droplets with different but completely miscible liquids are presented. The liquids consist of various aqueous mixtures of different nonvolatile diols and carbon acids with surface tensions ranging from 33 to 68 mN/m, contact angles between 9 degrees and 20 degrees, and viscosities from 1 to 12 cP. Two distinctly different coalescence behaviors, a delayed and a fast regime, are found. The transition between the two behaviors is remarkably sharp. It is found that the coalescence mode depends predominantly on the differences in the surface tensions of the two droplets. If the surface tension difference exceeds approximately 3 mN/m, the coalescence is delayed. If it is less, droplet fusion occurs fast. Within the investigated parameter space, the transition seems independent from droplet size, absolute values of the surface tensions, and viscosity. Certain aspects of the experimental findings are explained with the simple hydrodynamic model presented in a recent publication.

Temperature-dependent electron diffraction studies of cadmium arachidate monolayers and multilayers

Abstract Order and phase transitions of cadmium arachidate monolayers and multilayers were studied by electron diffraction. Both the monolayer and the 21-layer film undergo a thermotropic disordering at temperatures below the bulk melting temperatures known from differential scanning calorimetry measurements. This transition is marked by the disappearance of the peak intensities with the hexagonal diffraction pattern geometry remaining unchanged. The hexagonal diffraction pattern of the 21-layer film confirms epitactic deposition, and “hexagonal twinning” of the diffraction spots indicates chain tilt. The loss of intensity on heating is reversible for both the monolayer and the multilayers up to temperatures where the peaks fully disappear. Heating above these temperatures reorders the films partly into a polycrystalline structure.

Noncoalescence of Sessile Drops from Different but Miscible Liquids: Hydrodynamic Analysis of the Twin Drop Contour as a Self-Stabilizing Traveling Wave

Capillarity always favors drop fusion. Nevertheless, sessile drops from different but completely miscible liquids often do not fuse instantaneously upon contact. Rather, intermediate noncoalescence is observed. Two separate drop bodies, connected by a thin liquid neck, move over the substrate. Supported by new experimental data, a thin film hydrodynamic analysis of this state is presented. Presumably advective and diffusive volume fluxes in the neck region establish a localized and temporarily stable surface tension gradient. This induces a local surface (Marangoni) flow that stabilizes a traveling wave, i.e., the observed moving twin drop configuration. The theoretical predictions are in excellent agreement with the experimental findings.

An ellipsometric study of the surface freezing of liquid alkanes

Abstract The surface freezing of alkane/air interfaces was investigated by ellipsometry and surface tension measurements. It is observed as a small jump in the ellipsometric signal. This shift can be explained by the transition from an isotropic liquid surface to a well-ordered monolayer only through a fortituous cancellation of layering and anisotropy effects. Therefore, an alternative model is discussed which interprets the surface freezing as a transition from a nematic-like molecular ordering at the interface above the surface melting temperature to a smectic-like ordering below.

X‐ray reflectivity study of behenic acid Langmuir–Blodgett mono‐ and multilayers on SiO2 surfaces as‐deposited and after thermal treatment: Influence of substrate/film interactions on molecular ordering and film topology

Langmuir–Blodgett films of various layer numbers of behenic acid on SiO2 surfaces were investigated by x‐ray reflectivity before and after thermal treatment (annealing). The thicknesses of the monolayers at the substrate/film interface indicate that the molecules are oriented normal to the interface. For samples without further treatment, at some distance from the interface, three different layer spacings are found which correspond to molecular tilt angles of 21, 28, and 36 deg. The transition region of decreasing layer spacings extends roughly over the first five layers. The films are not in equilibrium after deposition. Their molecular packing and topology can be altered irreversibly by thermal treatment (annealing) at temperatures well below the main phase transition into an isotropic phase. Whereas the molecular ordering of the first monolayer is hardly affected by the heating, all other layer spacings are uniform after the heating, with molecular tilts of 36 deg. In the case of thick multilayers the ...

Sharp transition between coalescence and non-coalescence of sessile drops

Unexpectedly, under certain conditions, sessile drops from different but completely miscible liquids do not always coalesce instantaneously upon contact: the drop bodies remain separated in a temporary state of non-coalescence , connected through a thin liquid bridge. Here we investigate the transition between the states of instantaneous coalescence and temporary non-coalescence. Experiments reveal that it is barely influenced by viscosities and absolute surface tensions. The main system control parameters for the transition are the arithmetic means of the three-phase angles,

Delayed Coalescence Behavior of Droplets with Completely Miscible Liquids

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