Marcel Roth

Superhydrophobic surfaces by hybrid raspberry-like particles.

Surface roughness on different length scales is favourable for superhydrophobic behaviour of surfaces. Here we report (i) an improved synthesis for hybrid raspberry-like particles and (ii) a novel method to obtain superhydrophobic films of good mechanical stability. Polystyrene spheres with a diameter of 400 nm-1 microm are decorated with silica colloids < 100 nm in size, thus introducing surface asperities on a second length scale. To improve mechanical resistance, we then coated the polystyrene core and attached silica colloids with a smooth silica shell of 10 nm to 40 nm thickness. All three steps of this synthesis procedure can be sensitively tuned so that the average size and number of the silica colloids as well as the morphology of the resulting raspberry particles can be predicted. As the particles disperse in water, either monolayers can be prepared by dip coating or multilayers by drop casting. Although mechanically stable, the shells are porous enough to allow for leakage of molten or dissolved polystyrene from the core. In tetrahydrofuran vapour polystyrene bridges form between the particles that render the multilayer-film stable. Leaked polystyrene that masks some asperities can be removed by plasma cleaning. Surface roughness on larger scales can be tuned by the drying procedure. The films are hydrophobized by silanization with a semi-fluorinate silane.

The Softer the Better: Fast Condensation on Soft Surfaces

Condensation on soft elastic surfaces differs significantly from condensation on hard surfaces. On polymeric substrates with varying cross-linking density, we investigate the nucleation and the growth of condensing water drops. With increasing softness of the substrates, we find (1) increasing nucleation density, (2) longer relaxation times for drop shape equilibration after merging of two drops, and (3) prevention of merging on very soft surfaces. These effects lead to higher surface coverage and overall condensed volume on soft surfaces.

Colloidal aggregates tested via nanoindentation and quasi-simultaneous 3D imaging

The mechanical properties of aggregated colloids depend on the mutual interplay of inter-particle potentials, contact forces, aggregate structure and material properties of the bare particles. Owing to this variety of influences, experimental results from macroscopic mechanical testings were mostly compared to time-consuming, microscopic simulations rather than to analytical theories. The aim of the present paper was to relate both macroscopic and microscopic mechanical data with each other and simple analytical models. We investigated dense amorphous aggregates made from monodisperse poly-methyl methacrylate (PMMA) particles (diameter: 1.6

Micromechanical properties of colloidal structures

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Aggregation and Deformation Induced Reorganisation of Colloidal Suspension

m via nanoindentation in combination with confocal microscopy. The resulting macroscopic information was complemented by the three-dimensional aggregate structure as well as the microscopic strain field and strain tensor. The measured strain field and tensor were in reasonable agreement with the predictions from analytical continuum theories. Consequently, the measured force-depth curves could be analyzed within a theoretical framework that had been frequently used for nanoindentation of atomic matter such as metals, ceramics and polymers. The extracted values for hardness and effective Young’s modulus represented average values characteristic of the aggregate. On the basis of of these parameters we discuss the influence of the strength of particle bonds by introducing polystyrene (PS) between the particles.Graphical abstract

Viscoelastic rheology of colloid-liquid crystal composites.

The production, further processing as well as the product properties of nanostructured aggregates and colloidal films are specified by characteristics of the particles and particle structures. Especially, the effect of the colloidal structure on the micromechanical properties is of particular interest. The investigation of this structure-micromechanical property relationship was the main objective of this study. Therefore, the colloidal structure and micromechanical properties of silica model aggregates and colloidal PMMA films were analyzed and compared to theoretical considerations and simulations of the deformation and fracture behavior using the discrete elements method.

Measurement of rotation of individual spherical particles in cohesive granulates

Various mechanisms can lead to colloidal aggregation. Attractive interactions being the most prominent amongst them. In this chapter we describe the synthesis of well defined colloids with a smooth or rough surface, their mechanical characterization, controlled aggregation and the study of the mechanical and structural properties of the colloids and the formed aggregates. Depending on the interplay between the properties of the single colloids, the interactions between the colloids and the structure formed by the colloids, the macroscopic response of the system can greatly change.