Biomedical subjects

Hans Riegler

Publications and source records attributed to Hans Riegler.

4 recordsLinked to original sources

PubMed2006

Molecularly smooth cellulose surfaces for adhesion studies.

Cellulose is deposited on silicon wafer surfaces via spin coating from a solution of cellulose in dimethylacetamide (+7% lithium chloride). The resulting cellulose layers were analyzed by ellipsometry, AFM, FTIR, ICP-MS, X-ray reflectivity, and contact angle measurements. For cellulose concentrations below 0.07 wt% the wafer surfaces are covered with a network of cellulose fibrils. For concentrations between 0.07 and 0.5 wt%, closed films with thicknesses between 1.5 and approximately 10 nm are obtained. These films are molecularly smooth (rms roughness<2 nm). Higher concentrations result in thicker films with significantly rougher surfaces (rms roughness>2 nm). The cellulose layers were used to investigate cellulose/cellulose adhesion and their modification by polyelectrolytes. To this end the sticking behavior of cellulose beads was analyzed. It is demonstrated that the sticking of the beads depends on the type, amount, and adsorption symmetry of adsorbed polyelectrolyte. Low, incomplete polyelectrolyte coverage always enhances sticking, whereas for high coverage the symmetry of the polyelectrolyte coating is very important. In this case, sticking (adhesion) is enhanced if only one surface is covered with polyelectrolyte prior to contact. If both surfaces were fully covered with polyelectrolytes before contact, sticking (adhesion) is decreased.

Journal Article↗

PubMed2005

Reversible self-propelled droplet movement: a new driving mechanism.

Reversible self-propelled movements of droplets of long chain alkanes are presented and analyzed. Slightly above the bulk melting temperature, solid alkane multilayers melt into droplets which move in a self-avoiding, random path. While moving, the droplets consume the solid alkane and leave behind a widening groove. At temperatures slightly below bulk melting this process can be reversed. Now the droplets move backwards leaving behind a narrowing solid trail which is nourished by the shrinking droplet. The speeds of the droplets are analyzed quantitatively and the melting enthalpy is identified as energetic source for their movement.

Journal Article↗

PubMed2005

Fractal analysis methods for solid alkane monolayer domains at SiO2/air interfaces.

A systematic evaluation of various fractal analysis methods is essential for studying morphologies of finite and noisy experimental patterns such as domains of long chain alkanes at SiO(2)/air interfaces. The derivation of trustworthy fractal dimensions crucially relies on the definition of confidence intervals for the assumed scaling range. We demonstrate that the determination of the intervals can be improved largely by comparing the scaling behavior of different morphological measures (area, boundary, curvature). We show that the combination of area and boundary data from coarse-grained structures obtained with the box-counting method reveals clear confidence limits and thus credible morphological data. This also holds for the Minkowski density method. It also reveals the confidence range. Its main drawback, the larger swing-in period at the lower cutoff compared to the box-counting method, is compensated by more details on the scaling behavior of area, boundary, and curvature. The sandbox method is less recommendable. It essentially delivers the same data as box-counting, but it is more susceptible to finite size effects at the lower cutoff. It is found that the domain morphology depends on the surface coverage of alkanes. The individual domains at low surface coverage have a fractal dimension of approximately 1.7, whereas at coverages well above 50% the scaling dimension is 2 with a large margin of uncertainty at approximately 50% coverage. This change in morphology is attributed to a crossover from a growth regime dominated by diffusion-limited aggregation of individual domains to a regime where the growth is increasingly affected by annealing and the interaction of solid growth fronts which approach each other and thus compete for the alkane supply.

Journal Article↗

PubMed2005

Interaction forces between cellulose microspheres and ultrathin cellulose films monitored by colloidal probe microscopy-effect of wet strength agents.

Colloidal probe microscopy was employed to study forces between cellulose surfaces upon addition of a series of cationic copolymers in aqueous solution, as model compounds for wet strength agents. The content of quaternary ammonium groups and primary amines was systematically varied in the cationic polymers, to distinguish between the importance of electrostatical and H-bonding effects. Cellulose microspheres were glued at the apex of tipless microfabricated cantilevers and used as colloidal probes. Ultra thin cellulose films and cellulose fibres were employed as model surfaces. The cellulose films of a thickness of about 5 nm were spin-coated from cellulose solution onto silicon substrates. The root-mean-square-roughness (RMS) was 0.3-0.8 nm. The cationic model polymers were compared to Servamine, a polymer employed as standard wet strength resin in papermaking industries. Force versus separation measurements showed a detailed picture of adhesion and contact breaking. Relatively strong adhesion of the order of 0.3 mJ/m(2) was observed with Servamine within a range of approximately 10 nm. At larger distances weak bond breaking and elastic chain pulling were identified. When approaching the surface one to two small jump-in's possibly related to strong binding of Servamine and subsequent attraction could be found in the case of Servamine. In contrast, all the model copolymers showed only a weak adhesion of 8-30 micro/m(2), i.e., an order of magnitude less than that of Servamine and subsequent elastic rupture domains. The contour length, persistence length and characteristic rupture distances were calculated by means of applying the WLC model. Measurements against cellulose fibres obtained from the production process proved the relevance of the model systems.

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