arXiv subjects

Othmar Marti

Publications and source records attributed to Othmar Marti.

7 recordsLinked to original sources

arXiv2024

Detailed balance in non-equilibrium dynamics of granular matter: derivation and implications

Modelling the dynamics of dense granular media is a long standing challenge and essential to many natural phenomena and technological applications. Here, we trace back puzzling experimental observation of detailed-balanced steady states to self-organisation of the neighbour probability distribution. The emergence of detailed balance in non-equilibrium granular dynamics could constitute a major step toward better models of granular media, as well as provide more insight into non-equilibrium processes in general. We show analytically that DBSS emerges when a certain neighbour probability is uniform across the system. This condition leads to a conditional cell order distribution being independent of the condition. We then carry out rotational shear experiments, in which this condition is satisfied, and show that they give rise to robust detailed-balanced steady states. We also show that, when the unconditional cell order distribution maximises the entropy, it is determined by a single constant parameter that is characteristic of all cell transitions. These results illustrate the predictive power of recently proposed evolution equations, which pave the way to simpler models of the dynamics of planar granular systems.

cond-mat.soft↗

arXiv2019

Deal-Grove Oxidation and Nanoparticle Adhesion - an AFM Study

We study the formation of nanometer thick oxide layers around nanoparticles (plateaus) in a wet nitrogen environment over 32 days. We determine both the plateau height and the lateral force needed to remove the particles with an atomic force microscope (AFM). The height is well described by the Deal-Grove model for a thermally activated diffusion limited oxidation process. Furthermore, we observe that height and force appear to be correlated suggesting that the rise in adhesion forces is mainly governed by an oxidation process. Since the plateaus are the result of a change in the chemical structure, the surface remains permanently altered even after the removal of the nanoparticles. This observation is of great importance for many applications requiring smooth silicon surfaces, such as photolithography, where small contamination can cause the observed oxidation process and significantly increase the surface roughness.

cond-mat.mtrl-sci↗

arXiv2015

Comparison of nanomechanical properties of in vivo and in vitro keratin 8/18 networks

In our work we compare the mechanical properties of the extracted keratin cytoskeleton of pancreatic carcinoma cells with the mechanical properties of in vitro assembled keratin 8/18 networks. For this purpose we use microrheology measurements with embedded tracer beads. This method is a suitable tool, because the size of the beads compared to the meshsize of the network allows us to treat the network as a continuum. Observing the beads motion with a CCD-High-Speed-Camera then leads to the dynamic shear modulus. Our measurements show lower storage moduli with increasing distance between the rim of the nucleus and the bead, but no clear tendency for the loss modulus. The same measurement method applied to in vitro assembled keratin 8/18 networks shows different characteristics of storage and loss moduli. The storage modulus is one order of magnitude lower than that of the extracted cytoskeleton and the loss modulus is higher. We draw conclusions on the network topology of both keratin network types based on the mechanical behaviour.

physics.bio-ph↗

arXiv2014

Active multi-point microrheology of cytoskeletal networks

Active microrheology is a valuable tool to determine viscoelastic properties of polymer networks. Observing the beads response to the excitation of a reference leads to dynamic and morphological information of the material. In this work we present an expansion of the well-known active two-point microrheology. By measuring the response of multiple particles in the viscoelastic medium in response to the excitation of a reference particle we are able to determine the force propagation in the polymer network.

physics.bio-ph↗

arXiv2013

Deformation induced changes in surface properties of polymers investigated by scanning force microscopy

In this study the possibility of combining commercial Scanning Force Microscopes (SFM) with stretching devices for the investigation of microscopic surface changes during stepwise elongation is investigated. Different types of stretching devices have been developed either for Scanning Platform-SFM or for Stand Alone-SFM. Their suitability for the investigation of deformation induced surface changes is demonstrated. A uniaxially oriented polypropylene film is stretched vertically to its extrusion direction. The reorientation of its microfibrillar structure is investigated and correlated to macroscopic structural changes determined by taking a force-elongation curve. Microtome cuts of natural rubber filled with 15 PHR carbon black are stretched. Changes in topography, local stiffness and adhesive force are simultaneously reported by using a new imaging method called Pulsed Force Mode (PFM).

cond-mat.soft↗

arXiv2013

High speed nanotribology with quartz crystal resonators via Atomic Force Microscopy

Friction measurements in the range of several meters per second are still of great interests. With the atomic force microscopy (AFM), the oscilaltion situation of the quartz crystal resonators of 3MHz resonance frequency are studied. And the oscillation speed could reach up to several m/s. Then the friction measurements are carried out on the Fischer Pattern, which is prepared on the quartz crystal resonator. In this article, we present how to measure the oscillation speeds of the quartz crystal resonator and the friction measurements under different designs. Calibration method and the calculation methods are also discussed in details. Although there are errors in the AFM measurements due to the setup itself and the oscillating quartz which could highly affect the sharpness of the cantilever tip, the results indicate that the local friction coefficients with oscillation are higher than that without oscillation.

cond-mat.mes-hall↗

arXiv2013

Interaction of Epithelial Cells with Surfaces and Surfaces Decorated by Molecules

A detailed understanding of the interface between living cells and substrate materials is of rising importance in many fields of medicine, biology and biotechnology. Cells at interfaces often form epithelia. The physical barrier that they form is one of their main functions. It is governed by the properties of the networks forming the cytoskeleton systems and by cell-to-cell contacts. Different substrates with varying surface properties modify the migration velocity of the cells. On the one hand one can change the materials composition. Organic and inorganic materials induce differing migration velocities in the same cell system. Within the same class of materials, a change of the surface stiffness or of the surface energy modifies the migration velocity, too. For our cell adhesion studies a variety of different, homogeneous substrates were used (polymers, bio-polymers, metals, oxides). In addition, an effective lithographic method, Polymer Blend Lithography (PBL), is reported, to produce patterned Self-Assembled Monolayers (SAM) on solid substrates featuring two or three different chemical functionalities. This we achieve without the use of conventional lithography like e-beam or UV lithography, only by using self-organization. These surfaces are decorated with a Teflon-like and with an amino-functionalized molecular layer. The resulting pattern is a copy of a previously created self-organized polymer pattern, featuring a scalable lateral domain size in the sub-micron range down below 100 nanometers. The resulting monolayer pattern features a high chemical and biofunctional contrast with feature sizes in the range of cell adhesion complexes like e.g. focal adhesion points.

physics.bio-ph↗