arXiv subjects

Martin Brinkmann

Publications and source records attributed to Martin Brinkmann.

14 recordsLinked to original sources

arXiv2021

Rubbing and Drawing: Generic Ways to Improve the Thermoelectric Power Factor of Organic Semiconductors?

Highly oriented polymer films can show considerable anisotropy in the thermoelectric properties leading to power factors beyond those predicted by the widely obeyed power law linking the thermopower $S$ and the electrical conductivity $σ$ as $S\proptoσ^{-1/4}$. This has led to encouraging practical results with respect to the electrical conductivity, notwithstanding that the conditions necessary to enhance $σ$ and $S$ simultaneously are less clear. Here, kinetic Monte Carlo simulations are used to study the impact of structural anisotropy on the thermoelectric properties of disordered organic semiconductors. We find that stretching is a suitable strategy to improve the conductivity along the direction of strain, while the effect on the power factor depends on the morphology the polymer crystallizes. In general, crystalline polymers show a simultaneous increase in $σ$ and $S$ which is not the case for amorphous polymers. Moreover, we show that the trends resulting from simulations based on variable-range hopping are in good agreement with experiments and can describe the different functional dependencies in the $S$ versus $σ$ behaviour of different directions.

cond-mat.mtrl-sci↗

arXiv2021

High thermoelectric power factor of poly(3-hexylthiophene) through in-plane alignment and doping with a molybdenum dithiolene complex

Here we report a record thermoelectric power factor of up to 160 $μ$ W m-1 K-2 for the conjugated polymer poly(3-hexylthiophene) (P3HT). This result is achieved through the combination of high-temperature rubbing of thin films together with the use of a large molybdenum dithiolene p-dopant with a high electron affinity. Comparison of the UV-vis-NIR spectra of the chemically doped samples to electrochemically oxidized material reveals an oxidation level of 10%, i.e. one polaron for every 10 repeat units. The high power factor arises due to an increase in the charge-carrier mobility and hence electrical conductivity along the rubbing direction. We conclude that P3HT, with its facile synthesis and outstanding processability, should not be ruled out as a potential thermoelectric material.

physics.app-ph↗

arXiv2020

Detector Systems Engineering for Extremely Large Instruments

The scientific detector systems for the ESO ELT first-light instruments, HARMONI, MICADO, and METIS, together will require 27 science detectors: seventeen 2.5 $μ$m cutoff H4RG-15 detectors, four 4K x 4K 231-84 CCDs, five 5.3 $μ$m cutoff H2RG detectors, and one 13.5 $μ$m cutoff GEOSNAP detector. This challenging program of scientific detector system development covers everything from designing and producing state-of-the-art detector control and readout electronics, to developing new detector characterization techniques in the lab, to performance modeling and final system verification. We report briefly on the current design of these detector systems and developments underway to meet the challenging scientific performance goals of the ELT instruments.

astro-ph.IM↗

arXiv2019

Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors

Recent demonstrations of inverted thermal activation of charge mobility in polymer field-effect transistors have excited the interest in transport regimes not limited by thermal barriers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial molecular alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evidence converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process.

cond-mat.mtrl-sci↗

arXiv2016

Programmable Active Janus Droplets Driven by Water/Alcohol Phase Separation

We report the existence of self-propelled Janus droplets driven by phase separation, which are able to deliver cargo in a programmable manner. The self-propelling droplets are initially formed by a water/ethanol mixture in a squalane/monoolein solution, and evolve in up to three stages depending on ethanol concentration. In the first stage, the droplet propulsion is generated by Marangoni flow originating from the solubilization of ethanol in the oily phase. During this process the droplets absorb surfactant molecules; in combination with the continuous loss of ethanol this leads to a phase separation of the water/ethanol/monoolein mixture and the formation of Janus droplets, i.e. a water-rich droplet connected to an ethanol-rich droplet that is able to deliver cargo. We characterize the different evolution stages of self-propulsion by the flow field around the droplet that evolves from a weak pusher, over a neutral swimmer, to a dimer of neutral swimmers. Finally, we utilize this active system to deliver DNA as a cargo. Tuning the delay time before phase separation, by varying the chemical composition of the droplets, several different cargo delivery processes can be programmed.

cond-mat.soft↗

arXiv2016

Effects of slippage on the dewetting of a droplet

In many macroscopic dynamic wetting problems, it is assumed that the macroscopic interface is quasistatic, and the dissipation appears only in the region close to the contact line. When approaching the moving contact line, a microscopic mechanism is required to regularize the singularity of viscous dissipation. On the other hand, if the characteristic size of a fluidic system is reduced to a range comparable to the microscopic regularization length scale, the assumption that viscous effects are localized near the contact line is no longer justified. In the present work, such microscopic length is the slip length. We investigate the dewetting of a droplet using the boundary element method. Specifically, we solve for the axisymmetric Stokes flow with i) the Navier-slip boundary condition at the solid/liquid boundary, and ii) a time-independent microscopic contact angle at the contact line. The profile evolution is computed for different slip lengths and equilibrium contact angles. When decreasing the slip length, the typical nonsphericity first increases, reaches a maximum at a characteristic slip length $\tilde{b}_m$, and then decreases. Regarding different equilibrium contact angles, two universal rescalings are proposed to describe the behavior for slip lengths larger or smaller than $\tilde{b}_m$. Around $\tilde{b}_m$, the early time evolution of the profiles at the rim can be described by similarity solutions. The results are explained in terms of the structure of the flow field governed by different dissipation channels: viscous elongational flows for large slip lengths, friction at the substrate for intermediate slip lengths, and viscous shear flows for small slip lengths. Following the transitions between these dominant dissipation mechanisms, our study indicates a crossover to the quasistatic regime when the slip length is small compared to the droplet size.

physics.flu-dyn↗

arXiv2016

Deviation of viscous drops at chemical steps

We present systematic wetting experiments and numerical simulations of gravity driven liquid drops sliding on a plane substrate decorated with a linear chemical step. Surprisingly, the optimal direction to observe crossing is not the one perpendicular to the step, but a finite angle that depends on the material parameters. We computed the landscapes of the force acting on the drop by means of a contact line mobility model showing that contact angle hysteresis dominates the dynamics at the step and determines whether the drop passes onto the lower substrate. This analysis is very well supported by the experimental dynamic phase diagram in terms of pinning, crossing, sliding and sliding followed by pinning.

physics.flu-dyn↗

arXiv2016

Slip-Mediated Dewetting of Polymer Microdroplets

Classical hydrodynamic models predict that infinite work is required to move a three-phase contact line, defined here as the line where a liquid/vapor interface intersects a solid surface. Assuming a slip boundary condition, in which the liquid slides against the solid, such an unphysical prediction is avoided. In this article, we present the results of experiments in which a contact line moves and where slip is a dominating and controllable factor. Spherical cap shaped polystyrene microdroplets, with non-equilibrium contact angle, are placed on solid self-assembled monolayer coatings from which they dewet. The relaxation is monitored using \textit{in situ} atomic force microscopy. We find that slip has a strong influence on the droplet evolutions, both on the transient non-spherical shapes and contact line dynamics. The observations are in agreement with scaling analysis and boundary element numerical integration of the governing Stokes equations, including a Navier slip boundary condition.

cond-mat.soft↗

arXiv2016

Stochastic Rotation Dynamics simulations of wetting multi-phase flows

Multi-color Stochastic Rotation Dynamics (SRDmc) has been introduced by Inoue et al. as a particle based simulation method to study the flow of emulsion droplets in non-wetting microchannels. In this work, we extend the multi-color method to also account for different wetting conditions. This is achieved by assigning the color information not only to fluid particles but also to virtual wall particles that are required to enforce proper no-slip boundary conditions. To extend the scope of the original SRDmc algorithm to e.g. immiscible two-phase flow with viscosity contrast we implement an angular momentum conserving scheme (SRDmc+). We perform extensive benchmark simulations to show that a mono-phase SRDmc fluid exhibits bulk properties identical to a standard SRD fluid and that SRDmc fluids are applicable to a wide range of immiscible two-phase flows. To quantify the adhesion of a SRDmc+ fluid in contact to the walls we measure the apparent contact angle from sessile droplets in mechanical equilibrium. For a further verification of our wettability implementation we compare the dewetting of a liquid film from a wetting stripe to experimental and numerical studies of interfacial morphologies on chemically structured surfaces.

cond-mat.soft↗

arXiv2015

Liquid morphologies and capillary forces between three spherical beads

Equilibrium shapes of coalesced pendular bridges in a static assembly of spherical beads are computed by numerical minimization of the interfacial energy. Our present study focuses on generic bead configurations involving three beads, one of which is in contact to the two others while there is a gap of variable size between the latter. In agreement with previous experimental studies, we find interfacial `trimer' morphologies consisting of three coalesced pendular bridges, and `dimers' of two coalesced bridges. In a certain range of the gap opening we observe a bistability between the dimer and trimer morphology during shrinking and growth. The magnitude of the corresponding capillary forces in presence of a trimer or dimer depends, besides the gap opening only on the volume or Laplace pressure of liquid. For a given Laplace pressure, the capillary forces in presence of a trimer are slightly larger than the force of a single bridges at the same gap opening, which could explain the shallow maximum and plateau of the capillary cohesion of a wetting liquid for saturations in the funicular regime.

cond-mat.soft↗

arXiv2015

The Role of Contact Angle Hysteresis for Fluid Transport in Wet Granular Matter

The stability of sand castles is determined by the structure of wet granulates. Experimental data about the size distribution of fluid pockets are ambiguous about their origin. We discovered that contact angle hysteresis plays a fundamental role in the equilibrium distribution of bridge volumes, and not geometrical disorder as commonly conjectured, which has substantial consequences on the mechanical properties of wet granular beds, including a history dependent rheology and lowered strength. Our findings are obtained using a novel model where the Laplace pressures, bridge volumes and contact angles are dynamical variables associated to the contact points. While accounting for contact line pinning, we track the temporal evolution of each bridge. We observe a cross-over to a power-law decay of the variance of capillary pressures at late times and a saturation of the variance of bridge volumes to a finite value connected to contact line pinning. Large scale simulations of liquid transport in the bridge network reveal that the equilibration dynamics at early times is well described by a mean field model. The spread of final bridge volumes can be directly related to the magnitude of contact angle hysteresis.

cond-mat.soft↗

arXiv2014

Wetting heterogeneity in mixed-wet porous media controls flow dissipation

Pressure controlled displacement of an oil/water interface is studied in dense packings of functionalized glass beads with well-defined spatial wettability correlations. An enhanced dissipation is observed if the typical extension $ξ$ of the same-type wetting domains is smaller than the average bead diameter $d$. Three dimensional imaging using X-ray microtomography shows that the frequency $n(s)$ of residual droplet volumes $s$ for different $ξ$ collapse onto the same curve. This indicates that the additional dissipation for small $ξ$ is due to contact line pinning rather than an increase of capillary break-up/coalescence events.

physics.flu-dyn↗

arXiv2012

Impact of wettability correlations on multiphase flow through porous media

In the last decades, significant progress has been made in understanding the multiphase displacement through porous media with homogeneous wettability and its relation to the pore geometry. However, the role of wettability at the scale of the pore remains still little understood. In the present study the displacement of immiscible fluids through a two-dimensional porous medium is simulated by means of a mesoscopic particle approach. The substrate is described as an assembly of non-overlapping circular disks whose preferential wettability is distributed according to prescribed spatial correlations, from pore scale up to domains at system size. We analyze how this well-defined heterogeneous wettability affects the flow and try to establish a relationship among wettability-correlations and large-scale properties of the multiphase flow.

cond-mat.soft↗

arXiv2004

Ultrathin Organic Transistors on Oxide Surfaces

In recent years, thin-film organic field-effect transistors (OFETs) have begun to be considered as a possible alternative to the hydrogenated amorphous silicon thin-film transistors (a-Si:H TFT's) used in active matrix flat panel displays and other large-area electronics applications. Low-temperature processability, low-cost fabrication and compatibility with arbitrary substrates are some of the promising advantages of OFETs, among others. Of the many organic materials available, pentacene, in particular, is one of the leading candidates for use in current thin-film OFET architectures - this because of its excellent electrical characteristics and its resistance to atmospheric oxygen. In the recent literature, pentacene's transport properties, as well as transistor performance, have already been analyzed from the point of view of substrate treatments, pentacene evaporation rate and substrate temperature, electrode chemical nature and channel geometry. The results show that the morphology, crystal structure and molecular ordering of the first organic monolayer(s) at the pentacene/dielectric interface are essential determinants of carrier transport phenomena. To further investigate these interface effects, we have built a model organic field-effect transistor which consists essentially of a single layer of pentacene on an oxide substrate. Four-probe and two-probe transport measurements as a function of temperature and fields will be presented in relation with structural near-field observations. The experimental results suggest a simple two-dimensional model where the equilibrium between free and trapped carriers at the oxide interface determines the OFET characteristics and performance.

cond-mat.mtrl-sci↗