arXiv subjects

Florian Platten

Publications and source records attributed to Florian Platten.

5 recordsLinked to original sources

arXiv2026

Why the better conductor surges in impedance: a low-cost demonstration of the skin effect at line frequency

When alternating current flows through a conductor, induced internal eddy currents suppress current flow in the core and restrict it to a thin surface layer. Although this effect, known as the skin effect, is covered in virtually every undergraduate electrodynamics course, it is rarely explored experimentally without specialized and expensive equipment, and is sometimes even considered a mere high-frequency phenomenon. Here, we present a readily accessible, quantitative demonstration of the skin effect at line frequency using only standard laboratory supplies: an AC/DC power supply, basic multimeters, and laboratory support rods as cylindrical conductors. Based on a precise four-point Kelvin sensing scheme capable of resolving milliohm-range resistances, we exploit relative magnetic permeability rather than high frequency as the governing parameter to control skin depth. For non-magnetic stainless steel, line frequency represents an electrodynamically low frequency, resulting in identical AC and DC behaviour; whereas for ferromagnetic structural steel, high permeability compresses the skin depth to the millimeter scale, causing a sharp surge in AC impedance. This produces a counterintuitive outcome where the statically superior conductor suffers a marked relative impedance increase, whereas the poorer conductor remains unaffected, challenging common student expectations and catalyzing cognitive activation. Depending on the instructional setting, the approach can be implemented either as a quick lecture demonstration or as a quantitative experiment within an undergraduate laboratory course.

physics.ed-ph↗

arXiv2022

Protein solutions close to liquid-liquid phase separation exhibit a universal osmotic equation of state and dynamical behavior

Liquid-liquid phase separation (LLPS) of protein solutions is governed by highly complex protein-protein interactions. Nevertheless, it has been suggested that based on the extended law of corresponding states (ELCS), as proposed for colloids with short-range attractions, one can rationalize not only the thermodynamics, but also the structure and dynamics of such systems. This claim is systematically and comprehensively tested here by static and dynamic light scattering experiments. Spinodal lines, the isothermal osmotic compressibility $κ_\text{T}$ and the relaxation rate of concentration fluctuations $Γ$ are determined for protein solutions in the vicinity of LLPS. All these quantities are found to exhibit a corresponding-states behavior. This means that, for different solution conditions, these quantities are essentially the same if considered at similar reduced temperature or second virial coefficient. For moderately concentrated solutions, the volume fraction $ϕ$ dependence of $κ_\text{T}$ and $Γ$ can be consistently described by Baxter's model of adhesive hard spheres. The off-critical, asymptotic $T$ behavior of $κ_\text{T}$ and $Γ$ close to LLPS is consistent with the scaling laws predicted by mean-field theory. Thus, the present work aims at a comprehensive experimental test of the applicability of the ELCS to structural and dynamical properties of concentrated protein solutions.

cond-mat.soft↗

arXiv2022

Phase separation and dynamical arrest of protein solutions dominated by short-range attractions

The interplay of phase separation and dynamical arrest can lead to the formation of gels and glasses, which is relevant for such diverse fields as hard and soft condensed matter physics, materials science, food engineering and pharmaceutical industry. Here, the non-equilibrium states as well as the interactions of globular proteins are analyzed. Lysozyme in brine is chosen as a model system with short-range attractions. The metastable gas-liquid binodal and the dynamical arrest line as well as the second virial coefficient $B_2$ have been determined for various solution conditions by cloud-point measurements, optical microscopy, centrifugation experiments and light scattering. If temperature is expressed in terms of $B_2$, the binodals obtained under various conditions fall onto a master curve, as suggested by the extended law of corresponding states. Arrest lines for different salt concentrations overlap within experimental errors, whereas they do not overlap if the temperature axis is replaced by $B_2$. This indicates that the binodals are not sensitive to the details of the potential, but can be described by one integral parameter, i.e. $B_2$, whereas the arrest line appears governed by its attractive part. The experimental findings are supported by numerical results of the non-equilibrium self-consistent generalized Langevin equation theory.

cond-mat.soft↗

arXiv2021

Interactions in protein solutions close to liquid-liquid phase separation: Ethanol reduces attractions via changes of the dielectric solution properties

Ethanol is a common protein crystallization agent, precipitant, and denaturant, but also alters the dielectric properties of solutions. While ethanol-induced unfolding is largely ascribed to its hydrophobic parts, its effect on protein phase separation and inter-protein interactions remains poorly understood. Here, the effects of ethanol and NaCl on the phase behavior and interactions of protein solutions are studied in terms of the metastable liquid-liquid phase separation (LLPS) and the second virial coefficient $B_2$ using lysozyme solutions. Determination of the phase diagrams shows that the cloud-point temperatures are reduced and raised by the addition of ethanol and salt, respectively. The observed trends can be explained using the extended law of corresponding states as changes of $B_2$. The results for $B_2$ agree quantitatively with those of static light scattering and small-angle X-ray scattering experiments. Furthermore, $B_2$ values calculated based on inter-protein interactions described by the Derjaguin--Landau--Verwey--Overbeek (DLVO) potential and considering the dielectric solution properties and electrostatic screening due to the ethanol and salt content quantitatively agree with the experimentally observed $B_2$ values.

cond-mat.soft↗

arXiv2015

Extended law of corresponding states for protein solutions

The so-called extended law of corresponding states, as proposed by Noro and Frenkel [J. Chem. Phys. 113, 2941 (2000)], involves a mapping of the phase behaviors of systems with short-range attractive interactions. While it has already extensively been applied to various model potentials, here we test its applicability to protein solutions with their complex interactions. We successfully map their experimentally determined metastable gas--liquid binodals, as available in the literature, to the binodals of short-range square-well fluids, as determined by previous as well as new Monte Carlo simulations. This is achieved by representing the binodals as a function of the temperature scaled with the critical temperature (or as a function of the reduced second virial coefficient) and the concentration scaled by the cube of an effective particle diameter, where the scalings take into account the attractive and repulsive contributions to the interaction potential, respectively. The scaled binodals of the protein solutions coincide with simulation data of the adhesive hard-sphere fluid. Furthermore, once the repulsive contributions are taken into account by the effective particle diameter, the temperature dependence of the reduced second virial coefficients follows a master curve that corresponds to a linear temperature dependence of the depth of the square-well potential. We moreover demonstrate that, based on this approach and cloud-point measurements only, second virial coefficients can be estimated, which we show to agree with values determined by light scattering or by DLVO-based calculations.

cond-mat.soft↗