R. F. Neumann

Domain wall control in wire-tube nanoelements

The possibility of a three-state nanoelement, composed by a wire and a tube, is investigated by means of Monte Carlo simulations. The desired behavior may be identified by a step or plateau in the hysteresis curve, corresponding to a partial pinning of the domain wall at the interface between wire and tube sections. This step may be augmented in segmented nanoelements with large coercivity difference between the sections. Different possibilities, such as geometry and choice of materials, are explored.

Magnetic properties of La0.67Sr0.33MnO3/BiFeO3(001) heterojunctions: Chemically abrupt vs. atomic intermixed interface

Using first-principles density-functional calculations, we address the magnetic properties of the ferromagnet/antiferromagnet La0.67Sr0.33MnO3/BiFeO3(001) heterojunctions, and investigate possible driving mechanisms for a ferromagnetic (FM) interfacial ordering of the Fe spins recently observed experimentally. We find that the chemically abrupt defect-free La0.67Sr0.33MnO3/BiFeO3(001) heterojunction displays, as ground state, an ordering with compensated Fe spins. Cation Fe/Mn intermixing at the interface tends to favour, instead, a FM interfacial order of the Fe spins, coupled antiferromagnetically to the bulk La0.67Sr0.33MnO3 spins, as observed experimentally. Such trends are understood based on a model description of the energetics of the exchange interactions.

Stability of magnetic nanoparticles inside ferromagnetic nanotubes

During the last years great attention has been given to the encapsulation of magnetic nanoparticles. In this work we investigated the stability of small magnetic particles inside magnetic nanotubes. Multisegmented geometries were tested in order to optimize the stability of the particle inside the nanotubes. Our results evidenced that multisegmented nanotubes are more efficient to entrap the particles at temperatures up to hundreds of kelvins.

Fluid dynamics in porous media with Sailfish

In this work we show the application of Sailfish to the study of fluid dynamics in porous media. Sailfish is an open-source software based on the lattice-Boltzmann method. This application of computational fluid dynamics is of particular interest to the oil and gas industry and the subject could be a starting point for an undergraduate or graduate student in physics or engineering. We built artificial samples of porous media with different porosities and used Sailfish to simulate the fluid flow through in order to calculate permeability and tortuosity. We also present a simple way to obtain the specific superficial area of porous media using Python libraries. To contextualize these concepts, we test the Kozeny--Carman equation, discuss its validity and calculate the Kozenys constant for our artificial samples.

Confinement of magnetic nanoparticles inside multisegmented nanotubes by means of magnetic field gradients

The possibility of confining magnetic nanoparticles inside multisegmented nanotubes by using strong field gradients is considered by means of Monte Carlo simulations. The problem is reduced to the random walk performed by the nanoparticle on the energy landscape produced by the tube’s magnetic field. The role of tube material, number of segments, and spacer thickness in the amount of time spent by the particle inside the tube is examined, concluding that it is possible to control the encapsulation time by using different architectures.

Adsorption energy as a metric for wettability at the nanoscale

Wettability is the affinity of a liquid for a solid surface. For energetic reasons, macroscopic drops of liquid form nearly spherical caps. The degree of wettability is then captured by the contact angle where the liquid-vapor interface meets the solid-liquid interface. As droplet volumes shrink to the scale of attoliters, however, surface interactions become significant, and droplets assume distorted shapes. In this regime, the contact angle becomes ambiguous, and a scalable metric for quantifying wettability is needed, especially given the emergence of technologies exploiting liquid-solid interactions at the nanoscale. Here we combine nanoscale experiments with molecular-level simulation to study the breakdown of spherical droplet shapes at small length scales. We demonstrate how measured droplet topographies increasingly reveal non-spherical features as volumes shrink. Ultimately, the nanoscale droplets flatten out to form layer-like molecular assemblies at the solid surface. For the lack of an identifiable contact angle at small scales, we introduce a droplet’s adsorption energy density as a new metric for a liquid’s affinity for a surface. We discover that extrapolating the macroscopic idealization of a drop to the nanoscale, though it does not geometrically resemble a realistic droplet, can nonetheless recover its adsorption energy if line tension is included.

A Platform for Analysis of Nanoscale Liquids with an Array of Sensor Devices Based on Two-Dimensional Material

Analysis of nanoscale liquids, including wetting and flow phenomena, is a scientific challenge with far reaching implications for industrial technologies. We report the conception, development, and application of an integrated platform for the experimental characterization of liquids at the nanometer scale. The platform combines the functionalities of a two-dimensional electronic array of sensor devices with in situ application of highly sensitive optical microspectroscopy and atomic force microscopy. We demonstrate the performance capabilities of the platform with an embodiment based on an array of optically transparent graphene sensors. The application of electronic and optical sensing in the platform allows for differentiating between liquids electronically, for determining a liquids molecular fingerprint, and for monitoring surface wetting dynamics in real time. In order to explore the platforms sensitivity limits, we record topographies and optical spectra of individual, spatially isolated sessile oil emulsion droplets having volumes of less than ten attoliters. The results demonstrate that integrated measurement functionalities based on two-dimensional materials have the potential to push lab-on-chip based analysis from the microscale to the nanoscale.

Nanoscale Flow Chip Platform for Laboratory Evaluation of Enhanced Oil Recovery Materials

We present a lab-on-chip platform for the experimental evaluation of Enhanced Oil Recovery (EOR) methods from the nanoscale to the scale of reservoir rock pore networks. We have employed semiconductor process technology to build lab-on-chip flow devices with features at the nanometer scale that allow us to perform controlled flow experiments for calibrating multiscale flow models. The platform built on silicon semiconductor technology is highly customizable and allows for design adaptation of different physical model representations. The approach enables us to experimentally investigate and validate liquid flow in porous media below the micrometer scale and to deploy calibrated, multi-scale flow simulations in a digital representation of a given rock pore network. The chip implementations of the nanoscale, porous rock network enable systematic flow studies covering various parameters (e.g. effective porosity, viscosity, surface properties) under controlled conditions of physical parameters (e.g. temperature, pressure). High resolution optical microscopy measurement techniques enable us to track individual nanometer size fluorescent tags which allow us to directly determine fluid flow speeds even in sub-micrometer constrictions. We introduce the architecture of the flow chip, discuss how the flow experiments are performed and how the experimental results are used to calibrate the flow simulations. Ultimately, the calibrated flow simulations will be used for predicting the efficiency of a specific EOR agent for improving oil displacement in a pore scale network of reservoir rock.

Tailoring the nucleation of domain walls along multi-segmented cylindrical nanoelements

The magnetization reversal of three-segment cylindrical nanoelements comprising alternating nanowire and nanotube sections is investigated by means of Monte Carlo simulations. Such nanoelements may feature a three-state behaviour with an intermediate plateau in the hysteresis curve due to a metastable pinning of the domain walls (DWs) at the wire-tube interfaces. It turns out that vortex as well as transverse DWs contribute to the magnetization reversal. By varying the geometric parameters, the sequence, or the material of the elements the nucleation location of DWs, as well as their nucleation field, can be tailored. Especially interesting is the novel possibility to drive DWs coherently in the same or in opposite directions by changing the geometry of the hybrid nanoelement. This important feature provides additional flexibility to the construction of logical devices based on DW movement. Another prominent outcome is that DWs can be nucleated near the centre of the element and then traverse to the outer tips of the cylindrical structure when the applied field is increased, which also opens the possibility to use these three-segment nanoelements for the field-induced delivery of DWs as substitutes for large nucleation pads.