Molecular dynamics simulations of oxide memory resistors (memristors)

Reversible bipolar nano-switches that can be set and read electronically in a solid-state two-terminal device are very promising for applications. We have performed molecular-dynamics simulations that mimic systems with oxygen vacancies interacting via realistic potentials and driven by an external bias voltage. The competing short- and long-range interactions among charged mobile vacancies lead to density fluctuations and short-range ordering, while illustrating some aspects of observed experimental behavior, such as memristor polarity inversion.Comment: 15 pages, 5 figure

Self-assembly of Active Colloidal Molecules with Dynamic Function

Catalytically active colloids maintain non-equilibrium conditions in which they produce and deplete chemicals and hence effectively act as sources and sinks of molecules. While individual colloids that are symmetrically coated do not exhibit any form of dynamical activity, the concentration fields resulting from their chemical activity decay as $1/r$ and produce gradients that attract or repel other colloids depending on their surface chemistry and ambient variables. This results in a non-equilibrium analogue of ionic systems, but with the remarkable novel feature of action-reaction symmetry breaking. We study solutions of such chemically active colloids in dilute conditions when they join up to form molecules via generalized ionic bonds, and discuss how we can achieve structures with time dependent functionality. In particular, we study a molecule that adopts a spontaneous oscillatory pattern of conformations, and another that exhibits a run-and-tumble dynamics similar to bacteria. Our study shows that catalytically active colloids could be used for designing self-assembled structures that posses dynamical functionalities that are determined by their prescribed 3D structures, a strategy that follows the design principle of proteins

Electrowetting: from basics to applications

Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects¿rather than a unique one¿are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices

Tunable Casimir repulsion with three dimensional topological insulators

In this Letter, we show that switching between repulsive and attractive Casimir forces by means of external tunable parameters could be realized with two topological insulator plates. We find two regimes where a repulsive (attractive) force is found at small (large) distances between the plates, canceling out at a critical distance. For a frequency range where the effective electromagnetic action is valid, this distance appears at length scales corresponding to $1-\epsilon(\omega) (2/\pi)\alpha\theta$.Comment: 9 pages, 5 figures, published version with auxiliary material. Featured in Physical Review Focu

Linear actuator for a submersible water pump for use in boreholes

Both the theory and the test results show that the E-core electromagnet linear actuator, which is based on the variable reluctance principle, can generate a normal force in excess of 400kNm(^-2) when there is a flux density of IT within the airgap. When the actuator is used as a driver in a submersible water pump for use in boreholes the results show that the pump is capable of pumping up to 90% of the expected value. Pressures in excess of 10 Bar have been achieved, whilst the pump was operating at frequencies up to 30Hz. The flow rate was less than 0.21s ', however improvements to the pumping system are given, and the desired 1ls ' flow rate is achievable at a delivery head of 100m.The use of linear actuators for use in submersible water pumps is a relatively new technology, and as the demand for safe clean water increases, it provides for sustainable development. The actuator utilises a D C. supply with solar panels as the source, giving the potential for global use, particularly in developing countries (the South).The design of the driver can be optimised for selected parameters. However, the development of such drivers does have limitations, the overall diameter of the pump is restricted to that of the bore-hole size, 4 or 6 inches; further the length of the pump is dictated by the straightness of the bore-hole. Consequently, design tools, for the design of E-core Variable Reluctance Linear Actuators, (VRLA), are given