Non-equilibrium pair interactions in colloidal dispersions

We study non-equilibrium pair interactions between microscopic particles moving through a model shear-thinning fluid. Prior efforts to model pair interactions in non-Newtonian fluids have largely focused on constitutive models derived from polymer-chain kinetic theories focusing on conformational degrees of freedom, but neglecting the details of microstructural evolution beyond a single polymer length scale. To elucidate the role of strong structural distortion in mediating pair interactions in Brownian suspensions, we formulate and solve a Smoluchowski equation describing the detailed evolution of the particle configuration between and around a pair of microscopic probes driven at fixed velocity by an external force through a colloidal dispersion. To facilitate analysis, we choose a model system of Brownian hard spheres that do not interact hydrodynamically; while simple, this ‘freely draining’ model permits insight into connections between microstructure and rheology. The flow induces a non-equilibrium particle density gradient that gives rise to both viscous drag and an interactive force between the probes. The drag force acts to slow the centre-of-mass velocity of the pair, while the interactive force arising from osmotic pressure gradients can lead to attraction or repulsion, as well as deterministic reorientation of the probes relative to the external force. The degree to which the microstructure is distorted, and the shape of that distortion, depend on the arrangement of the probes relative to one another and their orientation to the driving force. It also depends on the magnitude of probe velocity relative to the Brownian velocity of the suspension. When only thermal fluctuations set probe velocity, the equilibrium depletion attraction is recovered. For weak forcing, long-ranged interactions mediated via the bath-particle flux give rise to entropic forces on the probes. The linear response is a viscous drag that slows forward motion; only the weakly nonlinear response can produce relative motion–attraction, repulsion or reorientation of the probes. We derive entropic coupling tensors, similar in ethos to pair hydrodynamic tensors, to describe this behaviour. The structural symmetry that permits this analogy is lost when forcing becomes strong, revealing instabilities in system behaviour. Far from equilibrium, the interactive force depends explicitly on the initial probe separation, orientation and strength of forcing; widely spaced probes interact through the distorted microstructure, whereas the behaviour of closely spaced probes is largely set by excluded-volume effects. In this regime, a pair of closely spaced probes sedimenting side-by-side tend to attract and reorient to permit alignment of their line-of-centres with the flow, while widely spaced probes fall without reorienting. Our results show qualitative agreement with experimental observations and provide a potential connection to the observed column instability in shear-thinning fluids.

Development of an Unmanned Aerial Vehicle for the Measurement of Turbulence in the Atmospheric Boundary Layer

This paper describes the components and usage of an unmanned aerial vehicle developed for measuring turbulence in the atmospheric boundary layer. A method of computing the time-dependent wind speed from a moving velocity sensor data is provided. The physical system built to implement this method using a five-hole probe velocity sensor is described along with the approach used to combine data from the different on-board sensors to allow for extraction of the wind speed as a function of time and position. The approach is demonstrated using data from three flights of two unmanned aerial vehicles (UAVs) measuring the lower atmospheric boundary layer during transition from a stable to convective state. Several quantities are presented and show the potential for extracting a range of atmospheric boundary layer statistics.

Surgical Experience Correlates with Performance on a Virtual Reality Simulator for Shoulder Arthroscopy

Background The traditional process of surgical education is being increasingly challenged by economic constraints and concerns about patient safety. Sophisticated computer-based devices have become available to simulate the surgical experience in a protected environment. As with any new educational tool, these devices have generated controversy about the validity of the training experience. Hypothesis Performance on a virtual reality simulator correlates with actual surgical experience. Study Design Controlled laboratory study. Methods Forty-three test subjects of various experience levels in shoulder arthroscopy were tested on an arthroscopy simulator according to a standardized protocol. Subjects were evaluated for time to completion, distance traveled with the tip of the simulated probe compared with a computer-determined optimal distance, average probe velocity, and number of probe collisions with the tissues. Results Subjects were grouped according to prior experience with shoulder arthroscopy. Comparing the least experienced with most experienced groups, the average time to completion decreased by 62% from 128.8 seconds to 49.2 seconds; path length and hook collisions were more than halved from 8.2 to 3.8 and 34.1 to 16.8, respectively; and average probe velocity more than doubled from 0.18 to 0.4 cm/second. There were no significant differences for any parameter tested between subjects with video game experience compared to those without. Conclusions The study demonstrated a close and statistically significant correlation between simulator results and surgical experience, thus confirming the hypothesis. Conversely, experience with video games was not associated with improved simulator performance. This indicates that the skill set tested may be similar to the one developed in the operating room, thus suggesting its use as a potential tool for future evaluation of surgical trainees. Clinical Relevance The results have implications for the future of orthopaedic surgical training programs, the majority of which have not embraced virtual reality technology for physician education.

Experimental Measurements of Velocity, Potential, and Temperature Distributions in Liquid Aluminum During Electromagnetic Stirring

An experimental technique has been developed to measure both axial and transverse velocities and temperature distribution in molten aluminum. Couette flow of liquid aluminum, lead, tin, and low melting alloy in cylindrical container was chosen for calibration of the experimental technique and the magnetic probe. Velocity and temperature profiles for liquid aluminum rotating in cylindrical container at different angular velocities are obtained for two different values of the depth. We determined that the velocity values increase with magnetic induction.

Velocity, Potential, and Temperature Distributions in Molten Metals During Electromagnetic Stirring: Part I — Experimental Measurements

An experimental technique has been developed to measure the local velocity in molten metals. Couette flow of liquid aluminum, lead, tin and low melting alloy in cylindrical container was chosen for calibration of the experimental technique and the magnetic probe. Velocity and temperature profiles for liquid aluminum rotating in cylindrical container at different angular velocities are obtained for two different values of the depth. We determined that the velocity values increase with magnetic induction, and the relationship between the normalized azimuthal velocity and the magnetic induction can be expressed by quadratic function.