Direct Measurement of Capillary Attraction between Floating Disks.

Two bodies resting at a fluid interface may interact laterally due to the surface deformations they induce. Here we use an applied magnetic force to perform direct measurements of the capillary attraction force between centimetric disks floating at an air-water interface. We compare our measurements to numerical simulations that take into account the disk's vertical displacement and spontaneous tilt, showing that both effects are necessary to describe the attraction force for short distances. We characterize the dependence of the attraction force on the disk mass, diameter, and relative spacing, and develop a scaling law that captures the observed dependence of the capillary force on the experimental parameters.

Characterization of Tensioned PDMS Membranes for Imaging Cytometry on Microraft Arrays.

Polydimethylsiloxane (PDMS) membranes can act as sensing elements, barriers, and substrates, yet the low rigidity of the elastomeric membranes can limit their practical use in devices. Microraft arrays rely on a freestanding PDMS membrane as a substrate for cell arrays used in imaging cytometry and cellular isolation. However, the underlying PDMS membrane deforms under the weight of the cell media, making automated analytical microscopy (and thus cytometry and cell isolation) challenging. Here we report the development of microfabrication strategies and physically motivated mathematical modeling of membrane deformation of PDMS microarrays. Microraft arrays were fabricated with mechanical tension stored within the PDMS substrate. These membranes deformed 20× less than that of arrays fabricated using prior methods. Modeling of the deformation of pretensioned arrays using linear membrane theory yielded ≤15% error in predicting the array deflection and predicted the impact of cure temperatures up to 120 °C. A mathematical approach was developed to fit models of microraft shape to sparse real-world shape measurements. Automated imaging of cells on pretensioned microarrays using the focal planes predicted by the model produced high quality fluorescence images of cells, enabling accurate cell area quantification (<4% error) at increased speed (13×) relative to conventional methods. Our microfabrication method and simplified, linear modeling approach is readily applicable to control the deformation of similar membranes in MEMs devices, sensors, and microfluidics.

Bouncing ball on a vibrating periodic surface.

We present an investigation of a partially elastic ball bouncing on a vertically vibrated sinusoidal surface. Following the work of McBennett and Harris [Chaos 26, 093105 (2016)], we begin by demonstrating that simple periodic vertical bouncing at a local minimum of the surface becomes unstable when the local curvature exceeds a critical value. The resulting instability gives rise to a period doubling cascade and results in persistent horizontal motion of the ball. Following this transition to horizontal motion, periodic "walking" states-where the ball bounces one wavelength over each vibration cycle-are possible and manifest for a range of parameters. Furthermore, we show that net horizontal motion in a preferred direction can be induced by breaking the left-right symmetry of the periodic topography.

The interaction of a walking droplet and a submerged pillar: From scattering to the logarithmic spiral.

Millimetric droplets may walk across the surface of a vibrating fluid bath, propelled forward by their own guiding or "pilot" wave field. We here consider the interaction of such walking droplets with a submerged circular pillar. While simple scattering events are the norm, as the waves become more pronounced, the drop departs the pillar along a path corresponding to a logarithmic spiral. The system behavior is explored both experimentally and theoretically, using a reduced numerical model in which the pillar is simply treated as a region of decreased wave speed. A trajectory equation valid in the limit of weak droplet acceleration is used to infer an effective force due to the presence of the pillar, which is found to be a lift force proportional to the product of the drop's walking speed and its instantaneous angular speed around the post. This system presents a macroscopic example of pilot-wave-mediated forces giving rise to apparent action at a distance.

Horizontal stability of a bouncing ball.

We present an investigation of a partially elastic ball bouncing on a vertically vibrated concave parabolic surface in two dimensions. In particular, we demonstrate that simple vertical motion, wherein the ball bounces periodically at the parabola's vertex, is unstable to horizontal perturbations when the parabolic coefficient defining the surface shape exceeds a critical value. The result is a new periodic solution where the ball bounces laterally over the vertex. As the parabola is further steepened, this new solution also becomes unstable which gives rise to other complex periodic and chaotic bouncing states, all characterized by persistent lateral motion.

The onset of chaos in orbital pilot-wave dynamics.

We present the results of a numerical investigation of the emergence of chaos in the orbital dynamics of droplets walking on a vertically vibrating fluid bath and acted upon by one of the three different external forces, specifically, Coriolis, Coulomb, or linear spring forces. As the vibrational forcing of the bath is increased progressively, circular orbits destabilize into wobbling orbits and eventually chaotic trajectories. We demonstrate that the route to chaos depends on the form of the external force. When acted upon by Coriolis or Coulomb forces, the droplet's orbital motion becomes chaotic through a period-doubling cascade. In the presence of a central harmonic potential, the transition to chaos follows a path reminiscent of the Ruelle-Takens-Newhouse scenario.

Interactions and pattern formation in a macroscopic magnetocapillary SALR system of mermaid cereal.

When particles are deposited at a fluid interface they tend to aggregate by capillary attraction to minimize the overall potential energy of the system. In this work, we embed floating millimetric disks with permanent magnets to introduce a competing repulsion effect and study their pattern formation in equilibrium. The pairwise energy landscape of two disks is described by a short-range attraction and long-range repulsion (SALR) interaction potential, previously documented in a number of microscopic condensed matter systems. Such competing interactions enable a variety of pairwise equilibrium states, including the possibility of a local minimum energy corresponding to a finite disk spacing. Two-dimensional (2D) experiments and simulations in confined geometries demonstrate that as the areal packing fraction is increased, the dilute repulsion-dominated lattice state becomes unstable to the spontaneous formation of localized clusters, which eventually merge into a system-spanning striped pattern. Finally, we demonstrate that the equilibrium pattern can be externally manipulated by the application of a supplemental vertical magnetic force that remotely enhances the effective capillary attraction.

Force and torque-free helical tail robot to study low Reynolds number micro-organism swimming.

Helical propulsion is used by many micro-organisms to swim in viscous-dominated environments. Their swimming dynamics are relatively well understood, but a detailed study of the flow fields is still needed to understand wall effects and hydrodynamic interactions among swimmers. In this letter, we describe the development of an autonomous swimming robot with a helical tail that operates in the Stokes regime. The device uses a battery-based power system with a miniature motor that imposes a rotational speed on a helical tail. The speed, direction, and activation are controlled electronically using an infrared remote control. Since the robot is about 5 cm long, we use highly viscous fluids to match the Reynolds number, Re, to be less than 0.1. Measurements of swimming speeds are conducted for a range of helical wavelengths, λ, head geometries, and rotation rates, ω. We provide comparisons of the experimental measurements with analytical predictions derived from resistive force theory. This force and torque-free neutrally buoyant swimmer mimics the swimming strategy of bacteria more closely than previously used designs and offers a lot of potential for future applications.

SurferBot: a wave-propelled aquatic vibrobot.

Nature has evolved a vast array of strategies for propulsion at the air-fluid interface. Inspired by a survival mechanism initiated by the honeybee (Apis mellifera) trapped on the surface of water, we here present theSurferBot: a centimeter-scale vibrating robotic device that self-propels on a fluid surface using analogous hydrodynamic mechanisms as the stricken honeybee. This low-cost and easily assembled device is capable of rectilinear motion thanks to forces arising from a wave-generated, unbalanced momentum flux, achieving speeds on the order of centimeters per second. Owing to the dimensions of the SurferBot and amplitude of the capillary wave field, we find that the magnitude of the propulsive force is similar to that of the honeybee. In addition to a detailed description of the fluid mechanics underpinning the SurferBot propulsion, other modes of SurferBot locomotion are discussed. More broadly, we propose that the SurferBot can be used to explore fundamental aspects of active and driven particles at fluid interfaces, as well as in robotics and fluid mechanics pedagogy.

Passive Control of Silane Diffusion for Gradient Application of Surface Properties.

Liquid lithography represents a robust technique for fabricating three-dimensional (3D) microstructures on a two-dimensional template. Silanization of a surface is often a key step in the liquid lithography process and is used to alter the surface energy of the substrate and, consequently, the shape of the 3D microfeatures produced. In this work, we present a passive technique that allows for the generation of silane gradients along the length of a substrate. The technique relies on a secondary diffusion chamber with a single opening, leading to a directional introduction of silane to the substrate via passive diffusion. The secondary chamber geometry influences the deposited gradient, which is shown to be well captured by Monte Carlo simulations that incorporate the passive diffusion and grafting processes. The technique ultimately allows the user to generate a range of substrate wettabilities on a single chip, enhancing throughput for organ-on-a-chip applications by mimicking the spatial variability of tissue topographies present in vivo.

Friction on water sliders.

A body in motion tends to stay in motion but is often slowed by friction. Here we investigate the friction experienced by centimeter-sized bodies sliding on water. We show that their motion is dominated by skin friction due to the boundary layer that forms in the fluid beneath the body. We develop a simple model that considers the boundary layer as quasi-steady, and is able to capture the experimental behaviour for a range of body sizes, masses, shapes and fluid viscosities. Furthermore, we demonstrate that friction can be reduced by modification of the body's shape or bottom topography. Our results are significant for understanding natural and artificial bodies moving at the air-water interface, and can inform the design of aerial-aquatic microrobots for environmental exploration and monitoring.

Optimized commercial desktop cutter technique for rapid-prototyping of microfluidic devices and application to Taylor dispersion.

Microfluidics provides a platform for efficient and transportable microanalysis, catalyzing advancements in fields such as biochemistry, materials science, and microbial ecology. While the analysis is cost-effective, standard device fabrication techniques are disproportionately expensive and specialized. A commercially available desktop cutting plotter provides an accessible method for rapidly fabricating microfluidic devices at extremely low costs. The optimized technique described in the present work enables fabrication of microchannels with dimensions as small as ∼100 µm. Straightness of channel walls is comparable to other common fabrication techniques but achieved here at a fraction of the cost and fabrication time. Solute dispersion experiments are performed using the rapidly prototyped channels to measure the effective dispersion coefficient in laminar flow through rectangular channels. The results of these experiments compare favorably to predictions from classical Taylor-Aris dispersion theory. This note provides all necessary tools for researchers and educators to seamlessly apply the desktop cutter fabrication technique. Materials list, fabrication instructions, and detailed channel characterization results are available in the supplementary material.

A first-principle mechanism for particulate aggregation and self-assembly in stratified fluids.

An extremely broad and important class of phenomena in nature involves the settling and aggregation of matter under gravitation in fluid systems. Here, we observe and model mathematically an unexpected fundamental mechanism by which particles suspended within stratification may self-assemble and form large aggregates without adhesion. This phenomenon arises through a complex interplay involving solute diffusion, impermeable boundaries, and aggregate geometry, which produces toroidal flows. We show that these flows yield attractive horizontal forces between particles at the same heights. We observe that many particles demonstrate a collective motion revealing a system which appears to solve jigsaw-like puzzles on its way to organizing into a large-scale disc-like shape, with the effective force increasing as the collective disc radius grows. Control experiments isolate the individual dynamics, which are quantitatively predicted by simulations. Numerical force calculations with two spheres are used to build many-body simulations which capture observed features of self-assembly.

Note: A versatile 3D-printed droplet-on-demand generator.

A versatile 3D-printed droplet-on-demand generator is presented for laboratory use in droplet impact and similar experiments. The design described and tested in the present work is modeled off of an existing design [Harris et al., Exp. Fluids 56, 83 (2015)] but is tested with an extended range of working fluids, and the manufacturing process is greatly simplified by 3D-printing the principal components. The present device is tested with de-ionized water and water-glycerol mixtures and was reliably able to produce single droplets-on-demand of diameters 0.65-1.32 mm with an overall variability of less than 1%. The computer-aided design (CAD) files, parts list, sample software, and circuit layout are available with this note, allowing for the device to be readily reproduced or adapted for a wide range of experimental applications.

The Diffusion of Passive Tracers in Laminar Shear Flow.

A simple method to experimentally observe and measure the dispersion of a passive tracer in a laminar fluid flow is described. The method consists of first injecting fluorescent dye directly into a pipe filled with distilled water and allowing it to diffuse across the cross-section of the pipe to obtain a uniformly distributed initial condition. Following this period, the laminar flow is activated with a programmable syringe pump to observe the competition of advection and diffusion of the tracer through the pipe. Asymmetries in the tracer distribution are studied and correlations between the pipe cross-section and the shape of the distribution is shown: thin channels (aspect ratio << 1) produce tracers arriving with sharp fronts and tapering tails (front-loaded distributions), while thick channels (aspect ratio ~1) present the opposite behavior (back-loaded distributions). The experimental procedure is applied to capillary tubes of various geometries and is particularly relevant to microfluidic applications by dynamical similarity.

How boundaries shape chemical delivery in microfluidics.

Many microfluidic systems-including chemical reaction, sample analysis, separation, chemotaxis, and drug development and injection-require control and precision of solute transport. Although concentration levels are easily specified at injection, pressure-driven transport through channels is known to spread the initial distribution, resulting in reduced concentrations downstream. Here we document an unexpected phenomenon: The channel's cross-sectional aspect ratio alone can control the shape of the concentration profile along the channel length. Thin channels (aspect ratio << 1) deliver solutes arriving with sharp fronts and tapering tails, whereas thick channels (aspect ratio ~ 1) produce the opposite effect. This occurs for rectangular and elliptical pipes, independent of initial distributions. Thus, it is possible to deliver solute with prescribed distributions, ranging from gradual buildup to sudden delivery, based only on the channel dimensions.

Issues and initiatives in the testing process in primary care physician offices.

BACKGROUND: Errors occur frequently in management of the testing process in primary care physicians' offices. These errors may result in significant harm to patients and lead to inefficient practice. Important issues are summarized for primary care clinicians and their offices toconsider in improving the management of the testing processes. METHODS: To identify published efforts to improve management of the testing process, a literature search was performed and the references from the identified articles were checked for additional studies. Descriptive studies, expert opinion pieces, and controlled trials were all included. Unpublished results of ongoing studies in laboratory testing errors in primary care practice are presented. RESULTS: A conceptual model of the testing process was developed, with identified general and specific errors that occur in the testing process. On the basis largely of descriptive studies, ways are described to reduce testing process errors and the harm resulting from these errors. CONCLUSIONS: Standardization of processes, computerized test tracking systems (especially those embedded in electronic medical records), and attention to human factors issues are likely to reduce errors and harm. These ideas need confirmation in well-designed randomized trials and quality improvement initiatives.

Database design to ensure anonymous study of medical errors: a report from the ASIPS Collaborative.

Medical error reporting systems are important information sources for designing strategies to improve the safety of health care. Applied Strategies for Improving Patient Safety (ASIPS) is a multi-institutional, practice-based research project that collects and analyzes data on primary care medical errors and develops interventions to reduce error. The voluntary ASIPS Patient Safety Reporting System captures anonymous and confidential reports of medical errors. Confidential reports, which are quickly de-identified, provide better detail than do anonymous reports; however, concerns exist about the confidentiality of those reports should the database be subject to legal discovery or other security breaches. Standard database elements, for example, serial ID numbers, date/time stamps, and backups, could enable an outsider to link an ASIPS report to a specific medical error. The authors present the design and implementation of a database and administrative system that reduce this risk, facilitate research, and maintain near anonymity of the events, practices, and clinicians.

Wavelike statistics from pilot-wave dynamics in a circular corral.

Bouncing droplets can self-propel laterally along the surface of a vibrated fluid bath by virtue of a resonant interaction with their own wave field. The resulting walking droplets exhibit features reminiscent of microscopic quantum particles. Here we present the results of an experimental investigation of droplets walking in a circular corral. We demonstrate that a coherent wavelike statistical behavior emerges from the complex underlying dynamics and that the probability distribution is prescribed by the Faraday wave mode of the corral. The statistical behavior of the walking droplets is demonstrated to be analogous to that of electrons in quantum corrals.

Variation in colorectal cancer screening steps in primary care: basis for practice improvement.

No published research has assessed the specific steps that primary care practices actually take to carry out screening for colorectal cancer (CRC). A written survey was distributed to clinicians and staff at 15 primary care practices to determine whether they perceived that personnel in their practices performed a series of 4 steps associated with screening colonoscopy and 7 steps associated with stool blood test screening. For each discrete step, the percentage of respondents from each practice who indicated that a given step is performed in that practice was calculated along with the mean of practice percentages. Survey results indicate wide variation in the degree to which these screening steps are performed across the 15 practices.  Variation was greater for steps that involved contacting nonresponders (reminders), scheduling, and rescheduling. Survey responses suggest substantial variation and much room for improvement in practice performance of evidence-based steps in the CRC screening process.