Microstructured surfaces, such as those inspired by nature, mediate surface interactions and are actively sought after to control wetting, adhesion, and friction. In particular, the rolling motion of spheres on microstructured surfaces in fluid environments is important for the transport of particles in microfluidic devices or in tribology. Here, we characterize the motion of smooth silicon nitride spheres (diameters 3-5 mm) as they roll down inclined planes decorated with hexagonal arrays of microwells and micropillars. For both types of patterned surfaces, we vary the area fraction of the micropatterned features from 0.04 to 0.96. We measure directly and independently the rotational and translational velocities of the spheres as they roll down planes with inclination angles that vary between 5 and 30°. For a given area fraction, we find that spheres have a higher translational and rotational velocity on surfaces with microwells than on micropillars. We rely on the model of Smart and Leighton [Phys. Fluids A 5, 13 (1993)] to obtain an effective gap width and coefficient of friction for all microstructured surfaces investigated. We find that the coefficient of friction is significantly higher for a surface with micropillars than that for one with microwells, likely due to the presence of interconnected drainage channels that provide additional paths for the fluid flow and favor solid-solid contact on the surface with micropillars. We find that while the effective gap width at a very low solid fraction is equal to the height of the patterned features, the effective separation decreases exponentially as the surface coverage of microstructures increases, with little measured differences between the two geometries. Superposition of resistance functions is used to relate the rapid decrease in the effective gap height with increase in the surface coverage observed in experiments.
Particle migration through gels, glasses, and other porous media provides selectivity, storage, and delivery of macromolecules and other particles that are critical to biological cell function, drug delivery, and water filtration. Modeling migration rates of solvent-borne colloids through such materials is thus essential in understanding and engineering the structure-transport relationship. However, most of these materials comprise an amorphously structured porous network not amenable to analytical modeling. Approaches to overcoming this challenge typically bypass interrogating the porous network by abstracting it away via mean-field models or by interrogating the solid features for a coarse estimate of porosity. While such approaches reduce analytical complexity significantly, resulting models cannot reveal interconnectedness of the void network, size-specific permeability, or insight into migration mechanisms. Other approaches aim to extract a network of void paths by approximating a medium as packing of monodisperse spheres and using traditional Voronoi decomposition, giving results that are accurate only when the constituent particles are monodisperse but strongly overpredict the passable size when the medium is made up of size-polydisperse particles, as is the case for colloidal gels, additive manufacturing, soil sediment, to name some examples. We use radical Voronoi decomposition to establish a network backbone of the porous microstructure, which accurately represents morphology for any degree of constituent polydispersity. We present an algorithm for endowing this network with the accurate size and shape and, from it, systematically deducing size-specific accessible branches. The result is a detailed permeability model for porous media of arbitrary microstructure that reveals material morphology, material phase, and size-specific permeability.
A gunshot wound to the mandible frequently creates a comminuted fracture that can be debilitating for the patient and challenging for the surgeon. In some instances, immediate open reduction and rigid fixation is not possible, and closed reduction with intermaxillary fixation is employed. This may lead to non-union or mal-union of the segments. This case report describes the management of mal-union of bilateral comminuted mandibular angle fractures secondary to a gunshot wound injury. The mandibular fractures were repaired using virtually planned patient specific reconstruction plates that included a specially designed crib cage to contain a bone graft. A cellular bone allograft—Vivigen (DePuy Synthes, Warsaw, IN)—was chosen as the bone grafting material. The patient was followed up for 7 months with normal functional status and mouth opening and without pain or signs of infection. This report demonstrates that using a virtually planned crib cage plate with cellular bone allograft can optimize surgical repair and bony healing of comminuted mandible fractures.
We investigate how the aspect ratio of micropillar or microwell arrays patterned on a surface affects the rolling and slipping motion of spheres under flooded conditions at low Reynolds numbers. We study arrays of rigid microstructures with aspect ratios varying over two orders of magnitude for surface coverages ranging from 0.04 to 0.96. We investigate how the surface features (dimensions, surface coverage, and geometry) individually impact the motion of the
Modulating the interaction potential between colloids suspended in a fluid can trigger equilibrium phase transitions as well as formation of non-equilibrium 'arrested states' such as gels and glasses. Faithful representation of such interactions are essential for using simulation to interrogate the microscopic details of non-equilibrium behavior, and for extrapolating observations to new regions of phase space that are difficult to explore in experiment. Although the extended law of corresponding states predicts equilibrium phases for systems with short-ranged interactions, it proves inadequate for equilibrium predictions of systems with longer-ranged interactions, and for predicting non-equilibrium phenomena in systems with either short-ranged or long-ranged interactions. These shortcomings highlight the need for new approaches to represent and disambiguate interaction potentials that replicate both equilibrium and non-equilibrium phase behavior. In this work, we use experiments and simulations to study a system with long-ranged thermoresponsive colloidal interactions and explore whether a resolution to this challenge can be found in regions of the phase diagram where temporal effects influence material state. We demonstrate that the conditions for non-equilibrium arrest by colloidal gelation are sensitive to both the shape of the interaction potential and the thermal quench rate. We exploit this sensitivity to propose a kinetics-based algorithm to extract distinct arrest conditions for candidate potentials that accurately selects between potentials that differ in shape but share the same predicted equilibrium structure. The method reveals that each potential has a quantitatively distinct arrest line, providing insight into how the shape of longer-ranged potentials influences the conditions for colloidal gelation.
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