Eric Loth

Inherently superoleophobic nanocomposite coatings by spray atomization.

We describe a technique to fabricate, for the first time, superoleophobic coatings by spray casting nanoparticle-polymer suspensions. The method involves the use of ZnO nanoparticles blended with a waterborne perfluoroacrylic polymer emulsion using cosolvents. Acetone is shown to be an effective compatibilizing cosolvent to produce self-assembling nanocomposite slurries that form hierarchical nanotextured morphology upon curing. Fabricated coating surface morphology is investigated with an environmental scanning electron microscope (ESEM), and surface wettability is characterized by static and dynamic contact angle measurements. The coatings can be applied to large and/or flexible substrates by spray coating with ease and require no additional surface treatments of commonly used hydrophobic molecules such as fluorosilanes; i.e., the nanocomposites are inherently superoleophobic. The superoleophobic nature of the coatings is also discussed within the framework of Cassie-Baxter and Wenzel wetting theories.

Recent advances in the mechanical durability of superhydrophobic materials

Large majority of superhydrophobic surfaces have very limited mechanical wear robustness and long-term durability. This problem has restricted their utilization in commercial or industrial applications and resulted in extensive research efforts on improving resistance against various types of wear damage. In this review, advances and developments since 2011 in this field will be covered. As such, we summarize progress on fabrication, design and understanding of mechanically durable superhydrophobic surfaces. This includes an overview of recently published diagnostic techniques for probing and demonstrating tribo-mechanical durability against wear and abrasion as well as other effects such as solid/liquid spray or jet impact and underwater resistance. The review is organized in terms of various types of mechanical wear ranging from substrate adhesion, tangential surface abrasion, and dynamic impact to ultrasonic processing underwater. In each of these categories, we highlight the most successful approaches to produce robust surfaces that can maintain their non-wetting state after the wear or abrasive action. Finally, various recommendations for improvement of mechanical wear durability and its quantitative evaluation are discussed along with potential future directions towards more systematic testing methods which will also be acceptable for industry.

A portable powered ankle-foot orthosis for rehabilitation

Innovative technological advancements in the field of orthotics, such as portable powered orthotic systems, could create new treatment modalities to improve the functional out come of rehabilitation. In this article, we present a novel portable powered ankle-foot orthosis (PPAFO) to provide untethered assistance during gait. The PPAFO provides both plantar flexor and dorsiflexor torque assistance by way of a bidirectional pneumatic rotary actuator. The system uses a portable pneumatic power source (compressed carbon dioxide bottle) and embedded electronics to control the actuation of the foot. We collected pilot experimental data from one impaired and three nondisabled subjects to demonstrate design functionality. The impaired subject had bilateral impairment of the lower legs due to cauda equina syndrome. We found that data from nondisabled walkers demonstrated the PPAFOs capability to provide correctly timed plantar flexor and dorsiflexor assistance during gait. Reduced activation of the tibialis anterior during stance and swing was also seen during assisted nondisabled walking trials. An increase in the vertical ground reaction force during the second half of stance was present during assisted trials for the impaired subject. Data from nondisabled walkers demonstrated functionality, and data from an impaired walker demonstrated the ability to provide functional plantar flexor assistance.

Superhydrophobic Nanocomposite Surface Topography and Ice Adhesion

A method to reduce the surface roughness of a spray-casted polyurethane/silica/fluoroacrylic superhydrophobic nanocomposite coating was demonstrated. By changing the main slurry carrier fluid, fluoropolymer medium, surface pretreatment, and spray parameters, we achieved arithmetic surface roughness values of 8.7, 2.7, and 1.6 μm on three test surfaces. The three surfaces displayed superhydrophobic performance with modest variations in skewness and kurtosis. The arithmetic roughness level of 1.6 μm is the smoothest superhydrophobic surface yet produced with these spray-based techniques. These three nanocomposite surfaces, along with a polished aluminum surface, were impacted with a supercooled water spray in icing conditions, and after ice accretion occurred, each was subjected to a pressurized tensile test to measure ice-adhesion. All three superhydrophobic surfaces showed lower ice adhesion than that of the polished aluminum surface. Interestingly, the intermediate roughness surface yielded the best performance, which suggests that high kurtosis and shorter autocorrelation lengths improve performance. The most ice-phobic nanocomposite showed a 60% reduction in ice-adhesion strength when compared to polished aluminum.

Conservative load transfer along curved fluid-solid interface with non-matching meshes

We investigate the effect of curvature on the accuracy of schemes used to transfer loads along the interface in coupled fluid-solid simulations involving non-matching meshes. We analyze two types of load transfer schemes for the coupled system: (a) point-to-element projection schemes and (b) common-refinement schemes. The accuracy of these schemes over the curved interface is assessed with the aid of static and transient problems. We show that the point-to-element projection schemes may yield inaccurate load transfer from the source fluid mesh to the target solid mesh, leading to a weak instability in the form of spurious oscillations and overshoots in the interface solution. The common-refinement scheme resolves this problem by providing an accurate transfer of discrete interface conditions across non-matching meshes. We show theoretically that the accurate transfer preserves the stability of the coupled system while maintaining the energy conservation over a reference interface. Finally, we introduce simple analytical error functions which correlate well with the numerical errors of the load transfer schemes.

Liquid repellent nanocomposites obtained from one-step water-based spray

Significant research efforts have been directed towards the development of novel superhydrophobic and superoleophobic coatings. However, it is still highly challenging to develop facile and environmentally friendly methods that lead to surfaces and coatings with efficient liquid repellency. Herein, we demonstrate for the first time, a one-step water-based spray method on heated metallic substrates for the preparation of water and oil repellent polymer nanocomposite coatings by using hydrophilic silica nanoparticles. These environmentally-friendly coatings exhibit static contact angles exceeding 150° and roll off angles less than 5° for both water and oil and can also successfully repel a wide variety of other liquids. The influence of the chemical composition and the surface texture on the wetting properties are discussed based on measurements from white light interferometry, electron microscopy and X-ray photoelectron spectroscopy. When the substrates were treated with an adhesive primer, the wear abrasion resistance of the coatings was enhanced. The effect of silica nanoparticle dispersion in the coatings on wear abrasion is also investigated. Certain nanocomposites were found to exhibit good abrasion resistance by retaining their water repellency for up to 60 abrasion cycles under 20.5 kPa applied pressure.

Transforming Anaerobic Adhesives into Highly Durable and Abrasion Resistant Superhydrophobic Organoclay Nanocomposite Films: A New Hybrid Spray Adhesive for Tough Superhydrophobicity

The authors report fabrication of tough nanostructured self-cleaning superhydrophobic polymer-organoclay films from anaerobic acrylic adhesives displaying strong adhesion to metal surfaces. Both industrial and bio-grade anaerobic adhesives such as bone cements could be used. Montmorillonite clay filled anaerobic adhesives were modified by blending with a water dispersed fluoro-methacrylic latex in solution to form abrasion resistant interpenetrating polymer network films upon spray casting. The adhesive films could cure by thermosetting in oxygen-rich environments. Very high contact angles with low hysteresis were also measured for acidic (pH 2) and basic (pH 11) aqueous buffer solutions indicating resistance to acidic and basic media.

Control of an oblique shock/boundary-layer interaction with aeroelastic mesoflaps

Aeroelastic mesoe aps for recirculating transpiration have been investigated in an effort to control shock/boundary-layer interactions (SBLIs) through passive cavity recirculation. The mesoe ap concept utilizes a matrix of small e apscovering an enclosed cavity that aredesigned to undergo local aeroelasticdee ection to achieve proper mass bleed or injection when subjected to gasdynamic pressure loading. Experiments were performed to investigate the applicability of the mesoe ap concept for oblique shock interaction by employing shadowgraph e ow visualizations, surface pressure measurements, and mean and e uctuating velocity measurements, along the spanwise midplane of the shock intersection. The experiments were conducted in a Mach 2.41 supersonic wind tunnel operating at a unit Reynolds number of 57 £106 mi1. With the thickest mesoe ap arrays in place, the leading shock formed at the location of the e rst e ap and the boundary-layer thickness at shock impingement was greaterduetoe owinjectionthroughtheupstreame aps.However,thethinnestmesoe aparraysyielded asomewhat reduced boundary-layer thickness downstream of the interaction as a result of the tangential bleeding by the last e aps. Stagnation pressure proe les for the thinnest arrays also showed improved recovery downstream of the SBLI as compared to thesolid-wall case. However, furtherstudy isneeded to investigate three-dimensional effectsand to determine whether this control strategy provides signie cant performance improvements for e ow conditions more consistent with actual inlets.

A pneumatic power harvesting ankle-foot orthosis to prevent foot-drop

BackgroundA self-contained, self-controlled, pneumatic power harvesting ankle-foot orthosis (PhAFO) to manage foot-drop was developed and tested. Foot-drop is due to a disruption of the motor control pathway and may occur in numerous pathologies such as stroke, spinal cord injury, multiple sclerosis, and cerebral palsy. The objectives for the prototype PhAFO are to provide toe clearance during swing, permit free ankle motion during stance, and harvest the needed power with an underfoot bellow pump pressurized during the stance phase of walking.MethodsThe PhAFO was constructed from a two-part (tibia and foot) carbon composite structure with an articulating ankle joint. Ankle motion control was accomplished through a cam-follower locking mechanism actuated via a pneumatic circuit connected to the bellow pump and embedded in the foam sole. Biomechanical performance of the prototype orthosis was assessed during multiple trials of treadmill walking of an able-bodied control subject (n = 1). Motion capture and pressure measurements were used to investigate the effect of the PhAFO on lower limb joint behavior and the capacity of the bellow pump to repeatedly generate the required pneumatic pressure for toe clearance.ResultsToe clearance during swing was successfully achieved during all trials; average clearance 44 ± 5 mm. Free ankle motion was observed during stance and plantarflexion was blocked during swing. In addition, the bellow component repeatedly generated an average of 169 kPa per step of pressure during ten minutes of walking.ConclusionThis study demonstrated that fluid power could be harvested with a pneumatic circuit built into an AFO, and used to operate an actuated cam-lock mechanism that controls ankle-foot motion at specific periods of the gait cycle.

Vortex Generators for a Normal Shock/Boundary Layer Interaction with a Downstream Diffuser

T HE interaction of a shock wavewith a turbulent boundary layer constitutes a fundamental problem of high-speed fluid mechanics. A detailed survey of past work on high-speed interactions has been carried out by Settles and Dolling [1] and Smits and Dussauge [2]. The shock interaction problem is particularly germane to the design of supersonic inlets. In such supersonic inlets, deceleration of the flow is achieved through a succession of oblique shock waves followed by a terminal normal shock. Boundary layers form on the inlet surfaces and interact with the shock system, giving rise to various shock/boundary-layer interactions (SBLIs). Each interaction of oblique/normal shock waves with the boundary layer causes stagnation pressure losses and downstream spatial distortions seen by the engine. An inlet must be carefully designed to minimize these losses and distortions during the compression process since they affect overall propulsion performance. In mixed-compression inlets, shock-induced separation can lead to engine unstart, which requires that the entire propulsion system undergo a restart sequence during flight. In external compression inlets, specifically axisymmetric configurations, a thick hubside boundary layer increases blockage and can decrease compressor performance. Thus, successfully controlling SBLIs has the potential to significantly improve supersonic inlet performance. As will be discussed in the following, various techniques of flow control for SBLIs have been proposed. However, it is often difficult to interpret the results because the flowfield may be too specific to an individual inlet configuration or too basic such that a relationship to inlet performance is not clear. To address this issue, a newwind-tunnel flowfield has been proposed [3] that captures much of the key shock boundary-layer interaction physics of supersonic external compression inlets. Thisflowfieldwill be used to study the novel flow control methods introduced herein. The conventionalflow control technique for SBLI conditions in an engine inlet employs a bleed of the boundary layer [4,5]. This bleed Presented as Paper 2010-4464 at the 40th AIAA Fluid Dynamics Conference and Exhibit, Chicago, IL, 28 June–1 July 2010; received 31 January 2011; revision received 26 July 2011; accepted for publication 11 August 2011. Copyright©2011 by theAmerican Institute ofAeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the