Rapid and Robust Surface Treatment for Simultaneous Solid and Liquid Repellency

A wide range of liquid and solid contaminants can adhere to everyday functional surfaces and dramatically alter their performance. Numerous surface modification strategies have been developed that can reduce the fouling of some solids or repel certain liquids but are generally limited to specific contaminants or class of foulants. This is due to the typically distinct mechanisms that are employed to repel liquids vs solids. Here, we demonstrate a rapid and facile surface modification technique that yields a thin film of linear chain siloxane molecules covalently tethered to a surface. We investigate and characterize the liquid-like morphology of these surfaces in detail as the key contributing factor to their anti-fouling performance. This surface treatment is extremely durable and readily repels a broad range of liquids with varying surface tensions and polarities, including water, oils, organic solvents, and even fluorinated solvents. Additionally, the flexible, liquid-like nature of these surfaces enables interfacial slippage, which dramatically reduces adhesion to various types of solids, including ice, wax, calcined gypsum, and cyanoacrylate adhesives, and also minimizes the nucleation of inorganic scale. The developed surfaces are durable and simple to fabricate, and they minimize fouling by both liquids and solids simultaneously.

Drag production mechanisms of filamentous biofilms

Biofilms were grown on smooth acrylic surfaces for nominal incubation times of three, five, and ten weeks in a flow loop at the University of Michigan. The biofilm covered surfaces were exposed to the turbulent flow in a high-aspect ratio, fully developed channel flow facility at height-based Reynolds numbers from Re_H ≈ 5,000 to 30,000. Measurements of the pressure drop along each fouled upper surface revealed that the friction drag increased from approximately 10% to 400%. The wide range in drag penalty was linked to variations in flow speed, the average thickness of the biofilms, and the level of film coverage over each surface through scaling parameters and empirical correlations. Rigid replicas of select biofilms were produced from time-averaged laser scans collected while the biofilm was subjected to flow. These rigid biofilm replicas experienced roughly half the drag increase of their compliant counterparts with the increase in friction spanning roughly 50% to 200%.

Bioinspired surfaces for turbulent drag reduction

In this review, we discuss how superhydrophobic surfaces (SHSs) can provide friction drag reduction in turbulent flow. Whereas biomimetic SHSs are known to reduce drag in laminar flow, turbulence adds many new challenges. We first provide an overview on designing SHSs, and how these surfaces can cause slip in the laminar regime. We then discuss recent studies evaluating drag on SHSs in turbulent flow, both computationally and experimentally. The effects of streamwise and spanwise slip for canonical, structured surfaces are well characterized by direct numerical simulations, and several experimental studies have validated these results. However, the complex and hierarchical textures of scalable SHSs that can be applied over large areas generate additional complications. Many studies on such surfaces have measured no drag reduction, or even a drag increase in turbulent flow. We discuss how surface wettability, roughness effects and some newly found scaling laws can help explain these varied results. Overall, we discuss how, to effectively reduce drag in turbulent flow, an SHS should have: preferentially streamwise-aligned features to enhance favourable slip, a capillary resistance of the order of megapascals, and a roughness no larger than 0.5, when non-dimensionalized by the viscous length scale.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

The acoustic emissions of cavitation bubbles in stretched vortices

Pairs of unequal strength, counter-rotating vortices were produced in order to examine the inception, dynamics, and acoustic emission of cavitation bubbles in rapidly stretching vortices. The acoustic signatures of these cavitation bubbles were characterized during their inception, growth, and collapse. Growing and collapsing bubbles often produced a sharp, broadband, pop sound. The spectrum of these bubbles, and the peak resonant frequency can generally be related to quiescent flow bubble dynamics and corresponding resonant frequencies. However, some elongated cavitation bubbles produced a short tonal burst, or chirp, with frequencies on the order of a few kilohertz. Theses frequencies are too low to be related to resonant frequencies of a bubble in a quiescent flow. Instead, the frequency content of the acoustic signal during bubble inception and growth is related to the volumetric oscillations of the bubble while it interacted with vortical flow that surrounds the bubble (i.e., the resonant frequency of the vortex-bubble system). A relationship was determined between the observed peak frequency of the oscillations, the highly stretched vortex properties, and the water nuclei content. It was found that different cavitation spectra could relate to different flow and fluid properties and therefore would not scale in the same manner.

Bragg cell laser intensity modulation: effect on laser Doppler velocimetry measurements

In most laser Doppler velocimetry (LDV) systems, the frequency of one of the two laser beams that intersect to create the probe volume is shifted with an acousto-optic element. It is shown here that Bragg shifting can impose a problematic fluctuation in intensity on the frequency-shifted beam, producing spurious velocity measurements. This fluctuation occurs at twice the Bragg cell frequency, and its relative amplitude to the time average intensity is a function of the ratio of the laser beam diameter to the Bragg cell acoustic wavelength. A physical model and a configuration procedure to minimize adverse effects of the intensity modulations are presented.

High-Reynolds-number turbulent-boundary-layer wall pressure fluctuations with skin-friction reduction by air injection

The hydrodynamic pressure fluctuations that occur on the solid surface beneath a turbulent boundary layer are a common source of flow noise. This paper reports multipoint surface pressure fluctuation measurements in water beneath a high-Reynolds-number turbulent boundary layer with wall injection of air to reduce skin-friction drag. The experiments were conducted in the U.S. Navy's Large Cavitation Channel on a 12.9-m-long, 3.05-m-wide hydrodynamically smooth flat plate at freestream speeds up to 20 ms and downstream-distance-based Reynolds numbers exceeding 200 x 10(6). Air was injected from one of two spanwise slots through flush-mounted porous stainless steel frits (approximately 40 microm mean pore diameter) at volume flow rates from 17.8 to 142.5 l/s per meter span. The two injectors were located 1.32 and 9.78 m from the model's leading edge and spanned the center 87% of the test model. Surface pressure measurements were made with 16 flush-mounted transducers in an "L-shaped" array located 10.7 m from the plate's leading edge. When compared to no-injection conditions, the observed wall-pressure variance was reduced by as much as 87% with air injection. In addition, air injection altered the inferred convection speed of pressure fluctuation sources and the streamwise coherence of pressure fluctuations.

Universal scaling for polymer chain scission in turbulence

We report that previous polymer chain scission experiments in strong flows, long analyzed according to accepted laminar flow scission theories, were in fact affected by turbulence. We reconcile existing anomalies between theory and experiment with the hypothesis that the local stress at the Kolmogorov scale generates the molecular tension leading to polymer covalent bond breakage. The hypothesis yields a universal scaling for polymer scission in turbulent flows. This surprising reassessment of over 40 years of experimental data simplifies the theoretical picture of polymer dynamics leading to scission and allows control of scission in commercial polymers and genomic DNA.

Hemolytic potential of hydrodynamic cavitation

The purpose of this study was to determine the hemolytic potentials of discrete bubble cavitation and attached cavitation. To generate controlled cavitation events, a venturigeometry hydrodynamic device, called a Cavitation Susceptibility Meter (CSM), was constructed. A comparison between the hemolytic potential of discrete bubble cavitation and attached cavitation was investigated with a single-pass flow apparatus and a recirculating flow apparatus, both utilizing the CSM. An analytical model, based on spherical bubble dynamics, was developed for predicting the hemolysis caused by discrete bubble cavitation. Experimentally, discrete bubble cavitation did not correlate with a measurable increase in plasma-free hemoglobin (PFHb), as predicted by the analytical model. However, attached cavitation did result in significant PFHb generation. The rate of PFHb generation scaled inversely with the Cavitation number at a constant flow rate, suggesting that the size of the attached cavity was the dominant hemolytic factor.

Determination of the in vivo cavitation nuclei characteristics of blood

Cavitation has been documented in the in vitro testing of blood-handling devices. To predict whether cavitation will occur clinically, the nuclei content of blood and the threshold pressure for activation of the in situ nuclei must be characterized. A single-pass flow apparatus is described for determining the nuclei characteristics of blood. The flow apparatus consists of a syringe pump and a venturi-geometry hydrodynamic device, called a cavitation susceptibility meter (CSM). Blood is accelerated through the throat of the CSM, thus exposing the nuclei in the blood to a well-defined pressure profile. The apparatus was used in an ex vivo sheep model for the determination of the in vivo nuclei characteristics of blood. The active nuclei concentration of in vivo blood was measured to be at most 2.7 nuclei per liter of plasma at a minimum throat pressure of -1610 mm Hg gauge (i.e., tension of 900 mm Hg). At this pressure, bubble stability theory predicts the active nuclei to have a radius on the order of 0.3 microm. Based on these results, in vitro studies to determine the cavitation potential of blood-handling devices must utilize test fluids that contain a minimum nuclei size distribution and concentration. It cannot be assumed that in vivo blood is nuclei rich, such that it will cavitate at or near vapor pressure.

Extreme negative pressure does not cause erythrocyte damage in flowing blood

In extracorporeal circulation, negative pressure is thought to be a source of hemolysis. This study was designed to investigate the effects of extreme negative pressure on flowing blood. The study model was pipe flow. The hemolysis generated by negative pressure driven flow was compared with that generated by positive pressure driven flow of equal magnitude to control for the hemolytic effect of shear stress. A series of pressures (720, 600, 500, -500, -600, and -720 mm Hg; n = 8) was tested for pipe diameters of 0.04 and 0.16 cm, with a length-to-diameter ratio of 500. The pressure difference across the pipe (deltaP) was equal to the magnitude of the applied pressure. The hemolysis was quantified by the modified index of hemolysis (MIH). For both pipe diameters, MIH was found to not depend on the deltaP or the blood collection day (multiple regression analysis, p = 0.50 and p = 0.63, respectively). There was no statistically significant difference between the MIH for equal deltaP generated by positive or negative pressure (p = 0.50) for both pipe diameters tested. MIH did depend upon the pipe diameter, with 0.04 cm having higher MIH at all pressures (p = 0.0003). Thus, negative pressure is not a significant hemolytic factor in flowing blood.