Air-fluid Interface Research Articles

Footprints on the beach: visualizing dilation-induced air entry

The dry fringe that appears beneath a footprint on a beach is a visual manifestation of dilation. Dilation causes the sand to attempt to increase in volume on shearing, resulting in negative pore pressure, and enhanced bearing capacity. Conventional theory cannot explain this increase in strength, suggesting instead that bearing capacity with the phreatic surface at the sand surface should be approximately half that of the dry case. To explore this apparent contradiction, ten load-controlled bearing capacity experiments were performed on transparent soil with phreatic surfaces ranging from 100 mm below to 30 mm above the footing. For phreatic surfaces deeper than the footing width, failure occurred within the dry material. In shallower phreatic surface conditions, bearing capacity increased as the phreatic surface approached the surface. Air entry was observed to initiate along the top surface of the saturated layer and extend to the depth of the bearing capacity mechanism. For the case of the phreatic and soil surfaces being coincident, the bearing capacity was observed to be more than double that of the dry case. In submerged cases bearing capacity was less than at-surface, as there was no air-fluid interface to enhance the development of negative pore pressure.

SOS-Inducing Drugs Trigger Nucleic Acid Release and Biofilm Formation in Gram-Negative Bacteria.

Our laboratory recently reported that induction of the SOS response, triggered by SOS-inducing drugs, was accompanied by a large release of DNA from enteric bacteria. The SOS response release had not previously been reported to include release of extracellular DNA from bacterial cells. We followed up on those observations in this current study and found that not just double-stranded DNA was being released, but also single-stranded DNA, RNA, and protein. SOS-inducing drugs also triggered formation of biofilm at the air-fluid interface on glass, and the biofilms contained DNA. We extended our study to test whether inhibitors of the SOS response would block DNA release and found that SOS inhibitors, including zinc salts, nitric oxide donors, and dequalinium, inhibited SOS-induced DNA release. The understanding that SOS-induced DNA release is associated with formation of biofilms increases our appreciation of the role of the SOS response in pathogenesis, as well as in emergence of new antibiotic resistance. Our findings with SOS inhibitors also suggest that regimens might be devised that could block the deleterious effects of the SOS response, at least temporarily, when this is desired.

Detachment forces during parallel-plate gap separation mediated by a simple yield-stress fluid.

In this work we have monitored the multiple stages of the normal traction force response of a yield-stress fluid confined between two circular parallel plates that are separated at constant velocity. At narrow initial gaps, the air-fluid interface suffers from the Saffman-Taylor instability, confirmed by visual inspection of fingering patterns imprinted on the fluid. At larger initial gaps, the fluid preserves the initially imposed circular symmetry of the confining plates, indicating the absence of instability. Due to the system characteristics and experimental environment, the multiple traction force contributions occurred in cascade, permitting us to isolate the adhesion responses associated with viscosity, capillarity, and yield stress. Employing a standard Herschel-Bulkley model, we assessed the scaling of the traction force in multiple regimes-specifically, evaluating the dependencies of the fingering to yield-stress transitions.

Coexistence of pulmonary hydatid cyst and tuberculosis in a child: a case report

BackgroundThe echinococcosis and tuberculosis of the lungs are both endemic in Algeria.The coexistence of echinococcosis and tuberculosis in the same lobe of the lung is exceptional.Case presentationA 10-year-old girl was admitted with a history of productive cough, fever, and chest pain.Chest examination revealed decreased air entry at left lower lobe. Chest X-ray demonstrated a round left lower lobe with an air fluid interface. Chest CT scan with intravenous contrast showed a lingual hydroaeric formation measuring 5×6 cm with a detached membrane image.A diagnosis of a complicated hydatid disease of the lung was made and the patient was subsequently started on oral albendazole before undergoing exploratory thoracotomy where partial cystectomy and the obliteration of the pericyst cavity. Histopathology was notable for echinococcosis membrane characteristics.After 2 months, the patient re-presented with complaints of cough, hemoptysis, and dyspnea.Radiographic images favored repermeabilization of bronchopleural fistulas.The management was the closing of the bronchial openings.Histological findings include image characteristic of the echinococcosis and tuberculosis.The patient continued on albendazole and an anti-tuberculosis regimen, with concurrent functional physiotherapy sessions.During the following 3-year follow-up, drastic clinical, radiographic, and functional improvement was noted.ConclusionConcurrent tuberculosis and echinococcosis infection should be taken into consideration when patients present with persistent bronchopleural fistula in endemic areas.A multidisciplinary approach of different medical teams facilitates diagnosis and rapid treatment.

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 the SurferBot: 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.

Young Man with Chest Pain

A 24-year-old man presented to the emergency department with right-sided chest pain after falling from a 2-wheeled scooter. His vital signs were unremarkable. He had tenderness on the right side of his chest, and sounds in the right lung were absent. A bedside ultrasound scan of the lung with the patient in a sitting position showed absent lung sliding and an air-fluid interface on the right side (Figure 1; M-mode pattern in Figure 2). The chest radiograph (Figure 3) confirmed the diagnosis.

Aerosol Generation in Ear Canal and Air‐Fluid Interface Suction

ObjectiveThe identification of aerosol-generating procedures (AGPs) is important during the current SARS-CoV-2 pandemic due to aerosol-mediated virus transmission. Aerosol measurement during clinical procedures using particle counting may be confounded by variable natural background aerosol levels or limited by partial volume sampling. The study objective was to quantify any significant aerosol generated from simulated suction clearance procedures.Study DesignProspective quantification of aerosol generation during clinical suction simulation.SettingClean chamber.MethodsWe created a clean environment for particle counting in a transparent neutralized polypropylene chamber. Air was passed through a HEPA 14 class filter to maintain a constant chamber inlet pressure. An optical particle counter was connected in line to the chamber exhaust vent to measure all of the vented particles. The chamber background count was 1 particle ≥0.3 µm per 15 minutes at a flow rate of 1 chamber air change per minute. We used this system to quantify very low aerosol counts generated from suction clearance of a silicone ear canal and at an open air-fluid interface.ResultsNo clinically significant aerosol generation was found by particle counting of the whole chamber air volume during simulated suction procedures.ConclusionSimulated ear suction clearance and air-fluid interface suction does not generate any significant aerosol. It appears likely that any aerosol potentially generated at the suction tube tip is entrained by incoming air flow. This is the first study to quantify aerosols generated by suction in a controlled environment; further research is required to determine its clinical implications.

How Much PEEP Does High Flow Deliver via Tracheostomy? A Literature Review and Benchtop Experiment

Background High flow tracheostomy (HFT) is a commonly used weaning and humidification strategy for tracheostomised patients, but little is known as to how much PEEP or mechanical benefit it offers. Patient anatomy and device characteristics differentiate it from high flow nasal cannula and the physiological effects observed. Objectives (1) To review the available literature on the effects of HFT on airway pressure and indices of gas exchange. (2) To quantify PEEP generated by a HFT circuit. Methods A randomised benchtop experiment was conducted, with a size 8 uncuffed Portex tracheostomy connected to an Optiflow™ with Airvo 2™ humidifier system. The tracheostomy tube was partially immersed in water to give rise to a column of water within the inner surface of the tube. An air fluid interface was generated with flows of 40 L/min, 50 L/min, and 60 L/min. The amount of potential PEEP (pPEEP) generated was determined by the distance the water column was pushed downward by the flow delivered. Findings. Overall 40 L/min, 50 L/min, and 60 L/min provided pPEEP of approximately 0.3 cmH2O, 0.5 cmH2O, and 0.9 cmH2O, respectively. There was a statistically significant change in pPEEP with change in flows from 40–60 L/min with an average change in pPEEP of 0.25–0.35 cmH2O per 10 L/min flow (p value <0.01). Interpretation. HFT can generate measurable and variable PEEP despite the open system used. The pPEEP generated with HFT is minimal despite statistically significant change with increasing flows. This pPEEP is unlikely to provide mechanical benefit in weaning patients off ventilatory support.

Boundary-layer profile of a singularly perturbed nonlocal semi-linear problem arising in chemotaxis

This paper is concerned with the stationary problem of an aero-taxis system with physical boundary conditions proposed by Tuval et al (2005 Proc. Natl Acad. Sci. 102 2277–82) to describe the boundary layer formation in the air–fluid interface in any dimensions. By considering a special case where fluid is free, the stationary problem is essentially reduced to a singularly perturbed nonlocal semi-linear elliptic problem. Denoting the diffusion rate of oxygen by ɛ > 0, we show that the stationary problem admits a unique classical solution of boundary-layer profile as ɛ → 0, where the boundary-layer thickness is of order ɛ. When the domain is a ball, we find a refined asymptotic boundary layer profile up to the first-order approximation of ɛ by which we find that the slope of the layer profile in the immediate vicinity of the boundary decreases with respect to (w.r.t.) the curvature while the boundary-layer thickness increases w.r.t. the curvature.

Assessing visible aerosol generation during vitrectomy in the era of Covid-19.

ObjectiveTo assess visible aerosol generation during simulated vitrectomy surgery.MethodsA model comprising a human cadaveric corneoscleral rim mounted on an artificial anterior chamber was used. Three-port 25 gauge vitrectomy simulated surgery was performed with any visible aerosol production recorded using high-speed 4K camera. The following were assessed: (1) vitrector at maximum cut rate in static and dynamic conditions inside the model, (2) vitrector at air–fluid interface in a physical model, (3) passive fluid–air exchange with a backflush hand piece, (4) valved cannulas under air, and (5) a defective valved cannula under air.ResultsNo visible aerosol or droplets were identified when the vitrector was used within the model. In the physical model, no visible aerosol or droplets were seen when the vitrector was engaged at the air–fluid interface. Droplets were produced from the opening of backflush hand piece during passive fluid–air exchange. No visible aerosol was produced from the intact valved cannulas under air pressure, but droplets were seen at the beginning of fluid–air exchange when the valved cannula was defective.ConclusionsWe found no evidence of visible aerosol generation during simulated vitrectomy surgery with competent valved cannulas. In the physical model, no visible aerosol was generated by the high-speed vitrector despite cutting at the air–fluid interface.

Optical Fiber Probe Viscometer Based on Hollow Capillary Tube

Viscosity measurements of a solution are crucial for many processes involving fluid flows. The current optical fiber viscometers are complex and, in some cases, provide indirect measurements of viscosity through other non-optical effects. We developed a miniaturized optical fiber probe capable of providing an optical interferometric measurement of the viscosity of small volumes of a liquid viscous medium (less than 50 pL). The probe consists of an air cavity with a small access hole for fluids, which resulted from a simple post-processing of a hollow capillary tube. The structure behaves as a two-wave interferometer, where the intensity of the signal is sensible to the position of the air-fluid interface inside the cavity. The fluid displacement over time is obtained by monitoring the signal intensity variations, at 1550 nm, during the process of removing the sensing head from a fluid solution. Multiple sucrose solutions with viscosities ranging from 2.01 to 16.1 mPa·s were used for calibration. The viscosity of the solution is measured through the fluid evacuation velocity in the first 300 ms of resolved oscillations during the evacuation process. Reproducibility measurements, the influence of temperature, and the access hole dimensions are also addressed. The application to biological fluids is important to be considered in future studies.

A simple capillary viscometer based on the ideal gas law.

We report a simple, inexpensive and user-friendly capillary viscometer based on the measurement of pressure drop in capillary tubing using the principle of ideal gas law. Viscosity is an important physical property of a fluid that provides molecular information of the fluid's behavior under flow conditions. Measuring viscosity, however, generally requires relatively large fluid volume samples and is expensive with commercial viscometers. Microfluidic viscometers at different levels of complexity can measure fluids at different flow rates with a small sample volume but the cost of commercially available microfluidic viscometers is still high. The reported capillary viscometer is cost-effective, uses small amounts of sample fluid and can measure viscosity under various shear rates. According to the Hagen–Poiseuille equation, the pressure drop of laminar flows in a capillary at a given flow rate is proportional to the viscosity of the fluid. When an enclosed air volume is connected to the upstream of the capillary, the pressure drop can be calculated with the change of the connected air volume, which is reflected by the displacement change of the air–liquid interface in the connecting capillary to the enclosed air volume. Based on these principles, the viscometer was assembled with readily accessible materials, and required no internal sensors or extensive programming. Measurements were successfully performed for five liquids including water, acetone, 2% fat milk, glycerin 30% and glycerin 40%. Except for acetone, the difference between measured and known viscosity was within 4% and highly consistent, well within the 13% uncertainty errors of readily accessible laboratory materials. Overall, the simple viscometer was easily assembled with low cost materials, was portable and accurate, and provided an alternative to expensive commercial viscometers. Finally, the simple capillary viscometer was a good outreach project for K-12 students to understand fluid behavior.

Numerical and experimental study of laser aided additive manufacturing for melt-pool profile and grain orientation analysis

Laser aided additive manufacturing (LAAM), a blown powder additive manufacturing process, can be widely adopted for surface modification, repair and 3D printing. A robust numerical model was developed to simulate convective fluid flow and balancing of surface tension forces at the air-fluid interface to predict melt-pool free surface curvature and solidified clad dimensions. The free surface physical interface was calculated using the Arbitrary Lagrangian Eulerian (ALE) moving mesh approach. Powder deposition efficiency was considered by activating mesh normal velocity at melted regions based on localized powder mass flux intensity from the discrete coaxial powder nozzles. The heat flux equation used for representing the laser heat source considered attenuation effect from the interaction between the powder jets and laser as well as heat sink effects of un-melted powder particles entering the melt-pool. The predicted thermal gradient directions agree well with grain growth orientations obtained from electron backscatter diffraction (ESBD) analysis in three different cross-sectional orientations. Experimental validation of clad width, height and melt-pool depth shows a maximum error of 10% for a wide range of processing parameters which consider the effects of varying laser power, laser scanning speed and powder feeding rate.

Self assembled transparent conducting network of multi-walled carbon nanotubes formed by evaporation

We develop a novel bubble-array technique to create a network pattern of multi-walled carbon nanotubes (MWCNT) on glass, through evaporation of a thin layer of the suspension of MWCNT. Spherical bubbles of approximately equal diameter (∼25μm) are created at the air-fluid interface. On complete evaporation of the solvent (1,2 dicloroethane), MWCNT self assemble to form a connected and roughly polygonal network. The glass surface thus becomes electrically conducting, while the gaps in the network allow considerable transmission of light in the visible range. The concentration of the MWCNT suspension has been varied to optimize the balance between sheet conductance and transmittance.

The Use of Membrane Fusion to Understand the Role of Lung Surfactant Protein B in Lipid Trafficking

Respiratory Distress Syndrome (RDS) is a disorder affecting premature babies who lack or have a complete absence of the essential lung surfactant components produced during proper fetal lung development. Composed of a mixture of lipids and proteins, Lung Surfactant lines the alveoli, providing a barrier against inhaled pathogens, lowering the surface tension at the air-fluid interface, and facilitating oxygen absorption. Specifically, Surfactant protein B (SP-B) has been identified as a major protein in surface tension reduction and lipid trafficking to the air-fluid interface of the alveoli. Previous studies have shown selective interactions with DPPC in SP-B penetration, possibly suggesting the flipping of DPPC to the air-fluid interface. We will study the phase separations and trafficking abilities of SP-B to better understand how it lowers surface tension. Using Fluorescence Resonance Energy Transfer with the fluorescent probes Rhodamine-PE and NBD-PE, we will monitor the predicted effects of SP-B in mediating membrane fusion and inducing phase separations. This method allows us to demonstrate the role of SP-B in lipid trafficking in a fast and cost effective manner with minimal sample consumption.

Growing a Chemical Garden at the Air-Fluid Interface.

Here we grow chemical gardens using a novel, quasi two-dimensional, experimental configuration. Buoyant calcium chloride solution is pumped onto the surface of sodium silicate solution. The solutions react to form a precipitation structure on the surface. Initially, an open channel forms that grows in a spiral. This transitions to radially spreading and branching fingers, which typically oscillate in transparency as they grow. The depth of the radial spreading, and the fractal dimension of the finger growth, are surprisingly robust, being insensitive to the pumping rate. The curvature of the channel membrane and the depth of the radially spreading solution can be explained in terms of the solution densities and the interfacial tension across the semipermeable membrane. These unusually beautiful structures provide new insights into the dynamics of precipitation structures and may lead to new technologies where structures are grown instead of assembled.

Long-chain Acylcarnitines Reduce Lung Function by Inhibiting Pulmonary Surfactant

The role of mitochondrial energy metabolism in maintaining lung function is not understood. We previously observed reduced lung function in mice lacking the fatty acid oxidation enzyme long-chain acyl-CoA dehydrogenase (LCAD). Here, we demonstrate that long-chain acylcarnitines, a class of lipids secreted by mitochondria when metabolism is inhibited, accumulate at the air-fluid interface in LCAD(-/-) lungs. Acylcarnitine accumulation is exacerbated by stress such as influenza infection or by dietary supplementation with l-carnitine. Long-chain acylcarnitines co-localize with pulmonary surfactant, a unique film of phospholipids and proteins that reduces surface tension and prevents alveolar collapse during breathing. In vitro, the long-chain species palmitoylcarnitine directly inhibits the surface adsorption of pulmonary surfactant as well as its ability to reduce surface tension. Treatment of LCAD(-/-) mice with mildronate, a drug that inhibits carnitine synthesis, eliminates acylcarnitines and improves lung function. Finally, acylcarnitines are detectable in normal human lavage fluid. Thus, long-chain acylcarnitines may represent a risk factor for lung injury in humans with dysfunctional fatty acid oxidation.

Dry Arthroscopy of the Elbow

Dry arthroscopy is attractive because it affords an unsurpassed clarity of view and minimizes swelling. The elbow is a challenging joint to assess arthroscopically; however, dry arthroscopy has some particular benefits in the elbow. The primary benefit is the quality of the tissue definition, but dry arthroscopy also increases the working time for surgery by reducing swelling and results in less postoperative discomfort for the patient. With dry arthroscopy, all joint surfaces are covered in synovial fluid, which reflects light, to provide a clearer image of the joint surfaces and depth of field. The air-fluid interface provides an uninterrupted appreciation of the synovial recesses and tissue perfusion. This article describes the technique and indications for dry elbow arthroscopy, which will allow other surgeons to reap the benefits of dry arthroscopy without the need for special equipment or changes in their basic technique.

Interferometric technique for nanoscale dynamics of fluid drops on arbitrary substrates

We demonstrate a simple interferometric probe to detect nanoscale dynamics of sessile fluid drops on arbitrary rough or flexible substrates. The technique relies on producing high-contrast Newton-ring like dynamical fringes by interference between a weak Fresnel reflection from the air-fluid interface of the drop and an air-glass interface of a convex lens placed above the drop in quasi-normal geometry. By analyzing the dynamical fringes, we observed 100–700 nm/s fluctuations in water drops evaporating on metal, leaves, insect wing, and sand paper due to their surface roughness. Similar fluctuations were also observed during spreading of non-volatile glycerin drops on various rough surfaces. Another application of the technique is demonstrated in precision measurement of change in evaporation rate of a water drop due to cooling of a metal substrate. This technique can be further miniaturized with a microscope objective with potential for wide applications.

A technique to functionalize and self-assemble macroscopic nanoparticle-ligand monolayer films onto template-free substrates.

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles. The simple, robust and scalable technique efficiently functionalizes metallic nanoparticles with thiol-ligands in a miscible water/organic solvent mixture allowing for rapid grafting of thiol groups onto the gold nanoparticle surface. The hydrophobic ligands on the nanoparticles then quickly phase separate the nanoparticles from the aqueous based suspension and confine them to the air-fluid interface. This drives the ligand-capped nanoparticles to form monolayer domains at the air-fluid interface. The use of water-miscible organic solvents is important as it enables the transport of the nanoparticles from the interface onto template-free substrates. The flow is mediated by a surface tension gradient and creates macroscopic, high-density, monolayer nanoparticle-ligand films. This self-assembly technique may be generalized to include the use of particles of different compositions, size, and shape and may lead to an efficient assembly method to produce low-cost, macroscopic, high-density, monolayer nanoparticle films for wide-spread applications.