In-field use of I-VED electrical impedance sensor for assessing post-dive decompression stress in humans.

Purpose: Ultrasound imaging is commonly used in decompression research to assess venous gas emboli (VGE) post-dive, with higher loads associated with increased decompression sickness risk. This work examines, for the first time in humans, the performance of a novel electrical impedance spectroscopy technology (I-VED), on possible detection of post-dive bubbles presence and arterial endothelial dysfunction that may be used as markers of decompression stress. Methods: I-VED signals were recorded in scuba divers who performed standardized pool dives before and at set time points after their dives at 35-minute intervals for about two hours. Two distinct frequency components of the obtained signals, Low-Pass Frequency-LPF: 0-0.5 Hz and Band-Pass Frequency-BPF: 0.5-10 Hz, are extracted and respectively compared to VGE presence and known flow-mediated dilation trends for the same dive profile for endothelial dysfunction. Results: Subjects with VGE counts above the median for all subjects were found to have an elevated average LPF compared to subjects with lower VGE counts, although this was not statistically significant (p=0.06), as well as significantly decreased BPF standard deviation post-dive compared to pre-dive (p=0.008). Conclusions: I-VED was used for the first time in humans and operated to provide qualitative in-vivo electrical impedance measurements that may contribute to the assessment of decompression stress. Compared to ultrasound imaging, the proposed method is less expensive, not operator-dependent and compatible with continuous monitoring and application of multiple probes. This study provided preliminary insights; further calibration and validation are necessary to determine I-VED sensitivity and specificity.

The role of air relative humidity on the wettability of Pseudomonas fluorescens AR11 biofilms.

Biofilms are complex porous materials formed by microorganisms, polysaccharides, proteins, eDNA, inorganic matter, and water. They are ubiquitous in various environmental niches and are known to grow at solid-liquid, solid-air and air-liquid interfaces, often causing problems in several industrial and sanitary fields. Their removal is a challenge in many applications and numerous studies have been conducted to identify promising chemical species as cleaning agents. While these substances target specific components of biofilm structure, the role of water content in biofilm, and how it can influence wettability and detergent absorption have been quite neglected in the literature. Estimating water content in biofilm is a challenging task due to its heterogeneity in morphology and chemical composition. In this study, we controlled water content in Pseudomonas fluorescens AR 11 biofilms grown on submerged glass slides by regulating environmental relative humidity after drying. Interfacial properties of biofilm were investigated by measuring wetting of water and soybean oil. The morphology of biofilm structure was evaluated using Confocal Laser Scanning Microscopy and Scanning Electron Microscopy. The results showed that biofilm water content has a significant and measurable effect on its wettability, leading to the hypothesis that a preliminary control of water content can play a crucial role in biofilm removal process.

Forced Wetting Properties of Bacteria-Laden Droplets Experiencing Initial Evaporation.

Microbial adhesion and spreading on surfaces are crucial aspects in environmental and industrial settings being also the early stage of complex surface-attached microbial communities known as biofilms. In this work, Pseudomonas fluorescens-laden droplets on hydrophilic substrates (glass coupons) are allowed to partially evaporate before running wetting measurements, to study the effect of evaporation on their interfacial behavior during spillover or splashing. Forced wetting is investigated by imposing controlled centrifugal forces, using a novel rotatory device (Kerberos). At a defined evaporation time, results for the critical tangential force required for the inception of sliding are presented. Microbe-laden droplets exhibit different wetting/spreading properties as a function of the imposed evaporation times. It is found that evaporation is slowed down in bacterial droplets with respect to nutrient medium ones. After sufficient drying times, bacteria accumulate at droplet edges, affecting the droplet shape and thus depinning during forced wetting tests. Droplet rear part does not pin during the rotation test, while only the front part advances and spreads along the force direction. Quantitative results obtained from the well-known Furmidge's equation reveal that force for sliding inception increases as evaporation time increases. This study can be of support for control of biofilm contamination and removal and possible design of antimicrobial/antibiofouling surfaces.

A New Phantom that Simulates Electrically a Human Blood Vessel Surrounded by Tissues: Development and Validation Against In-Vivo Measurements.

This study aims to develop a phantom that simulates the electrical properties of a human blood vessel surrounded by tissues, inside which bubbles can be infused to mimic Decompression Sickness (DCS) conditions. This phantom may be used to calibrate novel electrical methods for bubbles detection in humans and study bubble dynamics during DCS. It may contribute to the limitation of in-vivo trials and time/effort saving, while its use can be extended to other biomedical applications. To facilitate the design of the phantom, we perform first in-vitro measurements in a flow-loop and in-vivo measurements in a swine, in order to detect infused bubbles of a few tenths µm-representing Decompression Sickness conditions-in the test liquid flow and blood flow, respectively, by means of "I-VED" EU patented electrical impedance spectroscopy technique. Results show that the proposed phantom, consisting of a spongy specimen soaked in agar gel in the presence of electrolyte with a hole along it, simulates adequately the electrical properties of a human blood vessel surrounded by tissues. I-VED demonstrates pretty high sensitivity to sense micro-bubbles over the partially conductive vessel walls of the phantom or the isolated animal vein, as well as in the flow-loop: bubbles presence increases electrical impedance and causes intense signal fluctuations around its mean value.

Wetting and Imbibition Characteristics of Pseudomonas fluorescens Biofilms Grown on Stainless Steel.

This study aims to provide insights into biofilm resistance associated with their structural properties acquired during formation and development. On this account, the wetting and imbibition behavior of dehydrated Pseudomonas fluorescens biofilms grown on stainless steel electropolished substrates is thoroughly examined at different biofilm ages. A polar liquid (water) and a non-polar liquid (diiodomethane) are employed as wetting agents in the form of sessile droplets. A mathematical model is applied to appraise the wetting and imbibition performance of biofilms incorporating the evaporation of sessile droplets. The present results show that the examined biofilms are hydrophilic. The progressive growth of biofilms leads to a gradual increase of substrate surface coverage─up to full coverage─accompanied by a gradual decrease of biofilm surface roughness. It is noteworthy that just after 24 h of biofilm growth, the surface roughness increases about 6.7 times the roughness of the clean stainless steel surface. It is further found that the imbibition of liquid in the biofilm matrix is restricted only to the biofilm region under the sessile droplet. The lack of further capillary imbibition into the biofilm structure, beyond the droplet deposition region, implies that the biofilm matrix is not in the form of an extended network of interconnected micro/nanopores. All in all, the present results indicate a resilient biofilm structure to biocide penetration despite its hydrophilic nature.

Wetting properties of dehydrated biofilms under different growth conditions.

Biofilms are resilient to environmental conditions and often resistant even to strong disinfectants. It is crucial to investigate their interfacial properties, which can be effectively characterized by wetting analysis. Wetting phenomena on biofilm surfaces have been poorly investigated in literature, in particular a systematic study of wetting on real biofilm-coated substrates including the application of external body forces (forced wetting, i.e.: centrifugal and gravitational forces) is missing. The aim of this work is to study the role of nutrient and shear flow conditions on wetting properties of Pseudomonas fluorescens dehydrated biofilms, grown on glass substrates. An innovative device (Kerberos®), capable to study spreading/sliding behavior under the application of external body forces, is used here for a systematic analysis of wetting/de-wetting liquid droplets on horizontal substrates under the action of tangential forces. Results prove that, under different growth conditions, (i.e., nutrients and imposed flow), biofilms exhibit different wetting properties. At lower nutrient/shear flow conditions, biofilms show spreading/sliding behavior close to that of pure glass. At higher nutrient and shear flow conditions, droplets on biofilms show spreading followed by imbibition soon after deposition, which leads to peculiar droplet depinning during the rotation test. Wetting properties are derived as a function of the rotation speed from both top and side views videoframes through a dedicated image analysis technique. A detailed analysis of biofilm formation and morphology/topography is also provided here.

Wetting of Dehydrated Hydrophilic Pseudomonas fluorescens Biofilms under the Action of External Body Forces.

Wetting of dehydrated Pseudomonas fluorescens biofilms grown on glass substrates by an external liquid is employed as a means to investigate the complex morphology of these biofilms along with their capability to interact with external fluids. The porous structure left behind after dehydration induces interesting droplet spreading on the external surface and imbibition into pores upon wetting. Static contact angles and volume loss by imbibition measured right upon droplet deposition indicate that biofilms of higher incubation times show a higher porosity and effective hydrophilicity. Furthermore, during subsequent rotation tests, using Kerberos device, these properties dictate a peculiar forced wetting/spreading behavior. As rotation speed increases a long liquid tail forms progressively at the rear part of the droplet, which stays pinned at all times, while only the front part of the droplet depins and spreads. Interestingly, the experimentally determined retention force for the onset of droplet sliding on biofilm external surface is lower than that on pure glass. An effort is made to describe such complex forced wetting phenomena by presenting apparent contact angles, droplet length, droplet shape contours, and edges position as obtained from detailed image analysis.

On a generalized framework for turbulent collision frequency models in flotation: The road from past inconsistencies to a concise algebraic expression for fine particles.

Process modeling is a valuable tool for process design and optimization. Nonetheless, the extent of its use depends on the physical complexity of each particular application. Flotation is one of the most complex processes to model. In particular, in mechanical flotation cells, turbulent flow prevails and promotes bubble particle collisions. Many size and time scales of both hydrodynamic and physicochemical nature have to be resolved to model the process. The only way to achieve this is a combination of co-current (pulp and froth) and sequential multiscale modeling. A generalized framework for modeling the pulp phase from the device scale to thin film scale separating bubbles and particles is presented here. The core of the model is the term describing the collision frequency between bubbles and particles. Existing approaches to derive this term are reviewed and critically commented demonstrating several inconsistencies. A unified and consistent approach for deriving this collision frequency term is described overcoming all the inconsistencies of previous approaches. Specific results are presented for the case of flotation of fine particles, being practically the only case for which a simplified collision frequency expression of algebraic complexity can be derived.

The role of flow in bacterial biofilm morphology and wetting properties.

Biofilms are bacterial communities embedded in an extracellular matrix, able to adhere to surfaces. Different experimental set-ups are widely used for in vitro biofilm cultivation; however, a well-defined comparison among different culture conditions, especially suited to interfacial characterization, is still lacking in the literature. The main objective of this work is to study the role of flow on biofilm formation, morphology and interfacial properties. Three different in vitro setups, corresponding to stagnant, shaking, and laminar flow conditions (custom-made flow cell), are used in this work to grow single strain biofilms of Pseudomonas fluorescens AR 11 on glass coupons. Results show that flow conditions significantly influenced biofilm formation kinetics, affecting mass transfer and cell attachment/detachment processes. Distinct morphological patterns are found under different flow regimes. Static contact angle data do not depend significantly on biofilm growth conditions in the parametric range investigated in this work.

A critical review on turbulent collision frequency/efficiency models in flotation: Unravelling the path from general coagulation to flotation.

Flotation is a very important separation process in the mining industry. In addition, it finds important application as a water treatment process. The better design of flotation devices and operation strategies requires development of reliable and consistent mathematical models. Flotation is much more complex than typical unit processes, involving physicochemical interactions in small size scale and hydrodynamic interactions between bubbles, particles and liquid in a variety of size scales. The only feasible integrated approach to modeling the flotation process is by incorporating multiple scales. In the heart of such an approach there is a submodel for the frequency of collisions between bubbles and particles. Literature on this subject in the absence of intense turbulence is very extensive and there are many well-accepted models. Yet, the situation is quite different for the case of turbulence being an important collision mechanism, which is exactly what happens in industrial dispersed air flotation devices. The corresponding literature models are limited and focused on different aspects of the process. The present work is a review study attempting to unify these models in a general framework. The review is not restricted to presenting existing models but all relevant physical principles and fundamental theories are examined and assessed properly. In this respect, there are two levels of classification of literature material in this work. The first is a classification as general collision models and flotation models. It is important to critically examine the general collision models of engineering and physics literature since they constitute the basis on which flotation models rely. The second classification is with respect to the level of detail and sophistication of models (from distributed multibubble/multiparticle model to lumped models based on statistical theory of turbulence). The outcome of the work comes to the conclusion that existing models focus only to specific parts of the phenomena leading to bubble-particle collisions and for this, further improvement is needed to integrate them to obtain a better and more general picture. The material and comments presented in this work are meant as a decisive milestone towards this integration procedure.

Decompression induced bubble dynamics on ex vivo fat and muscle tissue surfaces with a new experimental set up.

Vascular gas bubbles are routinely observed after scuba dives using ultrasound imaging, however the precise formation mechanism and site of these bubbles are still debated and growth from decompression in vivo has not been extensively studied, due in part to imaging difficulties. An experimental set-up was developed for optical recording of bubble growth and density on tissue surface area during hyperbaric decompression. Muscle and fat tissues (rabbits, ex vivo) were covered with nitrogen saturated distilled water and decompression experiments performed, from 3 to 0bar, at a rate of 1bar/min. Pictures were automatically acquired every 5s from the start of the decompression for 1h with a resolution of 1.75µm. A custom MatLab analysis code implementing a circular Hough transform was written and shown to be able to track bubble growth sequences including bubble center, radius, contact line and contact angles over time. Bubble density, nucleation threshold and detachment size, as well as coalescence behavior, were shown significantly different for muscle and fat tissues surfaces, whereas growth rates after a critical size were governed by diffusion as expected. Heterogeneous nucleation was observed from preferential sites on the tissue substrate, where the bubbles grow, detach and new bubbles form in turn. No new nucleation sites were observed after the first 10min post decompression start so bubble density did not vary after this point in the experiment. In addition, a competition for dissolved gas between adjacent multiple bubbles was demonstrated in increased delay times as well as slower growth rates for non-isolated bubbles.

Circulatory bubble dynamics: from physical to biological aspects.

Bubbles can form in the body during or after decompression from pressure exposures such as those undergone by scuba divers, astronauts, caisson and tunnel workers. Bubble growth and detachment physics then becomes significant in predicting and controlling the probability of these bubbles causing mechanical problems by blocking vessels, displacing tissues, or inducing an inflammatory cascade if they persist for too long in the body before being dissolved. By contrast to decompression induced bubbles whose site of initial formation and exact composition are debated, there are other instances of bubbles in the bloodstream which are well-defined. Gas emboli unwillingly introduced during surgical procedures and ultrasound microbubbles injected for use as contrast or drug delivery agents are therefore also discussed. After presenting the different ways that bubbles can end up in the human bloodstream, the general mathematical formalism related to the physics of bubble growth and detachment from decompression is reviewed. Bubble behavior in the bloodstream is then discussed, including bubble dissolution in blood, bubble rheology and biological interactions for the different cases of bubble and blood composition considered.

A critical review of physiological bubble formation in hyperbaric decompression.

Bubbles are known to form in the body after scuba dives, even those done well within the decompression model limits. These can sometimes trigger decompression sickness and the dive protocols should therefore aim to limit bubble formation and growth from hyperbaric decompression. Understanding these processes physiologically has been a challenge for decades and there are a number of questions still unanswered. The physics and historical background of this field of study is presented and the latest studies and current developments reviewed. Heterogeneous nucleation is shown to remain the prime candidate for bubble formation in this context. The two main theories to account for micronuclei stability are then to consider hydrophobicity of surfaces or tissue elasticity, both of which could also explain some physiological observations. Finally the modeling relevance of the bubble formation process is discussed, together with that of bubble growth as well as multiple bubble behavior.

Effect of potato orientation on evaporation front propagation and crust thickness evolution during deep-fat frying.

UNLABELLED: The various theoretical approaches that have been proposed for modeling heat and mass transport during deep-fat frying of potatoes do not take into account the effect of potato orientation with respect to gravity. This can be partly attributed to lack of systematic experimental information at different orientations. The objective of the present work is to experimentally study the effect of potato orientation on the evaporation front propagation and crust thickness evolution and how this effect varies with frying conditions. To achieve this goal, a special device has been constructed which, among others, permits: (a) exposure of only one surface of a potato stick to hot oil, (b) rotation of this potato surface at 0°, 90°, and 180° with respect to gravity, and (c) accurate placement of miniature thermocouples under-but very close to-the exposed potato surface. Crust thickness is determined by 2 independent methods: (a) microphotography and (b) a micrometer. It is found that the evaporation front propagation and crust thickness evolution are different among the examined surface orientations. The fastest heat penetration and thickest crust are measured at vertical (90°) surfaces. The implications of this finding regarding potato texture and energy consumption are discussed. PRACTICAL APPLICATION: Understanding the role of surface orientation on the crust evolution and the propagation of the evaporation front inside the food is of particular value to: • Deterministic modeling efforts of the coupled heat and mass transfer phenomena during deep-fat frying, and • food industry; the present data suggest that crispier food is produced and less energy is consumed when the food is placed at a nonhorizontal position inside the fryer.

Evaporation front compared with crust thickness in potato deep-fat frying.

UNLABELLED: The various theoretical approaches that have been proposed for modeling heat and mass transport during deep-fat frying of potatoes provide a rather ambiguous view of the relation between the propagation of the evaporation front inside the food and the evolution of crust thickness. This can be partly attributed to the unavailability of detailed experimental information concerning the temperature field inside the developing crust to validate the models. The objective of the present work is to experimentally study the relation between crust thickness evolution and evaporation front propagation and how this varies with frying conditions. To achieve this goal, a special device has been constructed that permits (1) only 1 side of a potato stick to be exposed to hot oil, and (2) accurate and stable placement of miniature thermocouples in prescribed positions under but very close to the potato surface. Temperature recordings inside the developing crust allowed identification of different heating regimes during frying and a rough estimation of the evaporation front propagation. In addition, crust thickness was determined at intermittent time intervals by 2 independent methods (1) microphotography and (2) a micrometer. Comparison of the evaporation front propagation with crust thickness evolution indicates an interrelationship roughly up to the end of the boiling regime (bubble-end point). After this moment, the propagation of the evaporation front is faster than the evolution of crust thickness. PRACTICAL APPLICATION: Understanding the role of parameters that determine crust formation is of paramount importance since crust characteristics such as thickness and texture dictate the sensory perception of fried foods. This study aims to quantify the relationship between such parameters (that is, crust evolution and the propagation of the evaporation front inside the food) and to examine how frying conditions (oil temperature and frying duration) affect it. In addition, the present findings may be of particular value to deterministic modeling efforts on the coupled heat and mass transfer phenomena during deep-fat frying.

The effect of influent temperature variations in a sedimentation tank for potable water treatment--a computational fluid dynamics study.

A computational fluid dynamics (CFD) model is used to assess the effect of influent temperature variation on solids settling in a sedimentation tank for potable water treatment. The model is based on the CFD code Fluent and exploits several specific aspects of the potable water application to derive a computational tool much more efficient than the corresponding tools employed to simulate primary and secondary wastewater settling tanks. The linearity of the particle conservation equations allows separate calculations for each particle size class, leading to the uncoupling of the CFD problem from a particular inlet particle size distribution. The usually unknown and difficult to be measured particle density is determined by matching the theoretical to the easily measured experimental total settling efficiency. The present model is adjusted against data from a real sedimentation tank and then it is used to assess the significance of influent temperature variation. It is found that a temperature difference of only 1 degrees C between influent and tank content is enough to induce a density current. When the influent temperature rises, the tank exhibits a rising buoyant plume that changes the direction of the main circular current. This process keeps the particles in suspension and leads to a higher effluent suspended solids concentration, thus, worse settling. As the warmer water keeps coming in, the temperature differential decreases, the current starts going back to its original position, and, thus, the suspended solids concentration decreases.

Bubble dynamics during the non-isothermal degassing of liquids. Exploiting microgravity conditions.

This work reviews the up to date state of understanding of dynamic phenomena occurring when gas bubbles grow over submerged heated surfaces. Gas bubbles are produced on hot surfaces because the adjacent liquid layers become superheated causing local desorption of dissolved gases while the liquid far afield remains at low temperatures. Non-isothermal degassing is a very complex process combining heat and mass transport coupled with momentum exchange between the two phases. Difficulties due to buoyancy effects on gas bubbles as well as natural convection of hot liquid layers hindered its thorough investigation in terrestrial conditions and only recent microgravity data allowed serious progress to be made. To reduce the complexity, gas bubble growth on a heated wall was studied here separately from bubble lateral motion and coalescence. A complete mathematical formulation was provided but given the inability to solve the problem numerically with the present resources, a series of approximate solutions were attempted. The comparison between experimental observations and theoretical predictions revealed useful information regarding the governing mechanisms of bubble growth.

Modeling local flotation frequency in a turbulent flow field.

Despite the significance of turbulent fluid motion for enhancing the flotation rate in several industrial processes, there is no unified approach to the modeling of the flotation rate in a turbulent flow field. Appropriate modeling of the local flotation (bubble-particle attachment) rate is the basic constituent for global modeling and prediction of flotation equipment efficiency. Existing approaches for the local flotation rate are limited to specific set of conditions like high or low turbulence. In addition, the combined effects of buoyant bubble rise and/or particle gravity settling are usually ignored. The situation is even vaguer for the computation of collision and attachment efficiencies which are usually computed using the gravity induced velocities although the dominant mode of flotation is the turbulent one. The scope of this work is clear: the development of a general expression for the flotation rate in a turbulent flow field which will cover in a unified and consistent way all possible sets of the problem parameters. This is achieved by using concepts from statistical approach to homogeneous turbulence and gas kinetic theory.

Electrical conductance study of theta-liquid bridges.

This work investigates the behavior of small liquid bridges that are formed between two horizontal supporting surfaces, aligned at the vertical direction. The contact lines of the liquid bridges are not edge-pinned but free to move across the supporting surfaces with the contact angle as a parameter (theta-bridges). An a.c. electrical conductance technique coupled with high resolution optical images is used to characterize the geometrical details of constant volume liquid bridges when their length is increased gradually until rupture. A mathematical framework is developed for the identification of the geometrical characteristics of theta-liquid bridges explicitly from conductance data. Theoretical predictions show good agreement with measurements for most of the bridge lengths (separation distance between supports) except close to the rupture point where the bridge is highly stretched. It is further shown that for short and moderate separation distances the present model can be used with confidence to determine the bridge volume and neck radius from the electrical signal.