Numerical and Experimental Investigations of Horizontal Turbulent Particle-Liquid Pipe Flow.

A Eulerian-Eulerian computational fluid dynamics approach is used in conjunction with appropriate auxiliary models for turbulence and solid dynamic properties to study the complex turbulent flow of particle-liquid suspensions in a horizontal pipe. Numerical simulations of the detailed flow field are fully and successfully validated using a unique experimental technique of positron emission particle tracking. The study includes nearly neutrally buoyant as well as dense particles, ranging from small to large at low to high concentrations, conveyed by a Newtonian liquid. Results are analyzed in terms of radial particle and liquid velocity profiles as well as particle distribution in the pipe. The approach provides predictions with a high degree of accuracy. Particle behavior can be classified into three categories depending on their size and particle-liquid density ratio. An analysis of the forces governing the two-phase flow is used to interpret the phenomena observed.

On the clustering of bulk nanobubbles and their colloidal stability.

Bulk nanobubbles which are usually observed in pure water have a mean diameter typically around 100 nm. We use a combination of physical and chemical techniques to prove the hypothesis that the nanoentities observed in pure water are stable clusters of much smaller stable nanobubbles. The stability of bulk nanobubble clusters is affected by factors such as ionic strength or internal energy of the system. We show that bulk nanobubbles on the order of 100 nm exist in a stable cluster form in neutral or basic media, and dissociate into tiny primary nanobubbles on the order of 1 nm in acidic media, or in the presence of small amounts of salt. These new findings suggest that bulk nanobubbles which have a high surface energy unsurprisingly tend to behave in a similar manner to solid nanoparticles in terms of their agglomeration tendency, which is confirmed by the DLVO theory. The results will have important implications for our understanding and interpretation of the behaviour of bulk nanobubbles, in particular their interfacial and colloidal stability.

Response to "Comment on Bulk Nanobubbles or Not Nanobubbles: That is the Question".

Advanced techniques that combine high spatial resolution with chemical sensitivity to directly probe the observed nanoentities and provide direct evidence that they are truly gas-filled nanobubbles do not exist. Therefore, in our paper, we focused on providing, for the first time, multiple types of indirect evidence using a variety of physical and chemical techniques that the nanoentities are not due to contamination and, hence, they must be bulk nanobubbles (BNBs). It should be noted that such techniques require good experimental skills, sound protocols, good scientific expertise, and reliable equipment. While no single piece of indirect evidence on its own can be considered as conclusive proof, we estimate that our results combined provide strong evidence that bulk nanobubbles do exist and they are stable. The work presented in our paper is the culmination of a series of studies, and many authors have either directly or indirectly confirmed our findings. Nonetheless, in their Comment, Rak & Sedlak reject all of the work we reported. We here address their comments point by point and show that their criticisms are unwarranted and unfounded, as follows.

Correction: Proving and interpreting the spontaneous formation of bulk nanobubbles in aqueous organic solvent solutions: effects of solvent type and content.

Correction for 'Proving and interpreting the spontaneous formation of bulk nanobubbles in aqueous organic solvent solutions: effects of solvent type and content' by Ananda J. Jadhav et al., Soft Matter, 2020, 16, 4502-4511, DOI: 10.1039/D0SM00111B.

A Henry's law method for generating bulk nanobubbles.

A new technique for generating bulk nanobubble suspensions has been developed based on Henry's law which states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid. This principle which forms the basis of vacuum degasification has been exploited here to produce stable bulk nanobubbles in excess of 109 bubble mL-1 in pure water, through successive expansion/compression strokes inside a sealed syringe. We provide evidence that the observed nano-entities must be gas-filled nanobubbles by showing that: (i) they cannot be attributed to organic or inorganic impurities; (ii) they disappear gradually over time whilst their mean size remains unchanged; (iii) their number density depends on the concentration of dissolved gas in water and its solubility; and (iv) added sparging of gas enhances process yield. We study the properties of these nanobubbles including the effects of type of dissolved gas, water pH and the presence of different valence salts on their number density and stability. Given the potential of the technique for large scale production of nanobubble suspensions, we describe a successfully tested automated model and outline the basis for process scale-up.

Proving and interpreting the spontaneous formation of bulk nanobubbles in aqueous organic solvent solutions: effects of solvent type and content.

We show that the mixing of organic solvents with pure water leads to the spontaneous formation of suspended nano-entities which exhibit long-term stability on the scale of months. A wide range of solvents representing different functional groups are studied: methanol, ethanol, propanol, acetone, DMSO and formamide. We use various physical and chemical analytical techniques to provide compounded evidence that the nano-entities observed in all these aqueous solvent solutions must be gas-filled nanobubbles as they cannot be attributed to solvent nanodroplets, impurities or contamination. The nanobubble suspensions are characterized in terms of their bubble size distribution, bubble number density and zeta potential. The bubble number density achieved is a function of the type of solvent. It increases sharply with solvent content, reaching a maximum at an intermediate solvent concentration, before falling off to zero. We show that, whilst bulk nanobubbles can exist in pure water, they cannot exist in pure organic solvents and they disappear at some organic solvent-water ratio depending on the type of solvent. The gas solubility of the solvent relative to water as well as the molecular structure of the solvent are determining factors in the formation and stability of bulk nanobubbles. These phenomena are discussed and interpreted in the light of the experimental results obtained.

Bulk Nanobubbles or Not Nanobubbles: That is the Question.

Bulk nanobubbles are a novel nanoscale bubble system with unusual properties which challenge our understanding of bubble behavior. Because of their extraordinary longevity, their existence is still not widely accepted as they are often attributed to the presence of supramolecular structures or contaminants. Nonetheless, bulk nanobubbles are attracting increasing attention in the literature, but reports generally lack objective evidence that the observed nano-entities are indeed nanobubbles. In this paper, we use various physical and chemical analytical techniques to provide multiple evidence that the nano-entities produced mechanically in pure water by a continuous high-shear rotor-stator device or acoustic cavitation and spontaneously by water-ethanol mixing are indeed gas-filled domains. We estimate that the results presented here combined provide conclusive proof that bulk nanobubbles do exist and they are stable. This paper should help close the debate about the existence of bulk nanobubbles and, hence, enable the scientific community to rather focus on developing the missing fundamental science in this area.

Which Parameters Affect Biofilm Removal with Acoustic Cavitation? A Review.

Bacterial biofilms are a cause of contamination in a wide range of medical and biological areas. Ultrasound is a mechanical energy that can remove these biofilms using cavitation and acoustic streaming, which generate shear forces to disrupt biofilm from a surface. The aim of this narrative review is to investigate the literature on the mechanical removal of biofilm using acoustic cavitation to identify the different operating parameters affecting its removal using this method. The properties of the liquid and the properties of the ultrasound have a large impact on the type of cavitation generated. These include gas content, temperature, surface tension, frequency of ultrasound and acoustic pressure. For many of these parameters, more research is required to understand their mechanisms in the area of ultrasonic biofilm removal, and further research will help to optimise this method for effective removal of biofilms from different surfaces.

Discrete multi-physics: A mesh-free model of blood flow in flexible biological valve including solid aggregate formation.

We propose a mesh-free and discrete (particle-based) multi-physics approach for modelling the hydrodynamics in flexible biological valves. In the first part of this study, the method is successfully validated against both traditional modelling techniques and experimental data. In the second part, it is further developed to account for the formation of solid aggregates in the flow and at the membrane surface. Simulations of various types of aggregates highlight the main benefits of discrete multi-physics and indicate the potential of this approach for coupling the hydrodynamics with phenomena such as clotting and calcification in biological valves.

Effects of vessel size, cell sedimentation and haematocrit on the adhesion of leukocytes and platelets from flowing blood.

BACKGROUND: Leukocytes and platelets typically fulfil their functions through adhesion to the walls of vessels with different size, haematocrit and shear rate. OBJECTIVE: We aimed to investigate differential effects of these variables on leukocyte and platelet adhesion. METHODS: Blood with varying haematocrit was perfused at a range of wall shear rates through capillaries of depth 100 or 300 µm coated with P-selectin or collagen. RESULTS: Adhesion of leukocytes was much more efficient in the smaller capillaries, but was equal on the upper and lower surfaces and showed nearly identical shear rate dependence for either size of vessel. Platelets also adhered more efficiently in the smaller vessels (although the effect of size was not so great), and equally on upper and lower surfaces, but their adhesion was much less sensitive to increasing shear rate. In previous studies using vertically-orientated capillaries, leukocyte adhesion increased with increasing haematocrit (Am. J. Physiol.285 (2003), H229-H240). Here, in horizontal 100 µm capillaries, leukocyte adhesion was highly efficient at haematocrit of 10% but restricted to the lower surface. Adhesion decreased initially as haematocrit was increased to 30% and then increased slightly again at 40% haematocrit. Increasing haematocrit supported a monotonic increase in platelet adhesion in the horizontal capillaries. CONCLUSIONS: Platelets adhere efficiently over a wider range of sizes and shear rates, and at high haematocrit. Leukocytes adhere better in smaller vessels and at low haematocrit in horizontal vessels. The different behaviours may represent 'rheological adaptation' to functions in inflammation vs. haemostasis.

Comparative rheology of the adhesion of platelets and leukocytes from flowing blood: why are platelets so small?

We investigated rheological adaptation of leukocytes and platelets for their adhesive functions in inflammation and hemostasis, respectively. Adhesion and margination of leukocytes or platelets were quantified for blood perfused through capillaries coated with P-selectin or collagen, when flow rate, suspending phase viscosity, red cell aggregation, or rigidity was modified. Independent variation of shear rate and shear stress indicated that the ability of platelets to attach at higher levels than leukocytes was largely attributable to their smaller size, reducing their velocity before attachment, and, especially, drag after attachment. Increasing red cell aggregation increased the number of marginated and adhering leukocytes but inhibited platelet adhesion without effect on the number marginated. Increasing red cell rigidity tended to inhibit leukocyte adhesion but promote platelet adhesion. The effects on platelets may be explained by changes in the depth of the near-wall, red cell-depleted layer; broadening (or narrowing) this layer to greater (or less) than the platelet diameter would decrease (or increase) the normal force applied by red blood cells and make attachment less (or more) efficient. Thus different adhesive capabilities of leukocytes and platelets may arise from their differences in size, both directly because of influence on cell velocity and force experienced at the wall and indirectly through effects of size on margination in the bloodstream and interaction with the cell-free layer. In addition, red cell aggregation (of hitherto uncertain physiological significance) may be useful in promoting leukocyte adhesion in inflamed venules but inhibiting unwanted platelet deposition in veins.

Red cell aggregation as a factor influencing margination and adhesion of leukocytes and platelets.

Leukocytes and platelets must adhere to the wall of blood vessels to carry out their protective functions. Rheological factors influencing these processes are the delivery of the cells to the wall, referred to as margination, and the local shear rates and stresses at the wall. Margination requires leukocytes and platelets to be excluded from the central flow of the much more numerous red blood cells. This exclusion may be influenced by red cell aggregation. Red cell aggregation also influences development of plug flow in small vessels, which in turn modifies the wall shear rate and stress from those expected in ideal Poiseuille flow. Promotion of aggregation by added agents such as high molecular weight dextrans or by reduction in shear rate, increases margination of leukocytes and efficiency of attachment to the vessel wall. Interestingly, however, fewer studies exist for platelets, and these suggest that margination is actually promoted by increasing shear rate. Direct studies of the effects of red cell aggregation on platelets are required, but it appears that aggregation has different effects on delivery of platelets compared to leukocytes. These differences may represent adaptations for efficient adhesion of leukocytes and platelets in different regions of the circulation.