Cavitation in gas-saturated liquids

Oscillating gas bubbles can be created in a liquid by exposing it to ultrasound. These gas bubbles implode if the sound pressure is high enough. This process is called cavitation. Interesting phenomena take place during the collapse. The gas and vapour inside the bubble are compressed and reach temperatures of several thousand Kelvin and pressures of several hundred bars, a dense plasma is formed inside the bubble, light is emitted, reactive radical molecules are formed, and there is a liquid flow around the bubble which can be utilised for mixing or even scission of polymers by strain rates. An eroding jet is formed if the bubble collapses near a wall. The pressure and temperature are at ambient conditions around the bubble during this process. There is a plethora of applications which can be operated or intensified by cavitation, e.g. micro mixing, catalyst surface renewal and material synthesis, treatment of kidney stones, waste water treatment and other radical-induced chemistry such as polymerisation, and polymer weight distribution control. In this thesis, several forms of cavitation have been investigated. Special attention is paid to the influence of gas and vapour content in the cavitation bubble. The gas and vapour content of the bubble play a crucial role in the extent of the effects of cavitation. First and foremost the thermal properties adiabatic index and heat conductivity determine the maximum temperature during the collapse. Gases with a high adiabatic index, such as noble gases, and liquids with a low vapour pressure and a high adiabatic index, such as water or sulfuric acid, yield the highest hot spot temperatures and therefore the most intense effects. Some gases such as oxygen also participate in chemical reactions. Another difference between cavitation effects of several gases may be caused by changing gas solubility in the liquid. This changes the concentration gradient around the bubble, and the mass transport to and from the bubble. The effects of the gas on the cavitation process depend on the ultrasound frequency. At frequencies above 20 kHz air has a higher efficiency of radical formation than argon as a saturation gas. This has been measured by following the oxidation of potassium iodide to iodine spectroscopically. The carbon dioxide in the air contributes to this increase at low ultrasound input power. This is surprising since carbon dioxide addition in the cavitation bubble gas phase likely suppresses the hot spot temperature. The enhancement of radical production by carbon dioxide only occurs when it is present in low quantities (<1.5 vol%). A combination of the saturation gases argon (79 vol%), oxygen (20 vol%), and carbon dioxide (1 vol%) gives the highest radical production. Argon has a low heat capacity which results in high hot spot temperatures. Oxygen participates in new chemical reactions, which gives a higher probability of conversion of initial radicals towards the end product. The mechanism through which carbon dioxide enhances the radical production is speculative. It may be possible that the shape of the cavitation bubbles and micro bubble ejection is altered by enhanced mass transport of carbon dioxide into the bubble. Sometimes a high reactivity by radicals is undesired in an ultrasound process. An example of such a process is the study of scission of polymer-metal complexes by strain induced by a collapsing bubble. By mechanical scission of the complex the metal inside it becomes exposed and becomes available for catalytic reactions. This opens up interesting applications such as self-regenerating materials or materials that change colour when they are exposed to strain. Thermal activation and activation by radicals must be low compared to mechanical scission to be able to study the mechanical activation precisely. This has been done by changing the saturation gas from argon to methane or nitrogen. The radical production is 2 times higher under argon than under a nitrogen atmosphere, and 20 times higher under argon than under a methane atmosphere. The hot spot temperature correlates with the radical production. The scission percentages are roughly the same under these gases, indicating that mechanical scission is the most important mechanism of complex breakage. This is supported by model calculations of a collapsing bubble in a liquid saturated with these gases. An alternative method to create cavitation and its effects is by creating a flow through a constriction. The pressure inside the constriction drops to values sufficient to induce bubble growth. Subsequently the bubble collapses. Miniaturisation of this process is desired because it allows easy study in precisely manufactured geometries, and because the apparent frequency of the process increases. Experimental observations have been compared to computational fluid dynamics modeling results. This shows that it is important that the constriction exits in an unconfined area. If the constriction exit is confined, an extended low pressure region can occur, which induces excessive bubble growth and a non-violent implosion of the bubbles. Values of about 160 kHz in 0.75 mm capillaries are reached in 0.2 mm constrictions operated with a flow of 90 mL min-1, equal to a liquid velocity in the constriction of 59 m s-1. A radical production in the same range as those reported in previous work on hydrodynamic cavitation is measured with these settings. Chlorohydrocarbons have been added to the liquid to increase radical production. Simulations of the temperature of a collapsing bubble show a qualitative correlation with the radical production. A physical representation of the propagation of sound waves in a liquid-gas slug flow has been developed to be able to safely operate a gas-liquid micro separator. An undesired pressure increase by e.g. temporary blocking of an upstream microchannel can result in leaking of the liquid to the gas side. A safety pressure release valve and a safety capillary are incorporated into the process. The safety valve releases any undesired over pressure to the atmosphere. The safety capillary and the pressure pulse propagation speed are needed to be able to have enough time to open the valve. A second advantage of the developed equation is that, together with an equation for pressure drop, the gas bubble length and gas fraction inside the capillary can be calculated. This offers opportunities for online characterisation of slug flow in micro reactors

The evolution of early diagenetic processes at the Mozambique margin during the last glacial-interglacial transition

The Mozambique continental margin experienced large variations in sedimentation rates, primarily due to re-routing of sediment deposition from the Zambezi River during the last glacial-Holocene transition. As changes in sediment accumulation and organic matter deposition impose a strong control on the formation of authigenic minerals in the sediment, the distribution of these minerals may reflect the regional paleoenvironmental and paleoclimatic evolution. Combining geochemical analyses of porewaters and sediments with a reactive transport modeling approach, we reconstruct the depositional history and its effect on pyrite formation and other biogeochemical transformations at a site on the Mozambique margin over the past 27 kyr. Fitting the model to match the observed geochemical patterns, most importantly authigenic pyrite, allowed for the reconstruction of past sulfate-methane transition zone depth, which migrated in response to changes in the sediment accumulation and organic matter deposition. Changes in sediment deposition quickly affected organoclastic sulfate reduction and associated pyrite formation, but the effect on anaerobic methane oxidation and subsequent pyrite formation occurred with a lag on the order of thousands of years. Model results reveal a transition from high diagenetic reaction rates representative of near-shore depositional environments during the late glacial maximum, to a setting typical of offshore sediments with low reaction rates at the present day. Notably, the remnants of methane and dissolved iron pools produced in the past still shape the diagenetic processes at and below the sulfate-methane transition zone today. Since deglacial shelf-flooding and corresponding changes in sediment deposition occurred along continental margins worldwide, our analysis highlights the important role of non-steady state diagenesis in continental margin sediments and its relevance for paleoceanographic interpretation of sediment cores experiencing strong variations in sediment input

ESPNIC clinical practice guidelines: intravenous maintenance fluid therapy in acute and critically ill children- a systematic review and meta-analysis

PURPOSE Intravenous maintenance fluid therapy (IV-MFT) prescribing in acute and critically ill children is very variable among pediatric health care professionals. In order to provide up to date IV-MFT guidelines, the European Society of Pediatric and Neonatal Intensive Care (ESPNIC) undertook a systematic review to answer the following five main questions about IV-MFT: (i) the indications for use (ii) the role of isotonic fluid (iii) the role of balanced solutions (iv) IV fluid composition (calcium, magnesium, potassium, glucose and micronutrients) and v) and the optimal amount of fluid. METHODS A multidisciplinary expert group within ESPNIC conducted this systematic review using the Scottish Intercollegiate Guidelines Network (SIGN) grading method. Five databases were searched for studies that answered these questions, in acute and critically children (from 37 weeks gestational age to 18 years), published until November 2020. The quality of evidence and risk of bias were assessed, and meta-analyses were undertaken when appropriate. A series of recommendations was derived and voted on by the expert group to achieve consensus through two voting rounds. RESULTS 56 papers met the inclusion criteria, and 16 recommendations were produced. Outcome reporting was inconsistent among studies. Recommendations generated were based on a heterogeneous level of evidence, but consensus within the expert group was high. "Strong consensus" was reached for 11/16 (69%) and "consensus" for 5/16 (31%) of the recommendations. CONCLUSIONS Key recommendations are to use isotonic balanced solutions providing glucose to restrict IV-MFT infusion volumes in most hospitalized children and to regularly monitor plasma electrolyte levels, serum glucose and fluid balance

Expression of Msx1 and Dlx1 during Dumbo rat head development: Correlation with morphological features

The Dumbo rat possesses some characteristics that evoke several human syndromes, such as Treacher-Collins: shortness of the maxillary, zygomatic and mandibular bones, and low position of the ears. Knowing that many homeobox genes are candidates in craniofacial development, we investigated the involvement of the Msx1 and Dlx1 genes in the Dumbo phenotype with the aim of understanding their possible role in abnormal craniofacial morphogenesis and examining the possibility of using Dumbo rat as an experimental model for understanding abnormal craniofacial development. We studied the expression of these genes during craniofacial morphogenesis by RT-PCR method. We used Dumbo embryos at E12 and E14 and included the Wistar strain as a control. Semi-quantitative PCR analysis demonstrated that Msx1 and Dlx1 are expressed differently between Dumbo and Wistar rats, indicating that their low expression may underly the Dumbo phenotype

Nutritional support for children during critical illness: European Society of Pediatric and Neonatal Intensive Care (ESPNIC) metabolism, endocrine and nutrition section position statement and clinical recommendations

Abstract: Background: Nutritional support is considered essential for the outcome of paediatric critical illness. There is a lack of methodologically sound trials to provide evidence-based guidelines leading to diverse practices in PICUs worldwide. Acknowledging these limitations, we aimed to summarize the available literature and provide practical guidance for the paediatric critical care clinicians around important clinical questions many of which are not covered by previous guidelines. Objective: To provide an ESPNIC position statement and make clinical recommendations for the assessment and nutritional support in critically ill infants and children. Design: The metabolism, endocrine and nutrition (MEN) section of the European Society of Pediatric and Neonatal Intensive Care (ESPNIC) generated 15 clinical questions regarding different aspects of nutrition in critically ill children. After a systematic literature search, the Scottish Intercollegiate Guidelines Network (SIGN) grading system was applied to assess the quality of the evidence, conducting meta-analyses where possible, to generate statements and clinical recommendations, which were then voted on electronically. Strong consensus (> 95% agreement) and consensus (> 75% agreement) on these statements and recommendations was measured through modified Delphi voting rounds. Results: The final 15 clinical questions generated a total of 7261 abstracts, of which 142 publications were identified relevant to develop 32 recommendations. A strong consensus was reached in 21 (66%) and consensus was reached in 11 (34%) of the recommendations. Only 11 meta-analyses could be performed on 5 questions. Conclusions: We present a position statement and clinical practice recommendations. The general level of evidence of the available literature was low. We have summarised this and provided a practical guidance for the paediatric critical care clinicians around important clinical questions

The impact of the Cretaceous–Paleogene (K–Pg) mass extinction event on the global sulfur cycle: Evidence from Seymour Island, Antarctica

The Cretaceous–Paleogene (K–Pg) mass extinction event 66 million years ago led to large changes to the global carbon cycle, primarily via a decrease in primary or export productivity of the oceans. However, the effects of this event and longer-term environmental changes during the Late Cretaceous on the global sulfur cycle are not well understood. We report new carbonate associated sulfate (CAS) sulfur isotope data derived from marine macrofossil shell material from a highly expanded high latitude Maastrichtian to Danian (69–65.5 Ma) succession located on Seymour Island, Antarctica. These data represent the highest resolution seawater sulfate record ever generated for this time interval, and are broadly in agreement with previous low-resolution estimates for the latest Cretaceous and Paleocene. A vigorous assessment of CAS preservation using sulfate oxygen, carbonate carbon and oxygen isotopes and trace element data, suggests factors affecting preservation of primary seawater CAS isotopes in ancient biogenic samples are complex, and not necessarily linked to the preservation of original carbonate mineralogy or chemistry. Primary data indicate a generally stable sulfur cycle in the early-mid Maastrichtian (69 Ma), with some fluctuations that could be related to increased pyrite burial during the ‘mid-Maastrichtian Event’. This is followed by an enigmatic +4‰ increase in δ³⁴SCAS during the late Maastrichtian (68–66 Ma), culminating in a peak in values in the immediate aftermath of the K–Pg extinction which may be related to temporary development of oceanic anoxia in the aftermath of the Chicxulub bolide impact. There is no evidence of the direct influence of Deccan volcanism on the seawater sulfate isotopic record during the late Maastrichtian, nor of a direct influence by the Chicxulub impact itself. During the early Paleocene (magnetochron C29R) a prominent negative excursion in seawater δ³⁴S of 3–4‰ suggests that a global decline in organic carbon burial related to collapse in export productivity, also impacted the sulfur cycle via a significant drop in pyrite burial. Box modelling suggests that to achieve an excursion of this magnitude, pyrite burial must be reduced by >15%, with a possible role for a short term increase in global weathering rates. Recovery of the sulfur cycle to pre-extinction values occurs at the same time (∼320 kyrs) as initial carbon cycle recovery globally. These recoveries are also contemporaneous with an initial increase in local alpha diversity of marine macrofossil faunas, suggesting biosphere-geosphere links during recovery from the mass extinction. Modelling further indicates that concentrations of sulfate in the oceans must have been 2 mM, lower than previous estimates for the Late Cretaceous and Paleocene and an order of magnitude lower than today

Cavitation in gas-saturated liquids

Oscillating gas bubbles can be created in a liquid by exposing it to ultrasound. These gas bubbles implode if the sound pressure is high enough. This process is called cavitation. Interesting phenomena take place during the collapse. The gas and vapour inside the bubble are compressed and reach temperatures of several thousand Kelvin and pressures of several hundred bars, a dense plasma is formed inside the bubble, light is emitted, reactive radical molecules are formed, and there is a liquid flow around the bubble which can be utilised for mixing or even scission of polymers by strain rates. An eroding jet is formed if the bubble collapses near a wall. The pressure and temperature are at ambient conditions around the bubble during this process. There is a plethora of applications which can be operated or intensified by cavitation, e.g. micro mixing, catalyst surface renewal and material synthesis, treatment of kidney stones, waste water treatment and other radical-induced chemistry such as polymerisation, and polymer weight distribution control. In this thesis, several forms of cavitation have been investigated. Special attention is paid to the influence of gas and vapour content in the cavitation bubble. The gas and vapour content of the bubble play a crucial role in the extent of the effects of cavitation. First and foremost the thermal properties adiabatic index and heat conductivity determine the maximum temperature during the collapse. Gases with a high adiabatic index, such as noble gases, and liquids with a low vapour pressure and a high adiabatic index, such as water or sulfuric acid, yield the highest hot spot temperatures and therefore the most intense effects. Some gases such as oxygen also participate in chemical reactions. Another difference between cavitation effects of several gases may be caused by changing gas solubility in the liquid. This changes the concentration gradient around the bubble, and the mass transport to and from the bubble. The effects of the gas on the cavitation process depend on the ultrasound frequency. At frequencies above 20 kHz air has a higher efficiency of radical formation than argon as a saturation gas. This has been measured by following the oxidation of potassium iodide to iodine spectroscopically. The carbon dioxide in the air contributes to this increase at low ultrasound input power. This is surprising since carbon dioxide addition in the cavitation bubble gas phase likely suppresses the hot spot temperature. The enhancement of radical production by carbon dioxide only occurs when it is present in low quantities (<1.5 vol%). A combination of the saturation gases argon (79 vol%), oxygen (20 vol%), and carbon dioxide (1 vol%) gives the highest radical production. Argon has a low heat capacity which results in high hot spot temperatures. Oxygen participates in new chemical reactions, which gives a higher probability of conversion of initial radicals towards the end product. The mechanism through which carbon dioxide enhances the radical production is speculative. It may be possible that the shape of the cavitation bubbles and micro bubble ejection is altered by enhanced mass transport of carbon dioxide into the bubble. Sometimes a high reactivity by radicals is undesired in an ultrasound process. An example of such a process is the study of scission of polymer-metal complexes by strain induced by a collapsing bubble. By mechanical scission of the complex the metal inside it becomes exposed and becomes available for catalytic reactions. This opens up interesting applications such as self-regenerating materials or materials that change colour when they are exposed to strain. Thermal activation and activation by radicals must be low compared to mechanical scission to be able to study the mechanical activation precisely. This has been done by changing the saturation gas from argon to methane or nitrogen. The radical production is 2 times higher under argon than under a nitrogen atmosphere, and 20 times higher under argon than under a methane atmosphere. The hot spot temperature correlates with the radical production. The scission percentages are roughly the same under these gases, indicating that mechanical scission is the most important mechanism of complex breakage. This is supported by model calculations of a collapsing bubble in a liquid saturated with these gases. An alternative method to create cavitation and its effects is by creating a flow through a constriction. The pressure inside the constriction drops to values sufficient to induce bubble growth. Subsequently the bubble collapses. Miniaturisation of this process is desired because it allows easy study in precisely manufactured geometries, and because the apparent frequency of the process increases. Experimental observations have been compared to computational fluid dynamics modeling results. This shows that it is important that the constriction exits in an unconfined area. If the constriction exit is confined, an extended low pressure region can occur, which induces excessive bubble growth and a non-violent implosion of the bubbles. Values of about 160 kHz in 0.75 mm capillaries are reached in 0.2 mm constrictions operated with a flow of 90 mL min-1, equal to a liquid velocity in the constriction of 59 m s-1. A radical production in the same range as those reported in previous work on hydrodynamic cavitation is measured with these settings. Chlorohydrocarbons have been added to the liquid to increase radical production. Simulations of the temperature of a collapsing bubble show a qualitative correlation with the radical production. A physical representation of the propagation of sound waves in a liquid-gas slug flow has been developed to be able to safely operate a gas-liquid micro separator. An undesired pressure increase by e.g. temporary blocking of an upstream microchannel can result in leaking of the liquid to the gas side. A safety pressure release valve and a safety capillary are incorporated into the process. The safety valve releases any undesired over pressure to the atmosphere. The safety capillary and the pressure pulse propagation speed are needed to be able to have enough time to open the valve. A second advantage of the developed equation is that, together with an equation for pressure drop, the gas bubble length and gas fraction inside the capillary can be calculated. This offers opportunities for online characterisation of slug flow in micro reactors