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Cris Dwellingham
Use Droplet Precautions for patients known or suspected to be infected with pathogens transmitted by respiratory droplets that are generated by a patient who is coughing, sneezing, or talking.
When considering how people are infected and what can be done to prevent the infections, answers from many disciplines are sought: microbiology, epidemiology, medicine, engineering, and physics. There are many pathways to infection spread, and among the most significant from the epidemiological point of view is airborne transport. Microorganisms can become airborne when droplets are generated during speech, coughing, sneezing, vomiting, or atomization of feces during sewage removal. The fate of the droplets is governed by the physical principles of transport, with droplet size being the most important factor affecting their dispersion, deposition on surfaces and determining the survival of microorganisms within the droplets. In addition, physical characteristics of the indoor environment as well as the design and operation of building ventilation systems are of critical importance. Do we understand the mechanisms of infection spread and can we quantify the droplet dynamics under various indoor conditions? Unfortunately no, as this aspect of infection spread has attracted surprisingly little scientific interest. However, investigations of numerous cases in which a large number of people were infected show how critical the physics of microorganism spread can be. This paper reviews the state of knowledge regarding mechanisms of droplet spread and solutions available to minimize the spread and prevent infections.
Background: Influenza viruses are thought to be spread by droplets, but the role of aerosol dissemination is unclear and has not been assessed by previous studies. Oxygen therapy, nebulised medication and ventilatory support are treatments used in clinical practice to treat influenzal infection are thought to generate droplets or aerosols.
Objectives: Evaluation of the characteristics of droplet/aerosol dispersion around delivery systems during non-invasive ventilation (NIV), oxygen therapy, nebuliser treatment and chest physiotherapy by measuring droplet size, geographical distribution of droplets, decay in droplets over time after the interventions were discontinued.
Results: NIV using a vented mask produced droplets in the large size range (> 10 µm) in patients (p = 0.042) and coryzal subjects (p = 0.044) compared with baseline values, but not in normal controls (p = 0.379), but this increase in large droplets was not seen using the NIV circuit modification. Chest physiotherapy produced droplets predominantly of > 10 µm (p = 0.003), which, as with NIV droplet count in the patients, had fallen significantly by 1 m. Oxygen therapy did not increase droplet count in any size range. Nebulised saline delivered droplets in the small- and medium-size aerosol/droplet range, but did not increase large-size droplet count.
Conclusions: NIV and chest physiotherapy are droplet (not aerosol)-generating procedures, producing droplets of > 10 µm in size. Due to their large mass, most fall out on to local surfaces within 1 m. The only device producing an aerosol was the nebuliser and the output profile is consistent with nebuliser characteristics rather than dissemination of large droplets from patients. These findings suggest that health-care workers providing NIV and chest physiotherapy, working within 1 m of an infected patient should have a higher level of respiratory protection, but that infection control measures designed to limit aerosol spread may have less relevance for these procedures. These results may have infection control implications for other airborne infections, such as severe acute respiratory syndrome and tuberculosis, as well as for pandemic influenza infection.
A droplet in Preflight is a small application that runs a Preflight inspection on one or more PDFs that you drag onto the Droplet icon . You can save a droplet on the desktop or to another location on your computer.
Like droplets, a preflight action inspects multiple files simultaneously, separates successful files from problem files, and creates reports in designated locations. In addition, hot folders can convert multiple file types (JPEG, HTML, RTF, and so on) to PDF or to PDF/X using conversion settings you specify; inspect the files using specified profiles, and output them in any format Acrobat supports, including PDF and PostScript.
Lehman-McKeeman LD, Caudil D. 1992. Biochemical basis for mouse resistance to hyaline droplet nephropathy: Lack of relevance of the alpha 2u-globulin protein superfamily in this male rat-specific syndrome. Toxicol Appl Pharmacol 112:214-221.
Uwagawa S, Saito K, Nakayama A, Umihira M, Okuno Y. 1990. Comparison of hyaline droplets in rats with chronic progressive nephropathy and chemical-induced α2u-globulin nephropathy. J Toxicol Pathol 5:195-203.
Yamate J, Iwaki M, Nakatsuji S, Kuwamura M, Kotani T, Sakuma S. 1998. Lysozyme-containing renal tubular hyaline droplets in F344 rats bearing a rat fibrosarcoma-derived transplantable tumor. Toxicol Pathol 26:699-703.
Friction determines whether liquid droplets slide off a solid surface or stick to it. Surface heterogeneity is generally acknowledged as the major cause of increased contact angle hysteresis and contact line friction of droplets. Here we challenge this long-standing premise for chemical heterogeneity at the molecular length scale. By tuning the coverage of self-assembled monolayers (SAMs), water contact angles change gradually from about 10 to 110 yet contact angle hysteresis and contact line friction are low for the low-coverage hydrophilic SAMs as well as high-coverage hydrophobic SAMs. Their slipperiness is not expected based on the substantial chemical heterogeneity of the SAMs featuring uncoated areas of the substrate well beyond the size of a water molecule as probed by metal reactants. According to molecular dynamics simulations, the low friction of both low- and high-coverage SAMs originates from the mobility of interfacial water molecules. These findings reveal a yet unknown and counterintuitive mechanism for slipperiness, opening new avenues for enhancing the mobility of droplets.
Friction between a solid surface and water is present in everyday life in many ways. Surfaces such as regular window glass and rose petals have high friction resulting in droplets sticking even to vertical surfaces. Other surfaces, like lotus leaves and Teflon-coated kitchenware, have low friction allowing droplets to slide off even at small tilt angles. This latter feature of slipperiness is highly sought after1,2,3, for applications such as self-cleaning4, anti-icing5, microfluidics6 and heat transfer7.
Four distinct forms of droplet friction have been identified: contact line friction (CLF)8,9,10,11, viscous dissipation12, air resistance13, and electrostatic forces14. In general, CLF Fμ is the cause of the static friction of the droplet15 while the other forces arise only when the droplet has already started moving. CLF is shown to relate to droplet contact angles (CAs) via the following equation:
where A is a droplet contact area shape parameter, R is droplet size parameter, γ is liquid surface tension, and θREC and θADV are receding contact angle (RCA) and advancing contact angle (ACA), respectively9,10,16. The difference between θREC and θADV is called contact angle hysteresis (CAH)17, and its magnitude is directly related to the CLF.
It is widely acknowledged that droplet CAH originates from heterogeneities in surface topography and chemistry, as suggested theoretically in the early work by Pease18, Johnson and Dettre19, and Joanny and de Gennes8, and demonstrated experimentally for chemical patterns and topographies at the micrometre scale by Furuta et al.20 and Reyssat and Quéré21. The common understanding is that this effect of surface heterogeneities is valid in general, thus also when length scales are reduced to molecular scale, and Fadeev et al.22,23 and Bittoun et al.24 have experimentally shown that molecular-level heterogeneity of the surface affects CAH.
In this Article, we quantify systematically how molecular-scale chemical heterogeneity affects CAH and droplet CLF. As a model system, we use octyltrichlorosilane (OTS) self-assembled monolayer (SAM) grown on a flat, OH-terminated SiO2 surface. By varying the SAM growth time, we gradually varied its coverage (that is, its areal density) from OH-group-rich hydrophilic SiO2 substrate to a hydrophobic high-coverage SAM. CLF was observed to be high for the intermediate-coverage SAM that has a large fraction of OH groups and is thus chemically the most heterogeneous surface, and low for both low- and high-coverage SAMs that are chemically less heterogeneous yet still not completely homogeneous either (Fig. 1a). The finding that hydrophilic low-coverage SAM has low CLF is counterintuitive, and we performed molecular dynamics (MD) simulations for better understanding the SAM structure and droplet friction mechanisms. These findings help improve the performance of repellent coatings that benefit from low CLF. As an example, we achieved record-low CLF by minimizing chemical heterogeneity of a nanostructured superhydrophobic surface.
The experimental results indicate that CLF depends on the molecular-scale heterogeneity of the SAM surface: most heterogeneous intermediate-coverage SAMs have higher friction than low- and high-coverage SAMs. However, it is not clear based on the experiments why we get these CLF values at each coverage regime, and what is the mechanism of friction in each case. For low-coverage SAMs, MD simulations show the presence of a thin, continuous film of interfacial water on the accessible SiO2 surface, extending from the droplet (Fig. 5a). The film is tightly adhered to the SiO2 surface due to hydrogen bonding between water molecules and the surface OH groups. According to residence time analysis (Supplementary Note 14), this interfacial water is not fully confined, as it can move within the interfacial layer or out from it due to thermal fluctuations. These fluctuations occasionally decrease energy barriers for reordering the hydrogen bonding network of molecules in the first layers of water, which allows easier propagation of water molecules at the contact line37. The interfacial water thus acts like a lubricating layer for the droplet, enabling the very low CLF observed experimentally. MD simulations show how the droplet moves on the interfacial water layer of the low-coverage SAM (Supplementary Note 15 and Supplementary Video 5).
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