Patient generated aerosol in the context of ophthalmic surgery

Objective: To assess the patterns of patient generated aerosol in the context of ophthalmic surgery and ophthalmic examinations. To inform medical teams regarding potential hazards and suggest mitigating measures. Methods: Qualitatively, real-time time videography assessed exhalation patterns from simulated patients under different clinical scenarios using propylene glycol from an e-cigarette. Quantitatively, high-speed Schlieren imaging was performed to enable high resolution recordings analysable by MATLAB technical computing software. Results: Without a face mask, the standard prior to COVID 19, vapour was observed exiting through the opening in the drape over the surgical field. The amount of vapour increased when a surgical mask was worn. With a taped face mask, the amount of vapour decreased and with inclusion of a continuous suction device, the least amount of vapour was seen. These results were equivocal when the patient was supine or sitting upright. High-speed Schlieren imaging corroborated these findings and in addition showed substantial increase in airflow egress during coughing and with ill-fitting face masks. Conclusion: Advising patients to wear a surgical mask at the time of ophthalmic interventions potentially contaminants the ocular field with patient generated aerosol risking endophthalmitis. Surgeon safety can be maintained with personal protective equipment to mitigate the increased egress of vapour from the surgical drape and taping, with or without suction is advisable, whilst meticulous hygiene around lenses is required at the time of slit lamp examination.

A new argument against cooling by convective air eddies formed above sunlit zebra stripes

There is a long-lasting debate about the possible functions of zebra stripes. According to one hypothesis, periodical convective air eddies form over sunlit zebra stripes which cool the body. However, the formation of such eddies has not been experimentally studied. Using schlieren imaging in the laboratory, we found: downwelling air streams do not form above the white stripes of light-heated smooth or hairy striped surfaces. The influence of stripes on the air stream formation (facilitating upwelling streams and hindering horizontal stream drift) is negligible higher than 1–2 cm above the surface. In calm weather, upwelling air streams might form above sunlit zebra stripes, however they are blown off by the weakest wind, or even by the slowest movement of the zebra. These results forcefully contradict the thermoregulation hypothesis involving air eddies.

Schlieren Imaging of Single-Fin Missile models in a De-Laval Nozzle Test Section

The experimental study (Schlieren photography) to characterize the flow behavior around a semi-cylindrical missile model having a single planar and wrap-around fin surface is performed inside a modified De-Laval nozzle test section capable of sustaining an airflow at Mach number ~1.7M. The images obtained from this schlieren technique is compared with flow field contour images of the similar missile models at similar flow conditions. The experiments are performed on a modified two-walled glassed section to assist the Schlieren imaging. The test section is calibrated preliminary to the experiments to assure the supersonic fluid flow. A comparison of flow images around the two types of fins further helps in characterizing the flow in their vicinity.