As the restrictions imposed due to the COVID-19 pandemic are being gradually relaxed and people are resuming their usual activities, the configuration of two unmasked individuals at proximity having a face-to-face conversation poses a risk of transferring a high dose of a pathogen from an infected person to a susceptible one. To study this airflow and exchange between two individuals, we conduct fog flow visualization experiments and direct numerical simulations of colliding respiratory jets mimicking a short conversation. It is found that the vertical offset between the mouths of the speakers is an important parameter governing the propagation and evolution of the respiratory jets. A 'blocking effect' is observed at low offsets, which temporarily protects the susceptible speaker from the pathogen-loaded saliva droplets in the jet from the infected speaker. Whereas at large offsets, the interaction between the jets is minimum. At certain intermediate offsets, jet entrainment and inhaled breath, assist the pathogen-containing jets to propagate towards the susceptible speaker's mouth. Thus, the interaction of the respiratory jets permits air exchange to a varying degree depending upon the effectiveness of the blocking effect and jet entrainment.@en

Air exchange between people has emerged in the COVID-19 pandemic as the important vector for transmission of the SARS-CoV-2 virus. We study the airflow and exchange between two unmasked individuals conversing face-to-face at short range, which can potentially transfer a high dose of a pathogen, because the dilution is small when compared to long-range airborne transmission. We conduct flow visualization experiments and direct numerical simulations of colliding respiratory jets mimicking the initial phase of a conversation. The evolution and dynamics of the jets are affected by the vertical offset between the mouths of the speakers. At low offsets the head-on collision of jets results in a `blocking effect', temporarily shielding the susceptible speaker from the pathogen carrying jet, although, the lateral spread of the jets is enhanced. Sufficiently large offsets prevent the interaction of the jets. At intermediate offsets (8-10 cm for 1 m separation), jet entrainment and the inhaled breath assist the transport of the pathogen-loaded saliva droplets towards the susceptible speaker's mouth. Air exchange is expected, in spite of the blocking effect arising from the interaction of the respiratory jets from the two speakers.@en

Mixing and spreading of different liquids are omnipresent in nature, life and technology, such as oil pollution on the sea, estuaries, food processing, cosmetic and beverage industries, lab-on-a-chip devices, and polymer processing. However, the mixing and spreading mechanisms for miscible liquids remain poorly characterized. Here, we show that a fully soluble liquid drop deposited on a liquid surface remains as a static lens without immediately spreading and mixing, and simultaneously a Marangoni-driven convective flow is generated, which are counterintuitive results when two liquids have different surface tensions. To understand the dynamics, we develop a theoretical model to predict the finite spreading time and length scales, the Marangoni-driven convection flow speed, and the finite timescale to establish the quasi-steady state for the Marangoni flow. The fundamental understanding of this solutal Marangoni flow may enable driving bulk flows and constructing an effective drug delivery and surface cleaning approach without causing surface contamination by immiscible chemical species.

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