Philippe Bourrianne (MAE - Princeton Univ) - Droplets and air flows

Droplets are often noticed adhering to a substrate, such as raindrops stuck on eyeglasses. The capillary adhesion indeed overcomes the weight of a millimetric drop, which thus remain immobile on its solid substrate. In this seminar, we will discuss two situations where droplets are surrounded by air flows. We will demonstrate how air flows act on droplets and direct them.

First, we will study the case of levitating drops in an attempt to fully suppress the wetting of a solid by a liquid and its subsequent liquid-solid adhesion. Liquid levitation can even be achieved using the volatility of the liquid and creating a thin insulating vapor layer as early described by Leidenfrost in 1756. More recently, other strategies have been developed involving external fields such as air flows, Faraday waves or magnetic fields. We will describe a novel approach using active liquids able to sustain their own levitation in the absence of external forces at ambient temperature. We will first discuss the levitation of carbonated water drops deposited on superhydrophobic solid. In this regime, the drop self-generates the gas cushion which provides levitation. Finally, we will generalize this new regime of levitation to other active liquids.

Then, we will discuss how air flows covey aerosol droplets emitted by a person breathing, speaking or singing. By tracking the air exhaled by a person, we will describe the trajectories of possible pathogenic droplets responsible for the transmission of infectious respiratory diseases. We will focus our attention on the particular case of opera singers and musicians as orchestras and choir have been identified as possible clusters during pandemics. By characterizing the air flows in Opera, we will determine situations of higher risk within an orchestra.