Dynamics Of Gas-liquid Interfaces Under Impact

Impact phenomenon on the gas-liquid interface is omnipresent in nature and people’s life,with abundant flow phenomenon and complex flow mechanism,and has important research value and practical significance.In this dissertation,we explore three types of impact problems on the gas-liquid interface from the perspectives of liquid impact,solid impact,and gas impact based on experimental methods and theoretical analysis:the cavity dynamics of slippery liquid-infused porous spheres with special liquid surfaces impacting onto the water surface;the cavity generation and crown/jet evolution of hydrophilic solid spheres impinging the viscoelastic PEO solution surface;aerosol generation during airflow impinging on the tear film during the practical use of non-contact tonometer and its effect on the transmission of COVID-19.The main work and research results are as follows:(1)The formation of a stable gas cavity on the surfaces of solid bodies is essential for many practical applications,such as drag reduction and energy savings,owing to the transformation of the originally sticky solid-liquid interface into a free-slip liquid-vapor interface and the suppression of the shedding of unsteady vortex in the tail section.Here,we show that the simple infusion of a textured sphere with a smooth,slippery liquid layer can more easily create and sustain stable gas cavities in a liquid at lower impact velocities compared to a dry solid sphere at the same contact angle.To better visualize the underlying contact line dynamics,the collision of water entry of spheres and drops impact planes were first unified by a key parameter of curvature ratio based on systematic experimental results of drop impacting spheres.We further demonstrate that this unforeseen phenomenon mainly results from the preferential liquid lamella detachment from the slippery surface,which is induced by the higher viscosity of the lubricant relative to air.Kinematic discontinuity maintains the stability of the crown structure after liquid lamella separation.Benefiting from the lower cavity formation threshold,the drag reduction could be achieved over a wider parameter range.(2)The dynamics of hydrophilic spheres impinging on a pool of viscoelastic PEO solution was investigated and compared with the results of spheres impinging on a glycerol-water liquid with similar viscosity and surface tension,the addition of PEO polymer in water could significantly reduce the critical velocity of cavity generation and regulate the subsequent crown and jet evolution.The dynamic behaviors of cavity generation,contact line evolution,crown formation,jet fragmentation,and underwater cavity pinch-off were observed from three perspectives using high-speed cameras.The ejecta sheet and jet that emerged at the initial stage of the sphere impacting the liquid surface will pull and separate the upward climbing liquid film away from the sphere,thus reducing the critical velocity of cavity generation.A complete theoretical model to predict the critical velocity of cavity generation was derived based on the elastic force and the adsorption force of the attached wall flow.The different regimes of the cavities in viscoelastic PEO solutions were examined,and the diagrams of the regime divisions based on Bo numbers and We numbers are given.Finally,we depicted the crown and jet configurations,and explained the different results with the spheres impacting conventional Newtonian liquid surfaces.(3)Virus-laden aerosols are the primary route for SARS-CoV-2 transmission,generated from pathogenic body fluids of patients,like respiratory fluids,saliva,and tears.The non-contact tonometer measures intraocular pressure by flattening the ocular surface with high-speed air-puffs,which can assist doctors in diagnosing cataracts,glaucoma,and other eye diseases.Here,we investigate the mechanism of aerosol generation during the impact of high-speed airflow on the tear film of the ocular surface during the non-contact tonometer measurement.Highresolution photos provide visual evidence of aerosol generation by fluorescence photography.The size of the aerosol produced by the tonometer obeys a gamma distribution.The generation of aerosols is related to the instability of the tear film evolution and three stages of instability development are observed,including shear instability,Rayleigh-Taylor instability,and Rayleigh-Plateau instability.We conduct systematic experiments of high-speed airflow impacting liquid film on the model eye to verify the instability analysis with various liquid viscosities,airflow velocities,and gas densities.A method to suppress the generation of aerosols was proposed,and it was found that using high-viscosity hyaluronate sodium eyedrops can effectively reduce the amount of aerosol generated during non-contact tonometry measurements.