Enrichment of surface-active compounds in coalescing cloud drops I. Taraniuk 1, A. Kostinski 2, Y. Rudich 1 1 Department of Environmental Sciences, Weizmann Institute of Science, Rehovot 76100, Israel. 2 Michigan Technological University, USA. IAAR Tel Aviv

Questions we addressed How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? Why commonly assumed dilution wouldn’t block possible effects? What are these substances? Are they really present on drop’s surface?

Questions we addressed How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? Why commonly assumed dilution wouldn’t block possible effects? What are these substances? Are they really present on drop’s surface?

Cloud-and raindrops size distribution depend on surface tension of colliding drops

Evaporation of cloud- and raindrops depend on the presence of non-water molecules on the surface of drop

Drop-breakup and rate of fall depend on the surface tension of drop

Questions we addressed and answered How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? By modification of surface properties. Lets consider just one: surface tension. Why commonly assumed dilution wouldn’t block possible effects? What are these substances? Are they really present on drop’s surface?

Questions we addressed and answered How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? By modification of surface properties. Lets consider just one: surface tension. Why commonly assumed dilution wouldn’t block possible effects? What are these substances? Are they really present on drop’s surface?

Two main stages of drop growth in a cloud C loud C ondensation N uclei 0.01 μm Droplet 10 μm Droplet 50 μm Raindrop 1: Diffusion 2: Collision-coalescence 3000 μm

Two main stages of drop growth in a cloud C loud C ondensation N uclei 0.01 μm Droplet 10 μm Droplet 50 μm Raindrop 1: Diffusion 2: Collision-coalescence 3000 μm Pure water addition, increase of collective surface area and volume

Two main stages of drop growth in a cloud C loud C ondensation N uclei 0.01 μm Droplet 10 μm Droplet 50 μm Raindrop 1: Diffusion 2: Collision-coalescence 3000 μm Pure water addition, increase of collective surface area and volume No new water addition, collective volume and amounts of substances don’t change, surface area decreases - insoluble species concentration increases!

Two main stages of drop growth in a cloud C loud C ondensation N uclei 0.01 μm Droplet 10 μm Droplet 50 μm Raindrop 1: Diffusion 2: Collision-coalescence 3000 μm Pure water addition, increase of collective surface area and volume No new water addition, collective volume and amounts of substances don’t change, surface area decreases - insoluble species concentration increases! Coalescence concentrates water insoluble surfactants and leads to decrease of surface tension

Questions we addressed and answered How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? By modification of surface properties. Lets consider just one: surface tension. Why commonly assumed dilution wouldn’t block possible effects? There is no dilution at the stage of collision growth. There is enrichment! What are these substances? Are they really present on drop’s surface?

Questions we addressed and answered How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? By modification of surface properties. Lets consider just one: surface tension. Why commonly assumed dilution wouldn’t block possible effects? There is no dilution at the stage of collision growth. There is enrichment! What are these substances? Are they really present on drop’s surface?

Coalescence of drops in laboratory simulation:

Surface tension, mN - 1 m Number of coalescence events Surface tension decrease upon consecutive coalescence ( sessile drop with pendant drops preloaded by different surfactants) Adsorption isotherm for stearic acid

Questions we addressed and answered How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? By modification of surface properties. Lets consider just one: surface tension. Why commonly assumed dilution wouldn’t block possible effects? There is no dilution at the stage of collision growth. There is enrichment! What are these substances? They are surfactants; Water-insoluble are the most efficient because of their enrichment in coalescing drops. Are they really present and active on drop’s surface?

Questions we addressed and answered How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? By modification of surface properties. Lets consider just one: surface tension. Why commonly assumed dilution wouldn’t block possible effects? There is no dilution at the stage of collision growth. There is enrichment! What are these substances? They are surfactants; Water-insoluble are the most efficient because of their enrichment in coalescing drops. Are they really present and active on drop’s surface?

Sampling individual raindrops and measuring their surface tension

“Icicle” method

Surface area variation by varying drop volume cause variation of surface concentration of water-insoluble surfactants and consequently surface tension

Approximate surface area, cm Surface tension, mN -1 m Water-insoluble surfactants on raindrops decrease surface tension when surface area is decreased and uncover themselves

Questions we addressed and answered How could chemical substances affect the microphysics of clouds after activation of droplets, at the stage of collision growth? By modification of surface properties. Lets consider just one: surface tension. Why commonly assumed dilution wouldn’t block possible effects? There is no dilution at the stage of collision growth. There is enrichment! What are these substances? They are surfactants; Water-insoluble are the most efficient because of their enrichment in coalescing drops. Are they really present and active on drop’s surface? Yes, we found them acting at individual raindrops.

Conclusions Coalescence of drops concentrates water insoluble species on droplet’s surfaces and leads to decrease of surface tension. Water-insoluble surfactants were detected on the surface of individual raindrops. A chain of implications in cloud microphysics could be predicted and wait to be investigated.

Thank you! This work was supported, in part, by the US NSF this work was supported by NSF grant ATM and by the Israeli Science Foundation (Grant #162/05). We are grateful to W. Cantrell for helpful suggestions. Acknowledgements