Abstract
Abstract. The WeIzmann Supercooled Droplets Observation on Microarray (WISDOM) is a new setup for studying ice nucleation in an array of monodisperse droplets for atmospheric implications. WISDOM combines microfluidics techniques for droplets production and a cryo-optic stage for observation and characterization of freezing events of individual droplets. This setup is designed to explore heterogeneous ice nucleation in the immersion freezing mode, down to the homogeneous freezing of water (235 K) in various cooling rates (typically 0.1–10 K min−1). It can also be used for studying homogeneous freezing of aqueous solutions in colder temperatures. Frozen fraction, ice nucleation active surface site densities and freezing kinetics can be obtained from WISDOM measurements for hundreds of individual droplets in a single freezing experiment. Calibration experiments using eutectic solutions and previously studied materials are described. WISDOM also allows repeatable cycles of cooling and heating for the same array of droplets. This paper describes the WISDOM setup, its temperature calibration, validation experiments and measurement uncertainties. Finally, application of WISDOM to study the ice nucleating particle (INP) properties of size-selected ambient Saharan dust particles is presented.
Highlights
In mixed phase clouds, water droplets remain stable in a supercooled state below 273 K and ice nucleates spontaneously as droplets reach the homogeneous freezing temperature, below 236 K (Pruppacher et al, 1998)
A vertical temperature gradient may develop between the top of the device, in contact with the inner ambient of the cryostage, and the bottom of the device, which is in contact with the cooling silver block
The new setup WeIzmann Supercooled Droplets Observation on Microarray (WISDOM) is based on microfluidics technology and its detailed validation is presented
Summary
Water droplets remain stable in a supercooled state below 273 K and ice nucleates spontaneously as droplets reach the homogeneous freezing temperature, below 236 K (Pruppacher et al, 1998). Observations and modeling studies suggest that immersion freezing is the prominent mechanism for heterogeneous ice formation in mixed phase clouds (Rosenfeld and Woodley, 2000; Ansmann et al, 2008; Field et al, 2012; Nagare et al, 2016; Possner et al, 2017). Ice particles affect the radiative and microphysical properties of mixed phase clouds and Earth’s hydrological cycle. They can influence present and possibly future climate (Hoose and Möhler, 2012; IPCC, 2013). Studying ice formation in clouds is important, and yet, due to its complexity, this process is still not fully understood and presents a great challenge to laboratory and field researchers as well as for clouds and climate modelers (DeMott et al, 2010; Schnaiter et al, 2016; Ullrich et al, 2017)
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