Micro-thermography for imaging ice crystal growth and nucleation inside non-transparent materials

Ice crystal growth and nucleation rate measurements are usually done using visible light micros-copy in liquid and transparent samples. Yet, the understanding of important practical problems depends on monitoring ice growth inside solid materials. For example, how rapid ice growth leads to structural damage of food, or how the final structure of cementitious materials is affect-ed by ice during curing. Imaging crystal growth inside solid materials cannot be done with visi-ble light and is intrinsically more challenging than visible light imaging. Thermography is a technique that uses thermal (Infra-red) cameras to monitor temperature changes in a material, and it has been used to provide qualitative description of ice propagation and nucleation with a low spatial resolution. Here, we describe a method that uses a novel micro-thermography system to image ice nucleation, growth and melting inside non-transparent samples. This method relies on two major components: a cold stage with accurate temperature control (±0.001 ºC) and a thermal camera with high spatial and temperature resolution. Our experiments include imaging of ice formation and growth in pure water first, and inside plant leaves used as a model for a non-transparent material. Ice growth rate of 2.2 mm/s was measured inside a plant leaf at -12 ºC and ice nucleation in single plant cells was observed as a hot spot having a diameter of 160 µm. The results presented here provide experimental proof that high-quality imaging of ice growth is achievable, thus paving the way to quantitative measurements of ice growth kinetics and ice nu-cleation in solid materials.