This is a collaborative work with the group of Prof. D.A. Beysens at PMMH, ESPCI‑ParisTech, Paris (France). Actually, the experiments have been done in his laboratory. Frost formation occurs when humid air is in contact with a surface at a temperature well below the freezing point. We focus on the formation of a supercooled liquid condensate (called also Breath Figure [D. Beysens, C. R. Phys. 7 (2006) 1082, and references therein]) and its subsequent freezing. We perform the experiments on different surfaces at -9^oC by streaming water vapor air at 13^oC.

Our main contribution was in the experimental part (J. Guadarrama-Cetina) and in the analysis of the data (W. González-Viñas and J. Guadarrama-Cetina).

The main findings were that the condensation takes place as liquid water, and after some time in some places the water starts to freeze. When there is coexistence of supercooled liquid and solid water, the Wegener-Bergeron-Findeisen process [T. Bergeron, Int. Union of Geodesy and Geophysics 156 (1935)] takes place, which leads to evaporation of liquid water droplets. This phenomenon is driven by the difference in saturated vapor pressure between ice crystals and supercooled liquid water droplets. Hence, we observed that ice crystals grow at expenses of diminishing liquid droplets. If the mean distance between water droplets is smaller than a critical value, we show that the supercooled liquid droplets have not enough time to evaporate before an ice crystal hits a neighboring droplet. When this happens, a percolation phenomenon arises and the Breath Figure freezes quickly.

• • J. Guadarrama-Cetina et al. EPL 101 (2013), 16009.

  • J. Guadarrama-Cetina, Ph.D. thesis. Universidad de Navarra (2013).

We are grateful to Dennis Lamb for valuable insights on cloud physics and S. Berkowicz for a critical reading of the manuscript. We gratefully acknowledge the comments and suggestions of M. Robert. This work was partly supported by the Spanish Government (MEC grant no. FIS2011-24642). JGC acknowledges financial support from the ''Asociación de Amigos de la Universidad de Navarra''.

Snapshots of evaporation of water drops (A and B), growth of ice (see percolation path 0, 1, 2). The scale bar is 50 μm:

Snapshots with two percolation paths highlighted (0, 1, 2, 2' and 0, 1, 3, 3'). The scale bar is 50 μm: