The constant development of atmospheric close cells since a few decades permits today a wide range of in situ studies of nanomaterials in liquids, or liquids themselves in a Transmission Electron Microscope (TEM). However, Liquid Cell TEM fails to properly study liquid-vapor reactions of outermost importance in many societal subjects especially concerning water interactions with nanosystems, such as in the field of atmospheric chemistry or in pharmaceutical sciences. Such domains can fortunately be investigated in a dedicated Environmental TEM (FEI Titan ETEM), where the absence of sealing membranes around the sample allows direct exchanges between the vapor (gas) introduced into the microscope and a solid. Several studies exist of the hygroscopic behaviour of model or atmospheric aerosols in such ETEMs. A cryo-holder is used to cool down the specimen (typically in the range 0 to 18 °C) in order to reach the vapor-liquid equilibrium in the pressure range eligible in an ETEM (below 20 mbar).

We present here a new approach, where the sample is cooled by means of a micro-Peltier stage mounted on the type of a commercial MEMS-based ‘heating holder’ from DENSsolutionsTM, see Fig. 1a). This montage offers a bunch of advantages as compared to the above ETEM approach. One is its rapidity to reach a given relative humidity (linked to the temperature-pressure conditions), which allows to study very accurately the first stages of deliquescence (dissolution) or efflorescence (recrystallisation) of soluble solids, such as NaCl nanocubes as illustrated in Fig. b-d)*.

*Thanks are due to CLYM (www.clym.fr) for the access to the microscope. This study is supported by the French National Research Agency (ANR) under the ‘WATEM’ project n° ANR-20-CE42-0008.