The characterization of biological and inorganic materials by determining their three-dimensional structure in conditions closer to their native state is a major challenge of technological research. Environmental scanning electron microscopy (ESEM) provides access to the observation of hydrated samples in water environments. Here, we present a specific device for ESEM in the scanning transmission electron microscopy mode, allowing the acquisition of tilt-series suitable for tomographic reconstructions. The resolution which can be obtained with this device is first determined. Then, we demonstrate the feasibility of tomography on wet materials. The example studied here is hydrophilic mesoporous silica (MCM-41). Finally, the minimum thickness of water which can be detected is calculated from Monte Carlo simulations and compared with the resolution expected in the tomograms.

Two related aspects of nano-droplet condensation and droplets coalescence are studied for droplets on self-supported thin water films. The experiments are conducted in the environmental scanning electron microscope using wet scanning transmission electron microscopy. Favorable condensation sites are examined and in-situ position-controlled condensation experiments are conducted. The interaction among condensed multi-droplets as well as between a single droplet and the underneath nano-thick water film are dynamically examined with 10nm lateral resolution. The droplet round shape is reshaped to flat-like facets in-between droplets of 30-230 nm separation. Dynamic imaging of a few minutes duration shows a delayed coalescence effect, being explained by increased droplet-droplet electrostatic interaction relative to van der Waals interaction.