Fly Microdroplets Viewed Big: a Cryo-SEM Approach

There are numerous biological surfaces covered with secretory fluids, such as the adhesive pads of insects, sticky trichomes of plants, gustatory drops on plant leaves, lubricating coatings of vertebrate joints, etc. These fluids can be well visualised using light microscopy methods, however it is difficult to obtain detailed high magnification ultrastructural information on the morphology of these small-volume droplets. Conventional SEM preparation methods fail, because of the presence of vacuum and/or the instability of the subject fluid under the electron beam. Part of the problem can be resolved by the use of an Environmental SEM (ESEM) or an Atomic Force Microscopy (AFM), but the first method is restricted in its resolution and the second method has difficulty with resolving the complex geometry of the substrate and is rather slow for the analysis of these small, fast-evaporating droplets. Moreover, it is desirable in many applications to visualise the fluid behaviour at the contact interface between two surfaces, and this task cannot be resolved via the conventional application of either SEM or AFM methods. Herewith, we describe a Cryo-SEM approach successfully applied for visualisation of droplets of various fluids deposited on a substrate, or located at the interface between two surfaces.