The molecular- and nano-scale
structure of bulk materials can be imaged only by transmission electron
microscopy (TEM), a technique that requires high vacuum (low pressure). Until
now, this constraint made TEM incompatible with ‘wet’ materials, such as
molecules dispersed in a liquid or gels swollen by a solvent. Working at the
Materials Research Science and Engineering Center (MRSEC) on Polymers at UMass
Amherst, Hoagland, McCarthy, and Russell successfully imaged the fine structure
of wet polymeric materials prepared with ionic liquids. These non-volatile
liquids dissolve or solvate an interesting spectrum of ‘soft’ polymer systems
of fundamental and technological interest. A key objective is to refine the
approach so that the dynamics of such systems can be imaged in situ and in real time, accomplishing tasks
akin to those known from optical microscopy but at much smaller length
scales. Shown below are two images,
taken 1 second apart, of an ionic liquid spanning a 1-mm opening in a carbon film. Suspended in the liquid are polymer-coated nanoparticle tracers. In the interval between images,
the thin film, originally spread over the opening, has destabilized by breaking
into ~100-nm droplets that distribute around the rim. Nano- and micro-scale wetting transitions are
key to numerous technologies ranging from microfluidics to ultralyophobic surfaces, but these transitions have
never been visualized experimentally.
