Atomic surface structure made visible in viscous fluids
Stefan Weber, physicist at the Max Planck Institute for Polymer Research (MPIP), has presented a method to measure and image surface structures in different liquids with unprecedented resolution. His technique is based on measurements by atomic force microscopy (AFM) and enables to image details of atomic structures at surfaces. Until now this was only possible in highly fluid aqueous surroundings. Highly viscous liquids, e.g. oils or electrolytes, can hamper the imaging process.
Weber was now able to image the atomic structure of surface atoms in such a viscous environment - with surprisingly low noise and almost free of imaging artifacts. The liquid was even more than 30 times more viscous than water. In cooperation with researchers from University College Dublin (Ireland) Weber investigated the influence of the viscosity on the measurement noise. They published their results in the scientific journal Nanotechnology. "It is impressive to achieve such a high resolution in viscous liquids. These insights will help solving many practical challenges. Fuel cells or batteries, for example, contain highly viscous liquids," says Stefan Weber. His method could be used to investigate such systems on an atomic scale.
With modern AFM technology, atomic resolution imaging can be routinely achieved under certain conditions. Therefore an atomically sharp tip scans the surface of a material sample. The needle is a part of the so called cantilever that detects forces between tip and surface atoms. To achieve the highest possible resolution the needle is oscillating. Highly viscous liquids increase the damping of the cantilever motion which in turn leads to higher noise. This noise sets the ultimate limit to the resolution. Stefan Weber experimented with a highly viscous mixture of glycerol and water on a graphite surfaces in order to study the influence of imaging noise. "I was very surprised to get almost undisturbed images of the atomic structure of graphite", remembers Weber. He found that one of the reasons for the high resolution is that the amplitude of the tip motion is smaller than the diameter of the liquid molecules. This knowledge will help to address a number of unsolved questions in surface physics with high relevance, e.g. in energy applications such as batteries or fuel cells.
Since 2012, Stefan Weber is a project leader at the MPIP in the department of director Hans-Jürgen Butt. Already during his PhD and during his time as a Postdoc at UCD in Dublin, Weber worked with the AFM and developed new operation modes. At MPIP he has modified an AFM for low noise measurements in liquids and studies the principles of molecular self-organization at solid/liquid interfaces.