Height distribution and orientation of colloidal dumbbells near a wall

Microscopic particles found in engineering, materials science and soft matter physics are often confined: this restricts their motion and may therefore change their behavior. Spherical particles under confinement have been studied extensively, but less is known for particles that aren’t perfect spheres. In this work, together with Stefania Ketzetzi and coworkers, I’ve looked at how micron-sized colloidal dumbbell particles dance on top of a glass substrate.

Extracting 3D positions from 2D microscopy images using Digital Inline Holographic Microscopy. a) We use a simple microscope with an LED of a single wavelength, in this way, we can build a cheap but powerful holographic microscope. b) The dumbbell particles move on top of the glass substrate, we extract their height with respect to spheres that are fixed on the wall. In that way, we can determine their 3D position accurately. c) Example of a microscopy image with stuck particles in blue and moving particles in yellow. d) The 3D position is found by fitting a scattering model, the agreement between model and data is very good.

We’ve constructed a very cheap but powerful holographic microscope using a single-wavelength LED mounted on an existing microscope. In this way, we can fit a scattering model to the images and extract the precise 3D location of the particles. By using reference particles that are fixed on the glass substrate, we also determine the position and tilt of the substrate very accurately.

Schematics of the position and orientation of two dumbbell particles in time (left: 2.2 micrometer long, right: 4.2 micrometer long) as determined from our experiments.

From our experiments, we find that the interplay of gravitational and electrostatic forces causes the smaller dumbbells to show large out-of-plane rotations, while the larger dumbbell shows only small fluctuations. Surprisingly, the smaller dumbbell particles never lie completely flat with respect to the substrate! Our results highlight the complex behavior of non-spherical particles close to walls and we hope that this will aid in developing quantitative frameworks for arbitrarily-shaped particle dynamics in confinement. For the full story, you can find the article here:

Height distribution and orientation of colloidal dumbbells near a wall
Ruben W. Verweij, Stefania Ketzetzi, Joost de Graaf, and Daniela J. Kraft
Phys. Rev. E 102, 062608