Research Highlight of the Week – 12 October 2026
Particles move differently when a liquid is confined between nearby surfaces. Researchers from three CRC 1411 projects joined forces to investigate this change in a wedge-shaped cell whose wall spacing varied continuously. Direction-resolved dynamic light scattering and hydrodynamic analysis let them compare particle motion along the walls with motion across the liquid layer. The measurements show how the gap relative to particle size, and whether the walls attract or repel water, control the slowdown.
In simple words …A nanoparticle floating in water does not sit still. Water molecules bump into it from all sides, making it wander randomly. Scientists call this type of motion “diffusion”. Here, the researchers placed tiny glass beads in water between two glass walls and changed how far apart the walls were. They bounced a laser beam off the beads to see how quickly they move in different directions. As the gap became narrower, the balls moved more slowly along the walls. The important measure was the gap compared with the ball’s size. Motion away from a wall changed much less in the tested range. Walls that let water slide more easily also caused less slowing. These findings can help engineers understand particle movement through narrow channels and pores like those found in filters and materials used to separate mixtures.
What the team found
The team studied aqueous dispersions of spherical silica particles with diameters of around 100 and 300 nanometres. A wedge-shaped sample cell provided a controlled range of wall separations, while dynamic light scattering measured fluctuations associated with particle motion in directions parallel and perpendicular to the walls. As the gap narrowed, diffusion parallel to the walls slowed markedly. The results for both particle sizes followed the same pattern when expressed as the ratio of wall spacing to particle diameter. This shows that relative confinement, rather than the gap alone, governs the measured effect. Perpendicular diffusion stayed close to its unconfined value across the tested gaps, consistent with strong wall influence appearing mainly very near contact. The researchers also compared hydrophilic and hydrophobic walls. Parallel motion slowed less at hydrophobic surfaces, in line with greater liquid slip at those walls. Together, these measurements provide direction-specific evidence for hydrodynamic effects in confined particle transport.
The theoretical analysis connects these observations to the flow of liquid around moving particles: walls alter that flow and increase viscous resistance. The team also established a simple geometric relationship that combines the parallel and perpendicular diffusion components to reproduce measurements at intermediate angles. This provides a tested building block for future simulations of confined transport.
2/2Why this CRC 1411 collaboration matters—and where it leads
The CRC 1411 connection. The work combines precise thermophysical measurements with colloid synthesis, surface treatment and theoretical hydrodynamic analysis. It links particle size, wall spacing and surface chemistry to a transport property that matters in processing particulate products. For CRC 1411, this is a useful process-structure-property connection: measurements under controlled confinement can test models used to design pores in stationary phase materials for chromatography. The theoretical contribution links the measured directional differences to physical mechanisms and supplies a geometric model for future transport simulations.
Looking ahead. The next experiments can probe narrower gaps, higher particle concentrations and walls that are rough, charged or patterned. Comparing those data with simulations would show how far the present hydrodynamic explanation applies and help specify channel dimensions for a desired transport rate.
CRC 1411 co-authors of the study
Doctoral researchers: Pranay Kumar Chittem, Nicolás Salcedo Gálvez
Postdoctoral researcher: Paolo Malgaretti
Principal investigators: Jens Harting, Nicolas Vogel, Andreas P. Fröba
CRC projects involved
P. K. Chittem, N. S. Gálvez, P. Malgaretti, J. Harting, N. Vogel, P. S. Schmidt and A. P. Fröba, ‘Determination of particle diffusion in a confined fluid layer by dynamic light scattering,’ Colloids and Surfaces A: Physicochemical and Engineering Aspects 737, 139892 (2026). DOI: 10.1016/j.colsurfa.2026.139892.