Month: March 2023

3-D rotation tracking from 2-D images of spherical colloids with textured surfaces published in Soft Matter

3-D rotation tracking from 2-D images of spherical colloids with textured surfaces
Vincent Niggel, Maximilian R. Bailey, Carolina van Baalen, Nino Zosso, and Lucio Isa
Soft Matter, 19, 8790-8801 (2023)
doi: 10.1039/d3sm00076a
repository: https://doi.org/10.3929/ethz-b-000610754

Tracking the three-dimensional rotation of colloidal particles is essential to elucidate many open questions, e.g. concerning the contact interactions between particles under flow, or the way in which obstacles and neighboring particles affect self-propulsion in active suspensions. In order to achieve rotational tracking, optically anisotropic particles are required. We synthesise here rough spherical colloids that present randomly distributed fluorescent asperities and track their motion under different experimental conditions. Specifically, we propose a new algorithm based on a 3-D rotation registration, which enables us to track the 3-D rotation of our rough colloids at short time-scales, using time series of 2-D images acquired at high frame rates with a conventional wide-field microscope. The method is based on the image correlation between a reference image and rotated 3-D prospective images to identify the most likely angular displacements between frames. We first validate our approach against simulated data and then apply it to the cases of: particles flowing through a capillary, freely diffusing at solid–liquid and liquid–liquid interfaces, and self-propelling above a substrate. By demonstrating the applicability of our algorithm and sharing the code, we hope to encourage further investigations in the rotational dynamics of colloidal systems.

Presentation by Davide Breoni at the DPG Spring Meeting of the Condensed Matter Section in Dresden, Germany, 26th – 31st of March 2023

Davide presented his work at the 2023 DPG Spring Meeting of the Condensed Matter Section in Dresden, Germany. His presentation “Active Brownian Particles in a disordered motility environment“, focuses on the study of  Janus particles in a speckle light field.

Sorting of heterogeneous colloids by AC-dielectrophoretic forces in a microfluidic chip with asymmetric orifices published in Journal of Colloid and Interface Science

Sorting of heterogeneous colloids by AC-dielectrophoretic forces in a microfluidic chip with asymmetric orifices
Kai Zhao, Minghan Hu, Carolina van Baalen, Laura Alvarez and Lucio Isa
Journal of Colloid and Interface Science, 634, 921-929 (2023)
doi: 10.1016/j.jcis.2022.12.108
repository: https://www.research-collection.ethz.ch/handle/20.500.11850/589657

Hypothesis
The synthesis of compositionally heterogeneous particles is central to the development of complex colloidal units for self-assembly and self-propulsion. Yet, as the complexity of particles grows, synthesis becomes more prone to “errors”. We hypothesize that alternating-current dielectrophoretic forces can efficiently sort Janus particles, as a function of patch size and material, and colloidal dumbbells by size.

Experiments
We prepared Janus particles with different patch size and material by physical vapor deposition and colloidal dumbbells via capillarity-assisted particle assembly. We then performed sorting experiments in a microfluidic chip comprising electrodes with asymmetric orifices, specifically exploiting the dielectric contrast between different portions of the particles or their size difference to steer them towards different outlets.

Findings
We calculated that the DEP force for Janus particles may switch from positive to negative as a function of composition at a critical AC frequency, thus enabling sorting different particles crossing the electrodes’ region. The predictions are confirmed by optical microscopy experiments. We also show that intact and “broken” dumbbells can be simply separated as they experience different DEP forces. The integration of multiple asymmetric orifices leads a larger zone with high field gradient to increase separation efficiency and makes it a promising tool to select precise particle populations, isolating fractions with narrowly distributed characteristics.