Tag: Lucio Isa

Transition from scattering to orbiting upon increasing the fuel concentration for an active Janus colloid moving at an obstacle–decorated interface published in Soft Matter

Transition from scattering to orbiting upon increasing the fuel concentration for an active Janus colloid moving at an obstacle–decorated interface
Carolina van Baalen, William E. Uspal, Mihail N. Popescu, and Lucio Isa
Soft Matter, 19, 8790-8801 (2023)
doi: 10.1039/D3SM01079A
repository: https://doi.org/10.3929/ethz-b-000610754

Efficient exploration of space is a paramount motive for active colloids in practical applications. Yet, introducing activity may lead to surface-bound states, hindering efficient space exploration. Here, we show that the interplay between self-motility and fuel-dependent affinity for surfaces affects how efficiently catalytically-active Janus microswimmers explore both liquid–solid and liquid–fluid interfaces decorated with arrays of similarly-sized obstacles. In a regime of constant velocity vs. fuel concentration, we find that microswimmer–obstacle interactions strongly depend on fuel concentration, leading to a counter-intuitive decrease in space exploration efficiency with increased available fuel for all interfaces. Using experiments and theoretical predictions, we attribute this phenomenon to a largely overlooked change in the surface properties of the microswimmers’ catalytic cap upon H2O2 exposure. Our findings have implications in the interpretation of experimental studies of catalytically active colloids, as well as in providing new handles to control their dynamics in complex environments.

Tuning Electrostatic Interactions of Colloidal Particles at Oil-Water Interfaces with Organic Salts published in Physical Review Letters

Tuning Electrostatic Interactions of Colloidal Particles at Oil-Water Interfaces with Organic Salts
Carolina van Baalen, Jacopo Vialetto, and Lucio Isa
Phys. Rev. Lett. 131, 128202 (2023)
doi: 10.1103/PhysRevLett.131.128202
arxiv: 2305.01929

Monolayers of colloidal particles at oil-water interfaces readily crystallize owing to electrostatic repulsion, which is often mediated through the oil. However, little attempts exist to control it using oil-soluble electrolytes. We probe the interactions among charged hydrophobic microspheres confined at a water-hexadecane interface and show that repulsion can be continuously tuned over orders of magnitude upon introducing nanomolar amounts of an organic salt into the oil. Our results are compatible with an associative discharging mechanism of surface groups at the particle-oil interface, similar to the charge regulation observed for charged colloids in nonpolar solvents.

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.

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.

David visits the Soft Materials and Interfaces lab at ETH Zurich

Carolina (left) and David (right) in the micro 3D printing room. (Image by D. Bronte Ciriza.)
From the 11th to the 19th of December 2021, David Bronte Ciriza visited the Soft Materials and Interfaces lab at ETH Zurich. During this visit, David learnt different techniques to fabricate elongated microparticles, and together with  Carolina van Baalen produced the ones that will be tested under different optical landscapes in David’s secondment in UCL London. This visit has also served as an opportunity to meet other early stage researchers at ETH Zurich working in related topics, allowing to discuss different ideas in the fields of microfabrication, optical tweezers, and active matter.