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Self-propelled colloidal particles: single particle motion and collective behavior

Periodic Reporting for period 1 - SPCOLPS (Self-propelled colloidal particles: single particle motion and collective behavior)

Reporting period: 2016-09-01 to 2018-08-31

Active materials present us with interesting possibilities for the design of materials and devices, yet they also introduce some scientific and technological challenges. In particular, self-propelled colloidal particles or artificial microswimmers have been identified as a new class of matter with great potential, owing to their ability to mimic the collective motion of complex living systems, but also serve as model systems to study intrinsically out-of-equilibrium systems. Moreover, self-propelled particles (SPPs) can strikingly resemble the collective behavior of living microorganisms, by consuming internal energy or extracting energy from their local environment in order to generate their own motion. The present proposal aims at developing better model systems with tunable propulsion and intends to achieve this by two key ideas: (i) fluorescently labeled, refractive-index and density-matching active spherical particles,using tunable light control of the propulsion; (ii) fluorescently labeled self-propelled rods to study how shape anisotropy influences the collective motion. Systematic characterization of the proposed model systems will allow me to study when and how microscopic dynamics affect the macroscopic behavior of internally driven colloidal systems. Our results will shed light on how the dimensionality and shape affects the collective dynamics of SPPs. Potential applications lie in self-coating materials and there will be an increased understanding of the collective dynamics of active systems, with possible insights for biological systems.
The overall scientific aim of this proposal was to design, and fabricate in sufficient quantity of novel self-propelled particles with tunable propulsion in order to obtain a detailed real space insight on a single particle level in concentrated dispersion using confocal laser scanning microscopy. Systematic characterization of the proposed model systems will allow me to study when and how microscopic dynamics affect the macroscopic behavior of internal-driven colloidal systems. The research proposal consisted of two distinct but connected research topics. During the period of this project, the applicant was designed and developed three novel light-activated self-propelled particles (SPPs), namely light-activated Janus silica-titania, Janus titania-gold spherical particles, and iron-oxide tip-coated silica rods that give an exquisite control over the propulsion speed. Next, these systems were successfully used as model systems to study the collective behavior of micro-organisms.

The following projects were successfully completed:
i) Developed a novel, light-activated self-propelled particles (Janus TiO2-Au), in which we can, on demand, reverse the particle’s propulsion direction by exploiting the different photocatalytic activities. The control over the activity and the reversibility of direction by wavelength and intensity, combined with the interplay between attractions and repulsions of the individual units allows the colloidal assemblies to undergo both ‘fusion’ and ‘fission’ like transitions, mimicking aspects of multicellular behaviour.
ii) Studied active and passive mixtures, with an emphasis on the possibility to modulate structure of the passive particles by the presence of a few active ones.
iii) Developed light-activated self-propelled rods in order to mimic and study the collective behavior of collective motion of living microorganisms, i.e swarming motions, and bio-film formation.
iv) Confined self-propelled particles in cell-like environment (i.e. giant unilamellar vesicles).
v) Studied the dynamic interaction of active particles with the membrane.
One of the main scientific achievements of the project was developing novel light-driven self-propelled colloids, in which we can switch the propulsion direction on demand. The propulsion direction of an individual micro-swimmer can be reversed by exploiting the different catalytic activities of the particle under different wavelengths of light. Using two distinct wavelengths of light we can selectively activate which side of the particle triggers the decomposition reaction of the fuel molecules (H2O2) thereby controlling the propulsion direction.