Structure formation in confined colloidal rod-sphere mixtures

Institutions
  • Department of Chemistry
  • WG Nielaba (Statistical and Computational Physics)
Publications
  Baumgärtel, Philipp; Lüders, Anton; Nielaba, Peter (2022): Group formation and collective motion of colloidal rods with an activity triggered by visual perception Physical Review E. American Physical Society (APS). 2022, 106, 014603. ISSN 2470-0045. eISSN 2470-0053. Available under: doi: 10.1103/PhysRevE.106.014603

Group formation and collective motion of colloidal rods with an activity triggered by visual perception

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We investigate the formation of cohesive groups and the collective diffusion of colloidal spherocylinders with a motility driven by a simple visual perception model. For this, we perform Brownian dynamics simulations without hydrodynamic interactions. The visual perception is based on sight cones attached to the spherocylinders and perception functions quantifying the visual stimuli. If the perception function of a particle reaches a predefined threshold, an active component is added to its motion. We find that, in addition to the opening angle of the cone of sight, the aspect ratio of the particles plays an important role for the formation of cohesive groups. If the elongation of the particles is increased, the maximum angle for which the rods organize themselves into such groups decreases distinctly. After a system forms a cohesive group, it performs a diffusive motion, which can be quantified by an effective diffusion coefficient. For increasing aspect ratios, the spatial expansion of the cohesive groups and the effective diffusion coefficient of the collective motion increase, while the number of active group members decreases. We also find that a larger particle number, a smaller propulsion velocity of the group members, and a smaller threshold for the visual stimulus increase the maximum opening angle for which cohesive groups form. Based on our results, we expect anisotropic particles to be of great relevance for the adjustability of visual perception-dependent motility.

Origin (projects)

  Lüders, Anton; Zander, Ellen; Nielaba, Peter (2021): Microscopic diffusion coefficients of dumbbell- and spherocylinder-shaped colloids and their application in simulations of crowded monolayers The Journal of Chemical Physics. American Institute of Physics (AIP). 2021, 155(10), 104113. ISSN 0021-9606. eISSN 1089-7690. Available under: doi: 10.1063/5.0060063

Microscopic diffusion coefficients of dumbbell- and spherocylinder-shaped colloids and their application in simulations of crowded monolayers

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We explore the diffusion properties of colloidal particles with dumbbell and spherocylinder shapes using a hydrodynamic bead-shell approach and additional Brownian dynamics (BD) simulations. By applying the bead-shell method, we determine empirical formulas for the microscopic diffusion coefficients. A comparison of these formulas and established experimental and theoretical results shows remarkable agreement. For example, the maximum relative discrepancy found for dumbbells is less than 5%. As an application example of the empirical formulas, we perform two-dimensional (2D) BD simulations based on a single dumbbell or spherocylinder in a suspension of spheres and calculate the resulting effective long-time diffusion coefficients. The performed BD simulations can be compared to quasi-2D systems such as colloids confined at the interface of two fluids. We find that the effective diffusion coefficient of translation mostly depends on the sphere area fraction ϕ, while the effective diffusion coefficient of rotation is influenced by the aspect ratio and ϕ. Furthermore, the effective rotational diffusion constant seems to depend on the particle shape with the corresponding implementation of the interactions. In the resolution limit of our methods, the shape-dependent differences of the microscopic diffusion coefficients and the long-time diffusion constant of translation are negligible in the first approximation. The determined empirical formulas for the microscopic diffusion coefficients add to the knowledge of the diffusion of anisotropic particles, and they can be used in countless future studies.

Origin (projects)

    Stuckert, Rouven; Lüders, Anton; Wittemann, Alexander; Nielaba, Peter (2021): Phase behaviour in 2D assemblies of dumbbell-shaped colloids generated under geometrical confinement Soft Matter. Royal Society of Chemistry (RSC). 2021, 17, pp. 6519-6535. ISSN 1744-683X. eISSN 1744-6848. Available under: doi: 10.1039/D1SM00635E

Phase behaviour in 2D assemblies of dumbbell-shaped colloids generated under geometrical confinement

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The structure formation and the phase behaviour of monolayers of dumbbell-shaped colloids are explored. For this, we conduct Langmuir–Blodgett experiments at the air/water interface and conventional Brownian dynamic simulations without hydrodynamic interactions. Using Voronoi tessellations and the probability density of the corresponding shape factor of the Voronoi cells p(ζ), the influence of the area fraction ϕ on the structure of the monolayers is investigated. An increase of the area fraction leads to a higher percentage of domains containing particles with six nearest neighbours and a sharper progression of p(ζ). Especially in dense systems, these domains can consist of aligned particles with uniform Voronoi cells. Thus, the increase of ϕ enhances the order of the monolayers. Simulations show that a sufficient enhancement of ϕ also impacts the pair correlation function which develops a substructure in its first maxima. Furthermore, we find that reducing the barrier speed in the Langmuir–Blodgett experiments enhances the final area fraction for a given target surface pressure which, in turn, also increases the percentage of particles with six nearest neighbours and sharpens the progression of p(ζ). Overall, the experiments and simulations show a remarkable qualitative agreement which indicates a versatile way of characterising colloidal monolayers by Brownian dynamics simulations. This opens up perspectives for application to a broad range of nanoparticle-based thin film coatings and devices.

Origin (projects)

  Kalb, Julian; Dorman, James A.; Gerigk, Melanie; Knittel, Vanessa; Plüisch, Claudia Simone; Trepka, Bastian; Lehr, Daniela; Wittemann, Alexander; Polarz, Sebastian; Schmidt-Mende, Lukas (2018): Influence of substrates and rutile seed layers on the assembly of hydrothermally grown rutile TiO2 nanorod arrays Journal of Crystal Growth. 2018, 494, pp. 26-35. ISSN 0022-0248. eISSN 1873-5002. Available under: doi: 10.1016/j.jcrysgro.2018.05.004

Influence of substrates and rutile seed layers on the assembly of hydrothermally grown rutile TiO2 nanorod arrays

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Rutile TiO2 nanorod arrays (NRAs) are applicable in various prospective technologies. Hydrothermal methods present a simple technique to fabricate such NRAs. In this report, we present the fabrication of seed layers for the hydrothermal growth of rutile TiO2 nanorods via sputter deposition, electron-beam evaporation, and sol-gel method and study the influence of each on the growth behavior. To satisfy the requirements of numerous applications, p-type silicon, platinum, levitating carbon membranes, a template made of polystyrene spheres, and commercial fluorine tin oxide (FTO) were employed as substrates. We document the structural properties of the TiO2 seed layers and describe the relationship between the characteristics of the seed crystals, the growth evolution, and the appearance of as-grown nanorods. Various growth stages of rutile TiO2 nanorods are compared depending on whether they are grown on polycrystalline TiO2 or FTO seed layers. In both cases, a homogenous TiO2 bottom layer is formed at the seed layer/substrate interface, which is essential for electronic applications such as hybrid solar cells. Detached NRAs illustrate the effect of rutile FTO and TiO2 on the porosity of this bottom layer. Further details about the formation process of this layer are obtained from the growth on confined seed layers fabricated by electron-beam lithography.

Origin (projects)

Funding sources
Name Finanzierungstyp Kategorie Project no.
SFB third-party funds research funding program 591/16
Further information
Period: since 30.06.2020