Rotational diffusion of colloidal microspheres near flat walls.

Recently, colloids with an off-center fluorescent core and homogeneous composition have been developed to measure the rotational diffusivity of microparticles using 3D confocal microscopy in refractive index-matched suspensions. Here, we show that the same particles may be imaged using a standard fluorescence microscope to yield their rotational diffusion coefficients. Trajectories of the off-center core may be combined with known expressions for the correlation decay of particle orientations to determine an effective rotational diffusivity. For sedimented particles, we also find the rotational diffusivity about axes perpendicular and parallel to the interface by adding some bright field illumination and simultaneously tracking both the core and the particle. Trajectories for particles of different sizes yield excellent agreement with hydrodynamic models of rotational diffusion near flat walls, taking the sedimentation-diffusion equilibrium into account. Finally, we explore the rotational diffusivity of particles in crowded two-dimensional monolayers, finding a different reduction of the rotational motion about the two axes depending on the colloidal microstructure.

Trapping and manipulation of bubbles with holographic optical tweezers.

A methodology to manipulate bubbles and measure adhesion forces is presented and validated. Holographic optical tweezers are employed to establish a circular array of high intensity points to effectively trap a gas bubble within a liquid medium. This approach includes an efficient calibration protocol based on a theoretical framework for the calculation of optical forces using a ray tracing algorithm, which allows enhancing the versatility of optical manipulation to micro-objects with a lower refractive index than the surrounding medium. As an initial application, the adhesion force between two stable bubbles at different sizes is measured, finding a minimum when they have the same diameter.

Minimal numerical ingredients describe chemical microswimmers' 3-D motion.

The underlying mechanisms and physics of catalytic Janus microswimmers is highly complex, requiring details of the associated phoretic fields and the physiochemical properties of catalyst, particle, boundaries, and the fuel used. Therefore, developing a minimal (and more general) model capable of capturing the overall dynamics of these autonomous particles is highly desirable. In the presented work, we demonstrate that a coarse-grained dissipative particle-hydrodynamics model is capable of describing the behaviour of various chemical microswimmer systems. Specifically, we show how a competing balance between hydrodynamic interactions experienced by a squirmer in the presence of a substrate, gravity, and mass and shape asymmetries can reproduce a range of dynamics seen in different experimental systems. We hope that our general model will inspire further synthetic work where various modes of swimmer motion can be encoded via shape and mass during fabrication, helping to realise the still outstanding goal of microswimmers capable of complex 3-D behaviour.

Compression of colloidal monolayers at liquid interfaces: in situ vs. ex situ investigation.

The assembly of colloidal particles at liquid/liquid or air/liquid interfaces is a versatile procedure to create microstructured monolayers and study their behavior under compression. When combined with soft and deformable particles such as microgels, compression is used to tune not only the interparticle distance but also the underlying microstructure of the monolayer. So far, the great majority of studies on microgel-laden interfaces are conducted ex situ after transfer to solid substrates, for example, via Langmuir-Blodgett deposition. This type of analysis relies on the stringent assumption that the microstructure is conserved during transfer and subsequent drying. In this work, we couple a Langmuir trough to a custom-built small-angle light scattering setup to monitor colloidal monolayers in situ during compression. By comparing the results with ex situ and in situ microscopy measurements, we conclude that Langmuir-Blodgett deposition can alter the structural properties of the colloidal monolayers significantly.

Measurement of the capillary interaction force between Janus colloidal particles trapped at a flat air/water interface.

The capillary interaction force between spherical Janus particles trapped at the air-water interface is measured using a time-sharing optical tweezer (bond number ≪ 1). One face of the particles is hydrophilic, and the other one, hydrophobic. Measured force goes from almost pure quadrupolar to almost pure hexapolar interaction due to the three-phase contact line corrugation. Measured force curves are modeled as a sum of power laws, Ar-α + Br-ß + Cr-γ, obtained from an expansion in capillary multipoles. The mean values for the exponents of particle pairs of 3 µm are 〈α〉 = 5.05 ± 0.12, 〈ß〉 = 7.02 ± 0.03, and 〈γ〉 = 5.96 ± 0.03. For particles pairs of 5 µm, we find 〈α〉 = 5.02 ± 0.04, 〈ß〉 = 6.94 ± 0.06, and 〈γ〉 = 5.80 ± 0.05. In both cases, A < 0, B < 0, and C > 0.

Measurement of the force between uncharged colloidal particles trapped at a flat air/water interface.

The radial attraction between microspheres straddling at the air/water interface (Bond number ≪1), whose origin is the irregular shape of the contact line and its concomitant distortion of the water surface, is measured using two light beams of a time-sharing optical tweezer. The colloidal particles used to make the measurements are microspheres made of hydrophobically covered silica to reduce the electrostatic interactions to a minimum. The measured radial force goes as a quadrupolar power law, r-n, with n = 5.02 ± 0.18 and n = 5.04 ± 0.18 for particles of 3 µm and 5 µm, respectively. In both cases, the electrostatic interaction is negligible.