Colloidal electro-phoresis in the presence of symmetric and asymmetric electro-osmotic flow

CO2-induced drastic decharging of dielectric surfaces in aqueous suspensions

We study the influence of airborne CO2 on the charge state of carboxylate stabilized polymer latex particles suspended in aqueous electrolytes. We combine conductometric experiments interpreted in terms of Hessinger's conductivity model with Poisson-Boltzmann cell (PBC) model calculations with charge regulation boundary conditions. Without CO2, a minority of the weakly acidic surface groups are dissociated and only a fraction of the total number of counter-ions actually contribute to conductivity. The remaining counter-ions exchange freely with added other ions like Na+, K+ or Cs+. From the PBC-calculations we infer a corresponding pKa of 4.26 as well as a renormalized charge in reasonably good agreement with the number of freely mobile counter-ions. Equilibration of salt- and CO2-free suspensions against ambient air leads to a drastic de-charging, which exceeds by far the expected effects of to dissolved CO2 and its dissociation products. Further, no counter-ion-exchange is observed. To reproduce the experimental findings, we have to assume an effective pKa of 6.48. This direct influence of CO2 on the state of surface group dissociation explains our recent finding of a CO2-induced decrease of the ζ-potential and supports the suggestion of an additional charge regulation caused by molecular CO2. Given the importance of charged surfaces in contact with aqueous electrolytes, we anticipate that our observations bear substantial theoretical challenges and important implications for applications ranging from desalination to bio-membranes.

Electrokinetic effects of ambient and excess carbonization of dielectric surfaces in aqueous environments

The charge state of surfaces in contact with aqueous electrolytes is crucial for the performance and stability of dielectric surfaces in general and lyophobic colloids in particular. Thus far the role of adsorbed molecular CO2 remained largely unexplored. The aim of the present investigation is to study the de-charging and re-charging for two model surfaces upon addition of CO2 and/or 1:1 electrolytes (NaCl, HCl) under precisely controlled boundary conditions up to millimolar concentrations of additives. Starting from the salt- and CO2-free state, the ζ-potential magnitudes drop linearly with the logarithm of the CO2-concentrations over several orders of magnitude in CO2-concentrations. Hydrophobic Polystyrene nearly fully discharges, hydrophilic SiO2 reveals a 60% charge reduction. From the surface specific effects of instead adding NaCl or HCl, we discriminate and parameterize empirically the relative contribution of three individual mechanisms for decreasing the ζ-potential magnitudes (screening, pH-driven charge regulation, dielectric charge regulation) combining during CO2-addition. Moreover, depending on the achieved CO2-induced de-charging, the behavior upon subsequent addition of NaCl and HCl switches between two limiting cases. Screening dominates for surfaces in the native state without CO2, but a significant re-charging is observed for surfaces conditioned under excess CO2-concentrations.

Shaping the gradients driving phoretic micro-swimmers: influence of swimming speed, budget of carbonic acid and environment

pH gradient-driven modular micro-swimmers are investigated as a model for a large variety of quasi-two-dimensional chemi-phoretic self-propelled entities. Using three-channel micro-photometry, we obtain a precise large field mapping of pH at a spatial resolution of a few microns and a pH resolution of [Formula: see text] units for swimmers of different velocities propelling on two differently charged substrates. We model our results in terms of solutions of the three-dimensional advection-diffusion equation for a 1:1 electrolyte, i.e. carbonic acid, which is produced by ion exchange and consumed by equilibration with dissolved [Formula: see text]. We demonstrate the dependence of gradient shape and steepness on swimmer speed, diffusivity of chemicals, as well as the fuel budget. Moreover, we experimentally observe a subtle, but significant feedback of the swimmer's immediate environment in terms of a substrate charge-mediated solvent convection. We discuss our findings in view of different recent results from other micro-fluidic or active matter investigations. We anticipate that they are relevant for quantitative modelling and targeted applications of diffusio-phoretic flows in general and artificial micro-swimmers in particular.

Characterization of active matter in dense suspensions with heterodyne laser Doppler velocimetry

We present a novel approach for characterizing the properties and performance of active matter in dilute suspension as well as in crowded environments. We use Super-Heterodyne Laser-Doppler-Velocimetry (SH-LDV) to study large ensembles of catalytically active Janus particles moving under UV illumination. SH-LDV facilitates a model-free determination of the swimming speed and direction, with excellent ensemble averaging. In addition, we obtain information on the distribution of the catalytic activity. Moreover, SH-LDV operates away from walls and permits a facile correction for multiple scattering contributions. It thus allows for studies of concentrated suspensions of swimmers or of systems where swimmers propel actively in an environment crowded by passive particles. We demonstrate the versatility and the scope of the method with a few selected examples. We anticipate that SH-LDV complements established methods and paves the way for systematic measurements at previously inaccessible boundary conditions.

Non-monotonic concentration dependence of the electro-phoretic mobility of charged spheres in realistic salt free suspensions

Using super-heterodyne Doppler velocimetry with multiple scattering correction, we extend the optically accessible range of concentrations in experiments on colloidal electro-kinetics. Here, we measured the electro-phoretic mobility and the DC conductivity of aqueous charged sphere suspensions covering about three orders of magnitude in particle concentrations and transmissions as low as 40%. The extended concentration range for the first time allows the demonstration of a non-monotonic concentration dependence of the mobility for a single particle species. Our observations reconcile previous experimental observations made on other species over restricted concentration ranges. We compare our results to the state-of-the-art theoretical calculations using a constant particle charge and the carefully determined experimental boundary conditions as input. In particular, we consider the so-called realistic salt free conditions, i.e., we respect the release of counterions by the particles, the solvent hydrolysis, and the formation of carbonic acid from dissolved neutral CO₂. We also compare our results to previous results obtained under similarly well-defined conditions. This allows identification of three distinct regions of differing density dependence. There is an ascent during the build-up of double layer overlap, which is not expected by theory, an extended plateau region in quantitative agreement with theoretical expectation based on a constant effective charge and a sudden decrease, which occurs way before the expected gradual decrease. Our observations suggest a relation of the non-monotonic behavior to a decrease in particle charge, and we tentatively discuss possibly underlying mechanisms.

Demonstration of variable angle super-heterodyne dynamic light scattering for measuring colloidal dynamics

We demonstrate a prototype light scattering instrument combining a frequency domain approach to the intermediate scattering function from Super-Heterodyning Doppler Velocimetry with the versatility of a standard homodyne dynamic light scattering goniometer setup for investigations over a large range of scattering vectors. Comparing to reference experiments in correlation-time space, we show that the novel approach can determine diffusion constants and hence hydrodynamic radii with high precision and accuracy. Possible future applications are discussed shortly.

Numerical Study of Electro-Osmotic Fluid Flow and Vortex Formation

The phenomenon of electro-osmosis was studied by performing numerical simulations on the flow between parallel walls and at the nozzle microchannels. In this work, we propose a numerical approximation to perform simulations of vortex formation which occur after the passage of the fluid through an abrupt contraction at the microchannel. The motion of the charges in the solution is described by the Poisson-Nernst-Planck equations and used the generalized finite differences to solve the numerical problem. First, solutions for electro-osmotic flow were obtained for the Phan-Thien/Thanner model in a parallel walls channel. Later simulations for electro-osmotic flow were performed in a nozzle. The formation of vortices near the contraction within the nozzle was verified by taking into account a flow perturbation model.

Dynamics of Binary Active Clusters Driven by Ion-Exchange Particles

We present a framework to quantitatively predict the linear and rotational directed motion of synthetic modular microswimmers. To this end, we study binary dimers and characterize their approach motion as effective interactions within a minimal model. We apply this framework to the assembly of small aggregates composed of a cationic ion-exchange particle with up to five passive particles or anionic ion-exchange particles at dilute conditions. Particles sediment and move close to a substrate, above which the ion-exchange particles generate flow. This flow mediates long-range attractions leading to a slow collapse during which long-lived clusters of a few particles assemble. The effective interactions between unlike particles break Newton's third law. Depending on their symmetry, assemblies thus can become linear or circle swimmers, or remain inert (no directed motion).