An ion-exchange-resin-based microfluidic pump is introduced that utilizes trace amounts of ions to generate fluid flows. We show experimentally that our pump operates in almost deionized water for periods exceeding 24 h and induces fluid flows of μm s over hundreds of μm. This flow displays a far-field, power-law decay which is characteristic of two-dimensional (2D) flow when the system is strongly confined and of three-dimensional (3D) flow when it is not. Using theory and numerical calculations we demonstrate that our observations are consistent with electroosmotic pumping driven by μmol L ion concentrations in the sample cell that serve as 'fuel' to the pump. Our study thus reveals that trace amounts of charge carriers can produce surprisingly strong fluid flows; an insight that should benefit the design of a new class of microfluidic pumps that operate at very low fuel concentrations.

We report an experimental study on ion-exchange-based modular microswimmers in low-salt water. Cationic ion-exchange particles and passive cargo particles assemble into self-propelling complexes, showing self-propulsion at speeds of several micrometers per second over extended distances and times. We quantify the assembly and speed of the complexes for different combinations of ion-exchange particles and cargo particles, substrate types, salt types and concentrations, and cell geometries. Irrespective of the experimental boundary conditions, we observe a regular development of the assembly shape with increasing number of cargo. Moreover, the swimming speed increases stepwise upon increasing the number of cargo and then saturates at a maximum speed, indicating the active role of cargo in modular swimming. We propose a geometric model of self-assembly to describe the experimental observations in a qualitative way. Our study also provides some constraints for future theoretical modeling and simulation.

Abstract:We assemble charged colloidal spheres at deliberately chosen locations on a charged unstructured glass substrate utilizing ion exchange based electro-osmotic micro-pumps.Using microscopy, a simple scaling theory and Brownian Dynamics simulations, we systematically explore the control parameters of crystal assembly and the mechanisms through which they depend on the experimental boundary conditions. We demonstrate that crystal quality depends crucially on the assembly distance of the colloids. This is understood as resulting from the competition between inward transport by the electro-osmotic pump flow and the electro-phoretic outward motion of the colloids. Optimized conditions include substrates of low and colloids of large electro-kinetic mobility. Then a sorting of colloids by size is observed in binary mixtures with larger particles assembling closer to the ion exchanger beads. Moreover, mono-sized colloids form defect free single domain crystals which grow outside a colloid-free void with facetted inner crystal boundaries centred on the ion exchange particle. This works remarkably well, even with irregularly formed ion exchange resin splinters.

Frequency domain super-heterodyne laser light scattering is utilized in a low angle integral measurement configuration to determine flow and diffusion in charged sphere suspensions showing moderate to strong multiple scattering. We introduce an empirical correction to subtract the multiple scattering background and isolate the singly scattered light. We demonstrate the excellent feasibility of this simple approach for turbid suspensions of transmittance T 0.4. We study the particle concentration dependence of the electro-kinetic mobility in low salt aqueous suspension over an extended concentration regime and observe a maximum at intermediate concentrations.We further use our scheme for measurements of the self-diffusion coefficients in the fluid samples in the absence or presence of shear, as well as in polycrystalline samples during crystallization and coarsening. We discuss the scope and limits of our approach as well as possible future applications. PACS: if neededCorresponding author: Denis Botin dbotin@uni-mainz.de 2 INTRODUCTIONMultiple scattering (MS) strongly affects studies of turbid colloidal suspensions using Laser light scattering. Depending on the degree of MS, several different sophisticated approaches have been taken to use, correct for, or suppress MS in studies on suspension dynamics. At very large turbidities and small optical path lengths, suspension dynamics can be determined using diffusive wave spectroscopy (DWS) [1[2]]. In the regime of low to moderate multiple scattering, index matching is regularly employed [3, 4, 5], but it is not easily applicable in water based systems. Further, optical path lengths may be shortened by utilizing fibre optics [6]. In addition, cross correlation schemes [7] were pioneered by Phillies in the early eighties [8] and further developed to two-color or 2D cross correlation schemes [9, 10]. This way the methods and theory developed for (single) dynamic light scattering in the time domain [11] could be used also in turbid samples. Alternatively, in frequency domain, special mode selective heterodyne instrumentation was developed and applied to Brilluoin scattering [12,13,14]. Both cross correlation and mode selection, however, afford complex instrumentation and data evaluation schemes. A much simpler instrumentation is needed for the statistical analysis of heterodyne speckle fields taken at different times providing information about the velocity field in the fluid in a given plane perpendicular to the optical axis [15]. This information can be extracted, either by measuring their cross-correlation function or by recovering the power spectrum corresponding to the difference between the two speckle fields. Multiple scattering in this approach is minimized by using confocal geometry [16]. Yet another simple approach is path length resolved low coherence interferometry, which also is a static heterodyne technique [17,18]. There a certain small path length can be selected which corresponds to a single back-scattering event. Then only singly scattered light i...

We characterize the electro-phoretic motion of charged sphere suspensions in the presence of substantial electro-osmotic flow using a recently introduced small angle super-heterodyne dynamic light scattering instrument (ISASH-LDV). Operation in integral mode gives access to the particle velocity distribution over the complete cell cross-section. Obtained Doppler spectra are evaluated for electro-phoretic mobility, wall electro-osmotic mobility and particle diffusion coefficient. Simultaneous measurements of differing electro-osmotic mobilities leading to asymmetric solvent flow are demonstrated in a custom made electro-kinetic cell fitting standard microscopy slides as exchangeable sidewalls. Scope and range of our approach are discussed demonstrating the possibility of an internal calibration standard and using the simultaneously measured electro-kinetic mobilities in the interpretation of microfluidic pumping experiment involving an inhomogeneous electric field and a complex solvent flow pattern.freely suspended far off any walls [10,11,12].The standard method to measure the electro-osmotic mobility is the streaming potential [1, 3] but also microscopic determinations of suspension flow-profiles in standard or custom-made microelectro-phoresis cells have been performed [13,14,15]. We here adapt the latter approach but combine a suitable electro-kinetic cell with a recently introduced versatile version of LDV [16]. This is further motivated by the fact that in many technical applications and in particular contemporary micro-fluidic experiments both mobilities are needed simultaneously to interprete the experimental findings. A typical examples are micro-fluidic applications like electrophoretic [17], diffusio-phoretic [18] and osmotic trapping [19] or single particle electro-kinetic experiments of sedimented and optically trapped particles [20,21,22,23 ]. Here often inhomogeneous electric fields are employed which in addition may vary in time. In phoretic micro-swimming and electroosmotic pumping [24,25,26] typical experiments involve locally generated diffusio-electric fields. [27,28]. Also here we are relying on well known values for the electro-kinetic mobilities of substrates and transported particles. The electro-osmotic mobilities of several high purity materials are well documented in literature. However, for many standard materials like glass, quartz, PMMA or PDMS results show a large spread of values depending on cleaning and conditioning conditions. Moreover, in microfluidics and other applications, coated cell walls are frequently used, which introduces additional dependence of µeo on the preparation protocol followed. In this situation, it would be highly desirable to have a fast and reliable characterization method available, which in addition would also yield the particle mobility under exactly the same conditions.Our integral small angle super-heterodyne dynamic light scattering instrument (ISASH-LDV) was designed to determine the electro-phoretic mobilities of charged colloidal spheres in aqueous...