Uni- and Bidirectional Rotation and Speed Control in Chiral Photonic Micromotors Powered by Light.

Liquid crystalline polymers are attractive materials for untethered miniature soft robots. When they contain azo dyes, they acquire light-responsive actuation properties. However, the manipulation of such photoresponsive polymers at the micrometer scale remains largely unexplored. Here, uni- and bidirectional rotation and speed control of polymerized azo-containing chiral liquid crystalline photonic microparticles powered by light is reported. The rotation of these polymer particles is first studied in an optical trap experimentally and theoretically. The micro-sized polymer particles respond to the handedness of a circularly polarized trapping laser due to their chirality and exhibit uni- and bidirectional rotation depending on their alignment within the optical tweezers. The attained optical torque causes the particles to spin with a rotation rate of several hertz. The angular speed can be controlled by small structural changes, induced by ultraviolet (UV) light absorption. After switching off the UV illumination, the particle recovers its rotation speed. The results provide evidence of uni- and bidirectional motion and speed control in light-responsive polymer particles and offer a new way to devise light-controlled rotary microengines at the micrometer scale.

Correction: Electrokinetics and behavior near the interface of colloidal particles in non-polar dispersions.

Correction for 'Electrokinetics and behavior near the interface of colloidal particles in non-polar dispersions' by Manoj Prasad et al., Soft Matter, 2017, 13, 5604-5612, DOI: .

Correction: Space charge limited release of charged inverse micelles in non-polar liquids.

Correction for 'Space charge limited release of charged inverse micelles in non-polar liquids' by Manoj Prasad et al., Phys. Chem. Chem. Phys., 2016, 18, 19289-19298, DOI: .

Polarity-Dependent Adsorption of Inverse Micelles.

The adsorption of charged inverse micelles at the electrode-liquid interface has an important effect on field screening and on the voltage drop over diffuse double layers. Recently, we analyzed the behavior of inverse micelles in a nonpolar liquid close to this electrode-liquid interface. For the fluorocarbon/surfactant system under study, we are in the limit of slow adsorption and negligible desorption of inverse micelles on the electrodes. Upon applying a voltage step, this results in a measurable Stern layer buildup in the time range of hours clearly distinguishable from the diffuse double layer buildup, which happens in less than 1 s. This Stern layer buildup manifests itself by a shift in the voltage drop from the diffuse double layer to the Stern layer until the voltage drop over the Stern layers reaches the applied voltage, leaving a zero bulk field without the diffuse double layer. New measurements of the transients of Stern layer buildup show that the buildup of charges in the Stern layer is more complex. We explain the observed transient behavior by introducing an asymmetry in the adsorption rate of charged inverse micelles. We provide an equivalent electrical network, an analytical solution to explain the behavior in more detail, and simulations within the diffuse double layer limit for a range of adsorption rates.

Low coherence digital holography microscopy based on the Lorenz-Mie scattering model.

We demonstrate the use of low spatial and temporal coherence holography microscopy, based on the Lorenz-Mie model, using the standard tungsten-halogen lamp present in an inverted microscope. An optical model is put forward to incorporate the effect of spectral width and different incidence angles of the incident light determined by the aperture at the back focal plane of the condenser lens. The model is validated for 899 nm diameter polystyrene microspheres in glycerol, giving a resolution of 0.4% for the index of refraction and 2.2% for the diameter of the particles.

Electrokinetics and behavior near the interface of colloidal particles in non-polar dispersions.

The electrokinetics and charging of nonpolar colloidal dispersions subjected to a voltage are investigated by electric current and optical measurements. From electric current measurements in response to an alternating triangular voltage with a peak value of a few hundred volts, we find that polystyrene toner particles are compacted near the electrodes and their charge increases by more than a factor of 20. The important increase of charge is interpreted by a mechanism in which counter charges, which are originally at the particle surface, are desorbed. Optical measurements performed under a dc voltage of the order of a few hundred volts demonstrate that the charge of the particles can again decrease or even be inverted. These phenomena are attributed to the movement of counter charged species from the interface layers onto the surface of the particles. The findings of this study are relevant for electrophoretic displays and liquid toner printing.

Optical Manipulation of Single Magnetic Beads in a Microwell Array on a Digital Microfluidic Chip.

The detection of single molecules in magnetic microbead microwell array formats revolutionized the development of digital bioassays. However, retrieval of individual magnetic beads from these arrays has not been realized until now despite having great potential for studying captured targets at the individual level. In this paper, optical tweezers were implemented on a digital microfluidic platform for accurate manipulation of single magnetic beads seeded in a microwell array. Successful optical trapping of magnetic beads was found to be dependent on Brownian motion of the beads, suggesting a 99% chance of trapping a vibrating bead. A tailor-made experimental design was used to screen the effect of bead type, ionic buffer strength, surfactant type, and concentration on the Brownian activity of beads in microwells. With the optimal conditions, the manipulation of magnetic beads was demonstrated by their trapping, retrieving, transporting, and repositioning to a desired microwell on the array. The presented platform combines the strengths of digital microfluidics, digital bioassays, and optical tweezers, resulting in a powerful dynamic microwell array system for single molecule and single cell studies.

Different Types of Charged-Inverse Micelles in Nonpolar Media.

Over the last few years, the electrodynamics of charged inverse micelles (CIMs) in nonpolar liquids and the generation mechanism and properties of newly generated CIMs have been studied extensively for the model system of polyisobutylene succinimide in dodecane. However, the newly generated CIMs, which accumulate at the electrodes when a continuous voltage is applied, behave differently compared to the regular CIMs present in equilibrium in the absence of a field. In this work, we use transient current measurements to investigate the behavior of the newly generated CIMs when the field is reduced to zero or reversed. We demonstrate that the newly generated CIMs do not participate in the diffuse double layer near the electrode formed by the regular CIMs but form an interface layer at the electrode surface. A fraction of the newly generated negative CIMs can be released from this interface layer when the field there becomes zero. The findings of this study provide a better understanding of fundamental processes in nonpolar liquids and are relevant for applications such as electronic ink displays and liquid toner printing.

Space charge limited release of charged inverse micelles in non-polar liquids.

Charged inverse micelles (CIMs) generated during a continuous polarizing voltage between electrodes in the model system of polyisobutylene succinimide in dodecane do not populate a diffuse double layer like CIMs present in equilibrium (regular CIMs), but instead end up in interface layers. When the applied voltage is reversed abruptly after a continuous polarizing voltage step, two peaks are observed in the transient current. The first peak is due to the release of regular CIMs from the diffuse double layers formed during the polarizing voltage step, which is understood on the basis of the Poisson-Nernst-Planck equations. The second peak is due to the release of a small fraction of generated negative CIMs from the interface layer. A model based on space charge limited release of the generated negative CIMs from the interface layer is presented and the results of the model are compared with several types of measurements. For the situation in which the bulk is deprived of regular CIMs and neutral inverse micelles, the results of the model are in agreement with the experimental results. However, for the situation in which regular CIMs and neutral inverse micelles are present, the model shows discrepancies with the experiment for high voltages and high charge contents. These discrepancies are attributed to electrohydrodynamic flow caused by local variations in the electric field at the vicinity of the electrodes, which occur during the reversal voltage. Also the long term decrease of the amount of released generated CIMs is studied and it is found that the presence of regular CIMs and neutral inverse micelles speeds up the decrease. This study provides a deeper insight in the electrodynamics of CIMs and is relevant for various applications in non-polar liquids.

Methodological challenges of optical tweezers-based X-ray fluorescence imaging of biological model organisms at synchrotron facilities.

Recently, a radically new synchrotron radiation-based elemental imaging approach for the analysis of biological model organisms and single cells in their natural in vivo state was introduced. The methodology combines optical tweezers (OT) technology for non-contact laser-based sample manipulation with synchrotron radiation confocal X-ray fluorescence (XRF) microimaging for the first time at ESRF-ID13. The optical manipulation possibilities and limitations of biological model organisms, the OT setup developments for XRF imaging and the confocal XRF-related challenges are reported. In general, the applicability of the OT-based setup is extended with the aim of introducing the OT XRF methodology in all research fields where highly sensitive in vivo multi-elemental analysis is of relevance at the (sub)micrometre spatial resolution level.

Joule heating monitoring in a microfluidic channel by observing the Brownian motion of an optically trapped microsphere.

Electric fields offer a variety of functionalities to Lab-on-a-Chip devices. The use of these fields often results in significant Joule heating, affecting the overall performance of the system. Precise knowledge of the temperature profile inside a microfluidic device is necessary to evaluate the implications of heat dissipation. This article demonstrates how an optically trapped microsphere can be used as a temperature probe to monitor Joule heating in these devices. The Brownian motion of the bead at room temperature is compared with the motion when power is dissipated in the system. This gives an estimate of the temperature increase at a specific location in a microfluidic channel. We demonstrate this method with solutions of different ionic strengths, and establish a precision of 0.9 K and an accuracy of 15%. Furthermore, it is demonstrated that transient heating processes can be monitored with this technique, albeit with a limited time resolution.

Charging Dynamics of Aerosol OT Inverse Micelles.

Aerosol OT (AOT) is a commonly used surfactant and charging agent in nonpolar liquids. Properties such as the conductivity of AOT suspensions in nonpolar liquids and the behavior of charged AOT inverse micelles at interfaces have been studied recently, but still little is known about the generation dynamics of charged AOT inverse micelles. In this article, the generation dynamics of charged AOT inverse micelles in dodecane are investigated with transient current measurements. At low applied voltages, the generation rate is sufficiently fast to maintain the equilibrium concentration of charged inverse micelles, such that the current scales proportionally with the applied voltage. However, above a threshold voltage the current becomes limited by the generation of charged inverse micelles. Al2O3-coated electrodes are used to achieve these high-voltage current measurements while reducing surface generation currents. The dependency of the resulting generation-limited currents with the micelle concentration and the liquid volume is compatible with a bulk disproportionation mechanism. The measured currents are analyzed using a model based on drift, generation, and recombination of charged inverse micelles and the corresponding generation and recombination rates of charged AOT inverse micelles have been determined.

Characterizing and tracking individual colloidal particles using Fourier-Bessel image decomposition.

We use Fourier-Bessel Image Decomposition (FBID) of microscopy images to investigate the size, refractive index and 3-dimensional position of individual colloidal microspheres. With measurements of monodisperse polystyrene and poly(methyl methacrylate) particles we achieve a resolution of 1% in size and 0.2% in refractive index for a single image which is sufficient for accurate in situ characterization of polydisperse colloids. Also the binding of avidin molecules to individual biotinylated polystyrene particles is resolved. Finally, the FBID method offers a straightforward approach to 3-dimensional out-of-focus tracking. Here, the z-position of a freely diffusing particle is calculated by applying the statistics of Brownian motion to its set of Fourier-Bessel coefficients.

Single-mode air-clad liquid-core waveguides on a surface energy patterned substrate.

We demonstrate a new kind of single-mode micro-optical waveguide based on a liquid core on top of solid substrate and air cladding. The liquid is held in place by surface tension and patterned surface energy on the substrate. Due to the smooth nature of the liquid/air interface down to the molecular level, low scattering losses are expected. Losses were measured to be -6.0 and -7.8 dB/cm for, respectively, 12 and 9 µm wide waveguides.

Investigation of various types of inverse micelles in nonpolar liquids using transient current measurements.

Transient current measurements are used to characterize a wide variety of charge carriers in nonpolar liquids. The transient current method allows us to obtain both the concentration and mobility of charge carriers and therefore also the hydrodynamic radius using Stokes' law. In this article, five different surfactants in dodecane are investigated: OLOA11K, Solsperse13940, Span80, Span85, and AOT. We show that different types of currents are observed depending on the size of the inverse micelles. For large inverse micelles such as for OLOA11K, Solsperse13940, and Span80, the measurement of the transient current is straightforward because of the low steady-state current level. However, for small inverse micelles such as AOT and Span85, the current from the generation of charges is much larger such that high voltages, a small distance between the electrodes, and dielectric coatings on the electrodes are required to measure the signal related to the initially present charged inverse micelles. The estimated hydrodynamic radii of AOT and Span85, the two smallest inverse micelles, are in good agreement with the values reported in the literature. The comparison of the transient currents with simulations indicates that the dynamics of the charge transport are well-understood.

The micropatterning of layers of colloidal quantum dots with inorganic ligands using selective wet etching.

The micropatterning of layers of colloidal quantum dots (QDs) stabilized by inorganic ligands is demonstrated using PbS core and CdSe/CdS core/shell QDs. A layer-by-layer approach is used to assemble the QD films, where each cycle involves the deposition of a QD layer by dip-coating, and the replacement of the native organic ligands by inorganic moieties, such as OH(-) and S(2-), followed by a thorough cleaning of the resulting film. This results in a smooth and crack-free QD film on which a photoresist can be spun. The micropatterns are defined by a positive photoresist, followed by the removal of uncovered QDs by selective wet etching with an HCl/H3PO4 mixture. The resulting patterns can have submicron feature dimensions, limited by the resolution of the lithographic process, and can be formed on planar and 3D substrates. It is shown that the photolithography and wet etching steps have little effect on the photoluminescence quantum yield of CdSe/CdS QDs. Compared with the unpatterned CdSe/CdS QD film, only a 10% degradation in the quantum yield is observed. These results demonstrate the feasibility of the proposed micropatterning method to implement the large-scale device integration of colloidal quantum dots.

Single Elementary Charge Fluctuations on Nanoparticles in Aqueous Solution.

100 years ago, in 1923, the Nobel prize in physics was awarded for measurement of the unit charge. In addition to a profound impact on contemporary physics, this discovery has reshaped our understanding of charge-based interactions in chemistry and biology, ranging from oxidation and ionization to protein folding and metabolism. In a liquid, the discrete nature of the electric charge becomes prominent at the nanoscale when a charge carrier is exchanged between a molecule or a nanoparticle and the surrounding medium. However, our ability to observe the dynamics of such interactions at the level of a single elementary charge is limited due to the abundance of ions in water. Here, we report on the observation of single binding-unbinding events with elementary charge resolution at the surface of a nanoparticle suspended in water. Discrete steps in the electrical charge are revealed by analyzing the motion of optically trapped nanoparticles under the influence of an applied sinusoidal electric field. The measurements are sufficiently fast and long to observe individual (dis)charging events that occur on average every 3 s. Our results offer prospective routes for studying the dynamics of diverse chemical and biological phenomena on the nanoscale with elementary charge resolution.

Beyond Millikan: the dynamics of charging events on individual colloidal particles.

By measuring the stable charge on oil drops in air, Millikan demonstrated the discrete nature of electric charge. We extend his approach to the charge on solid-liquid interfaces, and focus on the dynamics of the discrete fluctuations. Our measurements are accurate and fast enough to observe changes of one elementary charge. Experiments over thousands of seconds yield information about the fast dynamics of electrochemical reactions, relevant for physicochemical and biological systems. As an example, we study (dis)charging processes on colloidal particles in a nonpolar liquid.

Transport of charged Aerosol OT inverse micelles in nonpolar liquids.

Surfactants such as Aerosol OT (AOT) are commonly used to stabilize and electrically charge nonpolar colloids in devices such as electronic ink displays. The electrical behavior of such devices is strongly influenced by the presence of charged inverse micelles, formed by excess surfactant that does not cover the particles. The presence of charged inverse micelles results in increased conductivity of the solution, affecting both the energy consumption of the device and its switching characteristics. In this work, we use transient current measurements to investigate the electrical properties of suspensions of the surfactant Aerosol OT in dodecane. No particles are added, to isolate the effect of excess surfactant. The measured currents upon application of a voltage step are found to be exponentially decaying, and can be described by an analytical model based on an equivalent electric circuit. This behavior is physically interpreted, first by the high generation rate of charged inverse micelles giving the suspension resistor like properties, and second by the buildup of layers of charged inverse micelles at both electrodes, acting as capacitors. The model explains the measurements over a large range of surfactant concentrations, applied voltages, and device thicknesses.

Impact of diffusion layers in strong electrolytes on the transient current.

Transient currents of electrolytes in response to a voltage step can reveal a lot about the behavior of charges present in an electrolyte. In this paper, electrolytes with high ionic strength are considered. In the limit of small voltage steps, the interpretation is straightforward as the equations describing the transient can be linearized. However, when high ion concentrations and voltage steps of the order of kT/q are considered, we find higher-order effects that occur simultaneously with the diffuse double layer charging. In this case, the diffuse double layer and the transient diffusion layer are coupled because of the screening of the field, leading to a -32 power law for the transient current.