Dirk van den Ende

Direct observation of ionic structure at solid-liquid interfaces: a deep look into the Stern Layer

The distribution of ions and charge at solid-water interfaces plays an essential role in a wide range of processes in biology, geology and technology. While theoretical models of the solid-electrolyte interface date back to the early 20th century, a detailed picture of the structure of the electric double layer has remained elusive, largely because of experimental techniques have not allowed direct observation of the behaviour of ions, i.e. with subnanometer resolution. We have made use of recent advances in high-resolution Atomic Force Microscopy to reveal, with atomic level precision, the ordered adsorption of the mono- and divalent ions that are common in natural environments to heterogeneous gibbsite/silica surfaces in contact with aqueous electrolytes. Complemented by density functional theory, our experiments produce a detailed picture of the formation of surface phases by templated adsorption of cations, anions and water, stabilized by hydrogen bonding.

The measurement of the shear-induced particle and fluid tracer diffusivities in concentrated suspensions by a novel method

The shear-induced particle self-diffusivity in a concentrated suspension (20%–50% solids volume fraction) of non-colloidal spheres (90 [mu]m average diameter) was measured using a new correlation technique. This method is based on the correlation between the positions of tracer particles in successive images and can be used to determine the self-diffusivity in non-colloidal suspensions for different time scales. These self-diffusivities were measured in the velocity gradient and vorticity directions in a narrow-gap Couette device for values of the strain [gamma][Delta]t ranging from 0.05 to 0.5, where [gamma] is the applied shear rate and [Delta]t is the correlation time. In both directions, the diffusive displacements scaled linearly with [gamma][Delta]t over the range given above and the corresponding diffusivities were found to be in good agreement with the experimental results of Leighton & Acrivos (1987a) and of Phan & Leighton (1993), even though these earlier studies were performed at much larger values of [gamma][Delta]t. The self-diffusivity in the velocity gradient direction was found to be about 1.7 times larger than in the vorticity direction. The technique was also used to determine the shear-induced fluid tracer by measuring the mean square displacement of 31.5 [mu]m diameter tracer particles dispersed in concentrated suspensions (30%–50% solids volume fraction) of much larger spheres (325 [mu]m average diameter). These fluid diffusivities were found to be 0.7 times the corresponding particle diffusivities when both were scaled with [gamma] a2 (2a = 325 [mu]m).

Shear-induced diffusion and rheology of noncolloidal suspensions: Time scales and particle displacements

The shear-induced self-diffusion and rheology of concentrated suspensions of noncolloidal hard spheres have been studied experimentally. The combined results provide an interesting physical picture. The projection of the trajectories of individual particles on the vorticity (z)–velocity (x) plane were determined through particle tracking. The particle trajectories turned out to be very useful for gaining qualitative insight into the microscopic particle motion. However, the technique is less suitable to obtain quantitative information. For a quantitative analysis of the particle displacements we measured the evolution of the ensemble averaged displacements as a function of time. The statistical analysis revealed two diffusion regimes, where 〈ΔzΔz〉 ∼ Δt. For large strain values (Δt>1) long-time self-diffusion was observed. The associated diffusion coefficient ∞ is in excellent agreement with literature data on shear-induced self-diffusion. On very short times (Δt≪1) a novel diffusive regime was discovered, characterized by a diffusion coefficient 0, which is significantly smaller than ∞ and grows monotonically with ϕ. 0 is detected on time scales on which the particle configuration is not changed significantly and thus it must represent the fluctuating motion of particles in the “cage” formed by their nearest neighbors. Finally, the rheology was studied with steady shear and oscillatory rheometry. The dynamic measurements in a controlled stress rheometer revealed that the viscoelastic response of the suspension is determined mainly by the amplitude of deformation. At small strain amplitudes γ0<1, the response is linear and a dynamic viscosity η′ is found, which is in excellent agreement with the high frequency limit η∞′ as reported in literature for colloidal hard sphere suspensions. Around γ0 = 1 the “cage” around a particle is deformed and a shear-induced microstructure is built. This leads to O(a) displacements of the particles and the viscoelastic response becomes strongly nonharmonic. Although the effect persists at large amplitudes, it becomes relatively small for γ0≫1. The microstructure is rearranged immediately after flow reversal and remains unchanged for the larger part of the period of oscillation. As a result a pseudolinear viscoelastic regime is found with a viscosity close to steady shear viscosity. Experiments show a correlation between the time scales controlling the 0/∞ diffusive behavior and the ones controlling the shear-induced changes in particle configuration as probed by the rheological measurements.

Electrowetting-controlled droplet generation in a microfluidic flow-focusing device

We studied the generation of aqueous microdrops in an oil–water flow-focusing device with integrated insulator-covered electrodes that allow for continuous tuning of the water wettability by means of electrowetting. Depending on the oil and water inlet pressures three different operating conditions were identified that shift upon applying a voltage: stable oil–water interface, drop generation, and laminar water jet formation. Full control over the drop generation is achieved within a well-defined range of inlet pressures, in quantitative agreement with a model based on the additive contributions from electrowetting and the local hydrostatic pressure at the junction. The tuning power of electrowetting is shown to increase upon device miniaturization, which makes this approach particularly attractive for flow control on the sub-micrometer scale.

Optofluidic lens with tunable focal length and asphericity

Adaptive micro-lenses enable the design of very compact optical systems with tunable imaging properties. Conventional adaptive micro-lenses suffer from substantial spherical aberration that compromises the optical performance of the system. Here, we introduce a novel concept of liquid micro-lenses with superior imaging performance that allows for simultaneous and independent tuning of both focal length and asphericity. This is achieved by varying both hydrostatic pressures and electric fields to control the shape of the refracting interface between an electrically conductive lens fluid and a non-conductive ambient fluid. Continuous variation from spherical interfaces at zero electric field to hyperbolic ones with variable ellipticity for finite fields gives access to lenses with positive, zero, and negative spherical aberration (while the focal length can be tuned via the hydrostatic pressure).

Shear-induced self-diffusion and microstructure in non-brownian suspensions at non-zero Reynolds numbers

This paper addresses shear-induced self-diffusion in a monodisperse suspension of non-Brownian particles in Couette flow by two-dimensional computer simulations following the lattice-Boltzmann method. This method is suited for the study of (many-particle) particulate suspensions and can not only be applied for Stokes flow, but also for flow with finite Reynolds number. At relatively low shear particle Reynolds numbers (up to 0.023), shear-induced diffusivity exhibited a linear dependence on the shear rate, as expected from theoretical considerations. Simulations at shear particle Reynolds numbers between 0.023 and 0.35, however, revealed that in this regime, shear-induced diffusivity did not show this linear dependence anymore. Instead, the diffusivity was found to increase more than linearly with the shear rate, an effect that was most pronounced at lower area fractions of 0.10 and 0.25. In the same shear regime, major changes were found in the flow trajectories of two interacting particles in shear flow (longer and closer approach) and in the viscosity of the suspension (shear thickening). Moreover, the suspended particles exhibited particle clustering. The increase of shear-induced diffusivity is shown to be directly correlated with this particle clustering. As for shear-induced diffusivity, the effect of increasing shear rates on particle clustering was the most intensive at low area fractions of 0.10 and 0.25, where the radius of the clusters increased from about 4 to about 7 particle radii with an increase of the shear Reynolds number from 0.023 to 0.35. The importance of particle clustering to shear-induced diffusion might also indicate the importance of other factors that can induce particle clustering, such as, for example, colloidal instability.

Microrheology: new methods to approach the functional properties of food

Three configurations have been developed to improve the understanding of structural element interactions in food material during deformation. The three configurations combine an inverted confocal scanning laser microscope (CSLM) and a cell that can apply to the sample a specific deformation: continuous shear, linear oscillatory shear and biaxial extension (compression). In the continuous shear and oscillatory shear configurations (OSCs), a zero-velocity plane is created in the sample by moving two plates in opposite direction, maintaining stable observation conditions of the structural behaviour under deformation. The OSC allows simultaneous application of CSLM and diffusing wave spectroscopy, a multiple light scattering technique. The third configuration (compression configuration) allows observation at a stagnation point during rheometric measurements. The configurations accept semi-liquid products (dressing, sauces, dairy products, etc.) for investigations in area such as aggregation, gelation, interactions at interface, coalescence, break-up.

Air cushioning in droplet impact. II. Experimental characterization of the air film evolution

A liquid drop approaching a solid surface deforms substantially under the influence of the ambient air which needs to be squeezed out before the liquid can actually touch the solid. We use nanometer- and microsecond-resolved dual wavelength interferometry described in Part I (also published in this issue) to reveal the complex spatial and temporal evolution of the squeezed air layer. In low-velocity droplet impact, i.e., We numbers of order unity, the confined air layer below the droplet develops two local minima in thickness. We quantitatively measure the evolution of the droplet bottom interface and find that surface tension determines the air film thickness below the first kink, after which fluid is diverted outward to form a second even sharper kink. Depending on We, one of the two kinks approaches the surface more closely forming liquid-solid contact. The early time spreading of liquid-solid contact is controlled by the capillary driving force and the inertia of the liquid. The cushioned air film geometry, i.e., a flat micrometer-thin gap, induces an increase of the spreading velocity; the contact area first spreads over the cushioned region, only then followed by radial spreading. This spreading mechanism can lead to the entrapment of one or more air bubbles.

Air cushioning in droplet impact. I. Dynamics of thin films studied by dual wavelength reflection interference microscopy

When a liquid droplet impacts on a solid surface, it not only deforms substantially but also an air film develops between the droplet and the surface. This thin air film—as well as other transparent films—can be characterized by reflection interference microscopy. Even for weakly reflecting interfaces, relative thickness variations of the order of tens of nanometers are easily detected, yet the absolute thickness is generally known only up to an additive constant which is a multiple of half of the wavelength. Here, we present an optical setup for measuring the absolute film thickness and its spatial and temporal behavior using a combination of a standard Hg lamp, an optical microscope, and three synchronized high-speed cameras to detect conventional side-view images as well as interferometric bottom view images at two different wavelengths. The combination of a dual wavelength approach with the finite coherence length set by the broad bandwidth of the optical filters allows for measuring the absolute thickness of transient air films with a spatial resolution better than 30 nm at 50 μs time resolution with a maximum detectable film thickness of approximately 8 μm. This technique will be exploited in Part II to characterize the air film evolution during low velocity droplet impacts.

Controlling flow patterns in oscillating sessile drops by breaking azimuthal symmetry

We study time-averaged flows within sessile drops that oscillate under the influence of an AC voltage applied in electrowetting configuration. We show that the average flow velocity in the azimuthal plane correlates with the eigenmodes of the drop in the polar direction and—most importantly—we demonstrate that the azimuthal symmetry of the flow fields can be broken by introducing pinning sites along the contact line of the drop. We anticipate that the controlled introduction of azimuthal vortices increases the mixing efficiency inside the droplet.