Michael H.G. Duits

Droplets Formation and Merging in Two-Phase Flow Microfluidics

Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed.

Suppressing the coffee stain effect: how to control colloidal self-assembly in evaporating drops using electrowetting

We study the influence of electrowetting on the formation of undesired solute residues, so-called coffee stains, during the evaporation of a drop containing non-volatile solvents. Electrowetting is found to suppress coffee stains of both colloidal particles of various sizes and DNA solutions at alternating (AC) frequencies ranging from a few Hertz to a few tens of kHz. Two main effects are shown to contribute to the suppression: (i) the time-dependent electrostatic force prevents pinning of the three phase contact line and (ii) internal flow fields generated by AC electrowetting counteract the evaporation driven flux and thereby prevent the accumulation of solutes along the contact line.

Microfluidic Technology in Vascular Research

Vascular cell biology is an area of research with great biomedical relevance. Vascular dysfunction is involved in major diseases such as atherosclerosis, diabetes, and cancer. However, when studying vascular cell biology in the laboratory, it is difficult to mimic the dynamic, three-dimensional microenvironment that is found in vivo. Microfluidic technology offers unique possibilities to overcome this difficulty. In this review, an overview of the recent applications of microfluidic technology in the field of vascular biological research will be given. Examples of how microfluidics can be used to generate shear stresses, growth factor gradients, cocultures, and migration assays will be provided. The use of microfluidic devices in studying three-dimensional models of vascular tissue will be discussed. It is concluded that microfluidic technology offers great possibilities to systematically study vascular cell biology with setups that more closely mimic the in vivo situation than those that are generated with conventional methods.

Anisotropic and hindered diffusion of colloidal particles in a closed cylinder.

Video microscopy and particle tracking were used to measure the spatial dependence of the diffusion coefficient (D(α)) of colloidal particles in a closed cylindrical cavity. Both the height and radius of the cylinder were equal to 9.0 particle diameters. The number of trapped particles was varied between 1 and 16, which produced similar results. In the center of the cavity, D(α) turned out to be 0.75D(0) measured in bulk liquid. On approaching the cylindrical wall, a transition region of about 3 particle diameters wide was found in which the radial and azimuthal components of D(α) decrease to respective values of 0.1D(0) and 0.4D(0), indicating asymmetrical diffusion. Hydrodynamic simulations of local drag coefficients for hard spheres produced very good agreement with experimental results. These findings indicate that the hydrodynamic particle-wall interactions are dominant and that the complete 3D geometry of the confinement needs to be taken into account to predict the spatial dependence of diffusion accurately.

Shear history dependence of the viscosity of aggregated colloidal dispersions

The shear history dependence of the viscosity of a depletion flocculated dispersion of colloidal spheres was studied using two different rheometrical geometries. The observed rheological behavior is found to depend on the geometry, due to effects of thixotropy and sedimentation. By comparing the results of a cone‐plate and a Couette geometry, we were able to obtain reliable data. The shear history dependence is explored by measuring a flow curve before and after subjecting the aggregated dispersion to a constant shear rate for one hour. The viscosity values of the flow curve after this hour turned out to be considerably lower than the initial flow curve. The results were interpreted with a microrheological model for fractal aggregation in shear flow. The drop in viscosity is attributed to a shear induced compaction of the aggregates. Combination of this model and the concept of compaction results in a satisfactory description of the experimental results.The shear history dependence of the viscosity of a depletion flocculated dispersion of colloidal spheres was studied using two different rheometrical geometries. The observed rheological behavior is found to depend on the geometry, due to effects of thixotropy and sedimentation. By comparing the results of a cone‐plate and a Couette geometry, we were able to obtain reliable data. The shear history dependence is explored by measuring a flow curve before and after subjecting the aggregated dispersion to a constant shear rate for one hour. The viscosity values of the flow curve after this hour turned out to be considerably lower than the initial flow curve. The results were interpreted with a microrheological model for fractal aggregation in shear flow. The drop in viscosity is attributed to a shear induced compaction of the aggregates. Combination of this model and the concept of compaction results in a satisfactory description of the experimental results.

A microfluidic platform for on-demand formation and merging of microdroplets using electric control

We discuss a microfluidic system in which (programmable) local electric fields originating from embedded and protected electrodes are used to control the formation and merging of droplets in a microchannel. The creation of droplets-on-demand (DOD) is implemented using the principle of electrowetting. Combined with hydrodynamic control, the droplet size and formation frequency can be varied independently. Using two synchronized DOD injectors, merging-on-demand (MOD) is achieved via electrocoalescence. The efficiency of MOD is 98% based on hundreds of observations. These two functionalities can be activated independently.

A new transient network model for associative polymer networks

A new model for the linear viscoelastic behavior of polymer networks is developed. In this model the polymer system is described as a network of spring segments connected via sticky points (as in the Lodge model). [Lodge, A. S., “A network theory of flow birefringence and stress in concentrated polymer solutions,” Trans. Faraday Soc. 52, 120–130 (1956).] An important extension (with respect to previous models) is that chain connectivity is taken into account. All segments that are located in between connected stickers are supposed to carry stress. The attachment and detachment of stickers is described with kinetic equations in which activation energies play a role. Simultaneous transitions involving groups of stickers are allowed. The model shows a strong dependence upon the number of segments per chain. Broad relaxation spectra can be obtained. The storage modulus can have more than one plateau corresponding with the fact that stress relaxation may need the breakup of several bonds.

Partial structure factors in colloidal silica mixtures determined with small‐angle neutron scattering contrast variation

Small‐angle neutron scattering experiments at various contrasts were performed on concentrated mixtures of colloidal spheres differing in size. Both colloidal components consisted of fairly monodisperse silica cores coated with a layer of octadecyl chains. Cyclohexane was used as dispersing medium; variation of the contrast was achieved by using mixtures of 1H ‐cyclohexane and 2H ‐cyclohexane. Scattered intensities were measured at three volume fractions up to 0.4, at equal partial volume fractions. The different contrast dependence of the scattering amplitudes of both colloids allowed us to calculate partial structure factors. This was done using a method which has not been reported previously. Describing the intraparticle structures with layered‐sphere models, and using a decoupling approximation, three partials were obtained from a system of linear equations. The scattering curves at the various contrasts constitute a consistent data set, at all volume fractions. Although the separate components intera...

The viscosity and sedimentation of aggregating colloidal dispersions in a Couette flow

In order to interpret the time dependence of the measured torque in a steady shear experiment on an aggregating dispersion, a microrheological model has been used in which two existing models are integrated. In this microrheological model, a theory for fractal aggregation in shear flow is combined with a theory for the sedimentation and resuspension of non‐colloidal hard spheres. The former theory describes the viscosity as a function of shear rate, while the latter predicts the stress increase in a Couette device due to sedimentation. The connection between the two theories is made by identifying the aggregate parameters with the hard sphere parameters (size and volume fraction). The parameters of the aggregates as a function of shear stress are obtained by measuring a flow curve before sedimentation effects become significant and fitting this curve with the fractal aggregation theory. During the sedimentation, the aggregate size and volume fraction become a function of both time and position in the rheo...

Droplet Manipulations in Two Phase Flow Microfluidics

Even though droplet microfluidics has been developed since the early 1980s, the number of applications that have resulted in commercial products is still relatively small. This is partly due to an ongoing maturation and integration of existing methods, but possibly also because of the emergence of new techniques, whose potential has not been fully realized. This review summarizes the currently existing techniques for manipulating droplets in two-phase flow microfluidics. Specifically, very recent developments like the use of acoustic waves, magnetic fields, surface energy wells, and electrostatic traps and rails are discussed. The physical principles are explained, and (potential) advantages and drawbacks of different methods in the sense of versatility, flexibility, tunability and durability are discussed, where possible, per technique and per droplet operation: generation, transport, sorting, coalescence and splitting.