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The formation of particle-counterion clusters through electrostatic interaction is studied in this work for a highly charge asymmetric colloidal suspension. The occurrence of such clustering is probed by the particle electrophoretic mobility, i.e., with the aid of a transport property. The results show that the effective charge manifesting under the presence of an external electric field is the renormalized charge predicted by an extension of the Debye-Huckel-Bjerrum theory to the fluid state of highly charged colloids.

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.

A direct consequence of the finite compressibility of a swollen microgel is that it can shrink and deform in response to an external perturbation. As a result, concentrated suspensions of these particles exhibit relaxation dynamics and rheological properties which can be very different with respect to those of a hard sphere suspension or an emulsion. We study the reduction in size of ionic microgels in response to increasing number of particles to show that particle shrinkage originates primarily from steric compression, and that the effect of ion-induced de-swelling of the polymer network is negligible. With increasing particle concentration, the single particle dynamics switch from those typical of a liquid to those of a super-cooled liquid and finally to those of a glass. However, the transitions occur at volume fractions much higher than those characterizing a hard sphere system. In the super-cooled state, the distribution of displacements is non-gaussian and the dependence of the structural relaxation time on volume fraction is describable by a Volger-Fulcher-Tammann function.

We investigate the phase and non-equilibrium behavior of suspensions comprised of swollen, ionic microgels as a function of particle stiffness. We use visual inspection of the samples, rheology and UV-visible spectroscopy to determine how the system phase changes with particle concentration for particles with different crosslinker content. Our results indicate that stiff particles exhibit all three phases observed in hard sphere suspensions: liquid, crystal and glass; however, the boundaries separating one phase from the other are different from the corresponding boundaries in hard sphere suspensions. In particular, the width of the liquid–crystal phase-coexistence region increases with decreasing particle stiffness. For particles with intermediate stiffness, the crystal phase disappears and the microgel suspension transitions from a liquid to a glassy state at certain particle concentration. For even softer particles, no glassy state is observed. Instead the system remains liquid within the experimentally accessed concentration range. These results emphasize the richness in behaviour that is brought about when deformable and compressible objects are considered instead of the more usual hard colloidal particles.

We introduce electro-coflow as a way to generate emulsion drops with an average size that can be larger, comparable, and smaller than the smallest geometric feature of the device. The method relies on using three immiscible liquids, two of them having a finite electrical conductivity. There are three regimes of operation that allow the steady generation of drops: dripping, electro-dripping, and an electrically dominated regime. We transit from one to the other by increasing the applied voltage and describe the changes in drop size by balancing the relevant forces in each regime.

We review our recent results on the use of hydrostatic pressure to change the size and the structure of microgel particles based on poly-(N-isopropylacrylamide). These changes are brought about through changes in the miscibility of the polymer in the solvent. Swelling induced by hydrostatic pressure can thus be thought of as an alternative to swelling induced by temperature, an interesting fact that can be exploited in the study of fundamental problems in soft condensed matter.

We study the structural properties of microgels made of poly(N-isopropylacrylamide) and acrylic acid as a function of hydrostatic pressure and temperature using small angle neutron scattering. Hydrostatic pressure induces particle deswelling by changing the mixing of the microgel with the solvent, similar to temperature. We extend this analogy to the structural properties of the particles and show that the form factor at a certain temperature is equal to the form factor at a certain hydrostatic pressure. We fit the results with an existent model for the microgel structure and carefully analyze the fitting procedure in order to obtain physically meaningful values of the free parameters in the model.

We perform extensive experiments with coflowing liquids in microfluidic devices and provide a closed expression for the drop size as a function of measurable parameters in the jetting regime that accounts for the experimental observations; this expression works irrespective of how the jets are produced, providing a powerful design tool for this type of experiments

Among the different ways of synthesizing fiber and tubular micro and nanostructures, some top-down methods resort to electro-hydrodynamic forces to smoothly stretch liquids interfaces down to such small size scales. The well-known electrospinning technique, commonly used to fabricate micro and nanofibers of a broad variety of materials, is now expanded to fabricate coaxial fibers upon the generation of electrified coaxial jets instead of single jets. We briefly report different types of micro and nano structures that may be fabricated with this new technique termed co-electrospinning.

We report an optical fiber tweezer based on high-index material for trapping and optical manipulation of microscale particles in water. The use of a high-index material increases the trapping force with respect to the more common silica, through tighter focusing of light. We demonstrate the potential of this simple and versatile device by trapping and rotating nematic liquid crystal drops. We monitor the rotation of the drop by detecting light modulation observed with the same fiber using backscattered light, which exhibits modulation in intensity due to the rotation of the drop; this further extends the capabilities of the fiber tweezers.

We report the fabrication of three-dimensional lattices of bipolar nematic liquid crystal droplets. The electro-optic properties of these materials consist of transitions between opaque, Bragg diffracting, and transparent states. These occur continuously at moderate electric (E) fields through bipolar axis rotation of the nematic droplets. An E field applied normal to the hexagonally close packed planes results in a Bragg diffraction pattern that is polarization independent. Application of in-plane fields allows switching between diffracting and transmitting states that depends on the polarization of the incident light.

Electro-hydro-dynamic (EHD) compound jets, with diameters in the micro and nanometric size range, from conical menisci of two co-flowing liquids, is a consolidated platform for the production of nanofibers with inner structure, in a process so-called coaxial electrospinning or co-electrospinning. In contrast to other multi-step template based procedures, the EHD methodology is much more simple and general since, firstly, a solid template is needless and, secondly, the process is seldom affected by the chemistry of the liquids. This gentle process allows selecting the liquid precursors depending on the application sought for the nanofibers. Here, we review different products obtained by this EHD technique: (1) solid and hollow carbon nanofibers from different precursors (polyacrylonitrile, polyvinylpyrrolidone and lignin), (2) nanofibers of biocompatible polymers encapsulating liquids in the form of beads, (3) spinning nanofibers of alginate and (4) in-fiber encapsulation of active microgels.

In this work, the aggregation of mesoscopic gel particles (soft colloids) has been experimentally investigated. The interaction between particles was controlled through the addition of salt, above the critical coagulation concentration, resulting in aggregation with finite bond energies. Attention has been paid to the structure of the clusters formed in the process as well as to the aggregation kinetics. The results indicate that the clusters are fractal and the kinetics of aggregation can be described through the dynamic scaling solution of the Smoluchowski equation. As the energy minimum increases in depth the resultant clusters pass from a very compact structure to typical diffusion-limited cluster aggregation (DLCA) fractal dimension values. In addition, the kinetics of growth change from those observed in reaction controlled aggregation to DLCA. These results can be explained within the framework of a reversible growth model, arising from the fact that aggregation takes place in an energy minimum of restricted depth. Moreover, they show that structure and kinetics decouple for such a soft sphere system, in contrast to what is encountered for DLCA and reaction-limited processes. Finally, an unexpected return to a reaction controlled aggregation kinetics was observed for sufficiently deep energy minima, which could be due to the polymerlike particularities of the soft particles considered in this work.