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Capillary-based microfluidics is a great technique to produce monodisperse and complex emulsions and particulate suspensions. In this review, the current understanding of drop and jet formation in capillary-based microfluidic devices for two primary flow configurations, coflow and flow-focusing is summarized. The experimental and theoretical description of fluid instabilities is discussed and conditions for controlled drop breakup in different modes of drop generation are provided. Current challenges in drop breakup with low interfacial tension systems and recent progress in overcoming drop size limitations using electro-coflow are addressed. In each scenario, the physical mechanisms for drop breakup are revisited, and simple scaling arguments proposed in the literature are introduced.

We apply an electric field to a moderately conducting liquid surrounded by another co-flowing liquid, all inside a glass-based microfluidic device, and study and classify the resulting emission modes. The viscosity and flow rate of the co-flowing liquid affect the number of modes observed in classical electrospray as well as their geometrical characteristics. In particular, we observe a two-dimensional whipping mode not described before. We also provide a qualitative description of some of the modes, including dripping, electrodripping, microdripping, the cone-jet mode, and both the two- and three-dimensional whipping modes.

Soluble oligonucleotides are typically introduced to bulk solution to promote hybridization activity on DNA-functionalized surfaces. Here, an alternative approach is explored by encapsulating secondary target strands inside semipermeable colloidal satellite assemblies, then triggering their release at 37 °C for subsequent surface hybridization activity. To prepare DNA-loaded satellite assemblies, uniform gelatin microspheres are fabricated using microfluidics, loaded with 15 base-long secondary DNA targets, capped with a polyelectrolyte bilayer, and finally coated with a monolayer of polystyrene microspheres functionalized with duplexes comprised of immobilized probes and soluble, 13 base-long hybridization partner strands. Once warmed to 37 °C, secondary DNA targets are released from the gelatin template and then competitively displace the shorter, original hybridization partners on the polystyrene microspheres.

We investigate microgels synthesized from $N$-isopropylacrylamide (NIPAM) copolymerized with a large mol% of acrylic acid, finding that when the acid groups are partially ionized at high temperatures, competition between ion-induced swelling and hydrophobic deswelling of poly(NIPAM) chains results in microphase separation. In cross-linked microgels, this manifests as a dramatic decrease in the ratio between the radius of gyration and the hydrodynamic radius to $\ensuremath{\sim}0.2$, indicating that almost all the mass of the microgel is concentrated near the particle center. We also observe a concurrent decrease of the polymer network length scale via small-angle neutron scattering, confirming the presence of a dense, deswollen core surrounded by a diffuse, charged periphery. We compare these results to those obtained for a system of charged ultralow-cross-linked microgels; the form factor shows a distinct peak at high $q$ when the temperature exceeds a threshold value. We successfully fit the form factor to theory developed to describe scattering from weakly charged gels in poor solvents, and we tie this behavior to charge segregation in the case of the cross-linked microgels.

We will discuss the defect structures that originate in nematic droplets with two or more handles. In these cases, the topology of the bounding surface requires the presence of defects. Our experiments elucidate where do these defects locate and how many of them populated the ground state of the system.

This study introduces an extremely stable attractive nanoscale emulsion fluid, in which the amphiphilic block copolymer, poly(ethylene oxide)-block-poly(e-caprolactone) (PEO-b-PCL), is tightly packed with lecithin, thereby forming a mechanically robust thin-film at the oil-water interface. The molecular association of PEO-b-PCL with lecithin is critical for formation of a tighter and denser molecular assembly at the interface, which is systematically confirmed by T2 relaxation and DSC analyses. Moreover, our suspension rheology studies also reflect the interdroplet attractions over a wide volume fraction range of the dispersed oil phase; this results in a percolated network of stable drops that exhibit no signs of coalescence or phase separation. We have figured out that this unique rheological behavior is attributed to the dipolar interaction between the phosphorylcholine groups of lecithin and the methoxy end groups of PEO-b-PCL. Finally, we demonstrate that our nanoemulsion system significantly enhances transdermal delivery efficiency due to its favorable attraction to the skin as well as high diffusivity of the nanoscale emulsion drops.

Coating a spherical colloid with a nematic liquid crystal causes frustration-induced defects in the crystal. The thickness of this coating can be used to systematically control the number and orientation of these defects, which could be useful for engineering the microstructure of colloidal materials.

In this paper, we present an in-depth study of a non-invasive method to determine the bulk modulus of microgel particles. The method relies on measuring the particle size as a function of external osmotic pressure using dynamic light scattering to measure the particle size and dextran solutions to change the osmotic pressure. An independent characterization of the dynamic response of the dextran solutions is required in order to obtain the microgel size in the microgel/dextran mixtures. The method works fine in most cases except for slightly crosslinked microgels, where the scattering from the particles is comparable to the scattering from the dextrans.

In this article we demonstrate that concentrated p-NIPA suspensions in a low temperature glassy state can liquefy and then solidify again as the temperature is raised across the LCST. Our system exhibits all the typical behavior of disordered colloidal suspensions, but the behavior is controlled by temperature. Below the LCST it shows the behavior typical of a colloidal glass, near the LCST it behaves like a liquid, while above the LCST it exhibits the properties typical of a colloidal gel. Moreover, we show that the elasticity of these suspensions exhibits critical-like behavior as a function of temperature both above and below the LCST, with a critical temperature that corresponds to the LCST. Our results thereby suggest interesting analogies between the glass and gel phases of these thermosensitive microgel particles.

The absorption and desorption of water, as well as the retention of certain molecules within a polymeric network, have special interest in a wide range of applications, including drug delivery, biosensing, chemical separation, catalysis, and optics. In this communication, we report the encapsulation by electrospinning of poly(N-isopropylacrylamide)-based monodisperse microgels within microfibers of crosslinked poly(vinylpyrrolidone), which is a hydrogel itself, up to concentrations of 40 wt.-% of the microgels. We present an optical characterization of these composite microfibers and quantify their absorbance, which can be as large as 17 times their de-swollen weight. Interestingly, this absorbance can be tuned by controlling the values of the temperature and the pH of the medium.

Tanaka’s approach to swelling kinetics of a solid gel sphere is extended to a spherical microgel shell. The boundary condition at the inner surface is obtained from the minimization of shear elastic energy. Temporal evolution of a shell is represented in a form of expansion over eigenfunctions of the corresponding diffusion equation. The swelling of Tanaka’s solid spherical gel is recovered as a special case of our general solution if the inner radius approaches zero. In another limiting case of a thin (balloon-like) shell, the set of eigenfunctions is reduced to a single exponential term. In the general case, a solid sphere swells slightly faster than the same sphere with an internal cavity. To test our theoretical model, we prepared monodisperse poly-N-isopropylacrylamide (PNIPAM) hydrogel shells using a microfluidic device. The temporal dependence of the inner and outer radii of the shell were measured, and the data were fitted to our theoretical model. As a result, we obtained the collective diffusion...

Mixtures of nonadsorbing polymer and colloidal particles exhibit a range of different morphologies depending on the particle and polymer concentrations and their relative size ratios. These can be very important for technological applications, where gelation can produce a weak solidlike structure that can help reduce phase separation, extending product shelf life. However, industrial products are typically formulated with polydisperse polymers, and the consequences of this on the phase behavior of the mixture are not known. We investigate the role of polymer polydispersity and show that a small amount of larger polymer in a distribution of nominally much smaller polymer can drastically modify the behavior. It can induce formation of a solidlike gel structure, abetted by the small polymer, but still allow further evolution of the phase separation process, as is seen with a monodisperse distribution of larger polymer. This coarsening ultimately leads to gravitational collapse. We describe the full phase behavior for polydisperse polymer mixtures and account for the origin of the behavior through measurements of the structure and dynamics and by comparing to the behavior with monodisperse polymers.

Glass formation in colloidal suspensions has many of the hallmarks of glass formation in molecular materials. For hard-sphere colloids, which interact only as a result of excluded volume, phase behaviour is controlled by volume fraction, phi; an increase in phi drives the system towards its glassy state, analogously to a decrease in temperature, T, in molecular systems. When phi increases above phi* approximately 0.53, the viscosity starts to increase significantly, and the system eventually moves out of equilibrium at the glass transition, phi(g) approximately 0.58, where particle crowding greatly restricts structural relaxation. The large particle size makes it possible to study both structure and dynamics with light scattering and imaging; colloidal suspensions have therefore provided considerable insight into the glass transition. However, hard-sphere colloidal suspensions do not exhibit the same diversity of behaviour as molecular glasses. This is highlighted by the wide variation in behaviour observed for the viscosity or structural relaxation time, tau(alpha), when the glassy state is approached in supercooled molecular liquids. This variation is characterized by the unifying concept of fragility, which has spurred the search for a 'universal' description of dynamic arrest in glass-forming liquids. For 'fragile' liquids, tau(alpha) is highly sensitive to changes in T, whereas non-fragile, or 'strong', liquids show a much lower T sensitivity. In contrast, hard-sphere colloidal suspensions are restricted to fragile behaviour, as determined by their phi dependence, ultimately limiting their utility in the study of the glass transition. Here we show that deformable colloidal particles, when studied through their concentration dependence at fixed temperature, do exhibit the same variation in fragility as that observed in the T dependence of molecular liquids at fixed volume. Their fragility is dictated by elastic properties on the scale of individual colloidal particles. Furthermore, we find an equivalent effect in molecular systems, where elasticity directly reflects fragility. Colloidal suspensions may thus provide new insight into glass formation in molecular systems.

We report on the use of hydrostatic pressure, P, to deswell thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgels and show that it can affect the polymer−solvent mixing as much as temperature, which is the traditional variable used to deswell pNIPAM particles. Interestingly, the microgel volume changes more gradually with pressure than it does with temperature. By comparing the pressure and temperature induced deswelling, we obtain the pressure dependence of the Flory solvency parameter, χ; it increases with P, indicating that pressure decreases the polymer−solvent miscibility. We interpret this increase in terms of the entropy change, △S, when a polymer−solvent contact is broken to form a solvent−solvent contact and find that |△S| also increases with P, consistent with previous experimental results with polymers and other gels. Hydrostatic pressure thus changes the entropic contribution of mixing, causing χ to increase and ultimately leading to particle deswelling.

In recent years, "smart" materials have been the focus of considerable interest, from both fundamental and applied perspectives. Polymer gels are within this category; they respond to specific environmental stimuli by changing their size. Thus, the internal structure, the refractive index, and the mechanical properties of the polymer network change. They are considered super absorbent materials, as they can absorb solvent up to several hundred times their own weight. They respond rapidly to local environmental variations, an important fact in device miniaturization and microsensor developments. As size changes are accompanied by changes in internal dimensions, microgels have found application as carriers of therapeutic drugs and as diagnostic agents. They have also been used as microreactors, optically active materials, for template synthesis of nanoparticles or fabrication of artificial muscle. In this paper we review a set of application based on the special features associated to this systems. Basic concepts on the physical-chemistry of gel swelling is first described, followed by different applications covering drug delivery, composite materials using polymer gels to modulate optical or magnetic and electrical properties, molecular imprinting, gel-based biosensors and polymer sensors and actuators used in the field of artificial muscles.

We describe new developments for the controlled fabrication of monodisperse emulsions using microfluidics. We use glass capillary devices to generate single, double, and higher order emulsions with exceptional precision. These emulsions can serve as ideal templates for generating well-defined particles and functional vesicles. Polydimethylsiloxane microfluidic devices are also used to generate picoliter-scale water-in-oil emulsions at rates as high as 10 000 drops per second. These emulsions have great potential as individual microvessels in high-throughput screening applications, where each drop serves to encapsulate single cells, genes, or reactants.

We present a robust and straightforward method for fabricating remarkably responsive hydrogel scaffolds consisting of submicron-sized microgel particles. We demonstrate that the microgel particles assemble either through bridging or depletion interactions to yield a structure that swells or deswells at a macroscopic level in much shorter times as compared to a bulk polymer gel of similar characteristics. This approach offers a new way of fabricating functional hydrogel scaffolds with controllable responsiveness to applied stimuli and excellent loading capability for a wide variety of materials, irrespective of chemistry, size, and shape.

The following article is based on the Symposium X presentation given by David A. Weitz (Harvard University) on April 11, 2007, at the Materials Research Society Spring Meeting in San Francisco. The article describes how simple microfluidic devices can be used to control fluid flow and produce a variety of new materials. Based on the concepts of coaxial flow and hydrodynamically focused flow, used alone or in various combinations, the devices can produce precisely controlled double emulsions (droplets within droplets) and even triple emulsions (double emulsions suspended in a third droplet). These structures, which can be created in a single microfluidic device, have various applications such as encapsulants for drugs, cosmetics, or food additives.

The presumed ground state of a nematic fluid confined in a cylindrical geometry with planar anchoring corresponds to that of an axial configuration, wherein the director, free of deformations, is along the long axis of the cylinder. However, upon confinement of lyotropic chromonic liquid crystals in cylindrical geometries, here we uncover a surprising ground state corresponding to a doubly twisted director configuration. The stability of this ground state, which involves significant director deformations, can be rationalized by the saddle-splay contribution to the free energy. We show that sufficient anisotropy in the elastic constants drives the transition from a deformation-free ground state to a doubly twisted structure, and results in spontaneous symmetry breaking with equal probability for either handedness. Enabled by the twist angle measurements of the spontaneous twist, we determine the saddle-splay elastic constant for chromonic liquid crystals for the first time., Chirality in molecular materials is commonly used to manipulate the polarization of light. Here, Nayani et al. observe the formation of doubly twisted structure in achiral chromonic liquid crystals when confined to a cylindrical capillary, which leads to spontaneous chiral breaking.

We present a theoretical study of the energetics of thin nematic shells with two charge-one-half defects and one charge-one defect. We determine the optimal arrangement: the defects are located on a great circle at the vertices of an isosceles triangle with angles of 66^{∘} at the charge-one-half defects and a distinct angle of 48^{∘}, consistent with experimental findings. We also analyze thermal fluctuations around this ground state and estimate the energy as a function of thickness. We find that the energy of the three-defect shell is close to the energy of other known configurations having two charge-one and four charge-one-half defects. This finding, together with the large energy barriers separating one configuration from the others, explains their observation in experiments as well as their long-time stability.

We investigate poly(N-isopropylacrylamide) (pNIPAM) microgels randomly copolymerized with large mol % of protonated acrylic acid (AAc), finding that above the lower critical solution temperature the presence of the acid strongly disrupts pNIPAM's collapse, leading to unexpected new behavior at high temperatures. Specifically, we see a dramatic increase in the ratio between the radius of gyration and the hydrodynamic radius above the theoretical value for homogeneous spheres, and a corresponding increase of the network length scale, which we attribute to the presence of a heterogeneous polymer distribution that forms due to frustration of pNIPAM's coil-to-globule transition by the AAc. We analyze this phenomenon using a Debye-Bueche-like scattering contribution as opposed to the Lorentzian term often used, interpreting the results in terms of mass segregation at the particle periphery.

Microgel particles based on poly (N-isopropylacrylamide) have been shown to display an initial swelling behavior, followed by a collapse, with increasing concentration of added poly(ethylene oxide), PEO, chains. This paper considers the thermodynamic reasons for the observed expansion and subsequent shrinkage of the particles. At low concentrations of PEO, the free chains permeate into the microgel particles and cause an increase in osmotic pressure, expanding the particles. At higher concentrations of PEO, the particles are saturated and an increase in osmotic pressure in the external phase causes the particles to collapse again. The calculated magnitude of swelling and the effect of PEO molecular weight are, at least qualitatively, in agreement with the experimental observations reported elsewhere.

In this work, we experimentally determine the translational diffusion coefficient D t of charged polypeptides, poly- l -lysine (PLL) molecules, that change conformation from random-coil to α-helix or β-sheet upon increasing the solvent pH and temperature, respectively. We measure the scattered light component having the same polarization direction as the incident light, to construct the field autocorrelation function and obtain D t , further estimating the PLL dimensions in the random-coil and α-helix structures. We use a commercial high molecular weight PLL sample that is widely used in many practical applications.

Propagation through a distorting obstacle may significantly influence the amplitude, phase and polarization state of a light beam. This potentially has consequences for the behaviour of the optical angular momentum of light. We experimentally study how both the spin and orbital angular momentum (OAM) of light behaves upon passage through microscopic optically trapped particles. Particles trapped with Gaussian and, separately, Bessel light beams in two spatially distinct sample chambers are studied with trapped objects in the first chamber acting as distorting obstacles. The Bessel beam can reconstruct its spatial form and this shows reconstruction of both spin and OAM over extended distances.

In this work, we study experimentally the effect of an external osmotic pressure πext on the swelling of ionic mesoscopic gels in the weak screening limit, where the Debye screening length is larger than the mesh size of the gel. Variations in the osmotic pressure were induced by adding dextran to the solution. The results show that ionic microgels do not respond to πext below a given value of the normal stress; above this value the system de-swells with increasing osmotic pressure. The start of de-swelling is set by the gel charge density. The Flory thermodynamic theory for ionic gels captures the essential characteristics of the de-swelling behavior; in particular, it predicts with fairly good accuracy the value of πext at which de-swelling begins. Finally, due to the colloidal character of the gels, we observe that the system flocculates by a depletion interaction mechanism at high dextran concentrations.

Weapply an electric field to amoderately conducting liquid surrounded by another coflowing liquid, all inside a glass-based microfluidic device, to study nonaxisymmetric instabilities.Wefind that the bending of the electrified jet results in a steady-state, helicoidal structure with a constant opening angle. Remarkably, the characteristic phase speed of the helicoidalwave only depends on the charge carried by the jet in the helicoidal region and its stability critically depends on the properties of the coflowing liquid. In fact, the steady-state helical structure becomes chaotic when the longest characteristic time is that of the inner liquid rather than that of the outer coflowing liquid. We also perform a numerical analysis to show that the natural preference of the jet is to adopt the conical helix structure observed experimentally.

We study the form factor of thermoresponsive microgels based on poly(N-isopropylacrylamide) at high generalized volume fractions, ζ, where the particles must shrink or interpenetrate to fit into the available space. Small-angle neutron scattering with contrast matching techniques is used to determine the particle form factor. We find that the particle size is constant up to a volume fraction roughly between random close packing and space filling. Beyond this point, the particle size decreases with increasing particle concentration; this decrease is found to occur with little interpenetration. Noteworthily, the suspensions remain liquid-like for ζ larger than 1, emphasizing the importance of particle softness in determining suspension behavior.

We study the stability of toroidal droplets inside a yield stress material. Similar to toroidal droplets in a viscous liquid, the slenderness of the torus controls whether it breaks into spherical droplets or grows thicker towards its center to coalesce onto itself and form a single spherical droplet. However, unlike tori generated in a viscous liquid, the elasticity of the outer medium can prevent either or both of these instabilities; this depends on the slenderness of the torus. Interestingly, we find that the value of the tube radius needed to prevent breakup is always larger than the value of the radius of the handle to prevent growth. This reflects the different deformations experienced by the yield stress material in either process. A simple model balancing the surface tension stress, which drives the evolution of the torus, and the yield stress, which favors its stability, accounts for all of our observations.

In this work, the effects of salt concentration over the swelling of ionic mesoscopic gels will be studied theoretically and verified with experiments. We will restrict ourselves to the weak screening limit where the Debye screening length is larger than the mesh size of the gel. Under this condition, direct electrostatic interactions are negligible and the swelling is driven by the osmotic pressure of the ions. The swelling response of a mesoscopic gel is strongly dependent on the ionization degree of the gel. In particular, a maximum in the size–salt concentration curve appears for a partially ionized gel, when the salt concentration equals the network charge concentration. This maximum is removed for a totally ionized polymer network. Despite these facts, a charge independent asymptotic behavior between size and salt concentration becomes apparent, at sufficiently high values of the latter. The Flory–Huggins mean-field approach together with the Donnan relations describe the observed swelling adequately, thus accounting for the essential physics of the problem. As a final remark, an experimental method for determining gel charge will be inferred from the results.

In this work, the aggregation of microgel particles has been investigated, paying special attention to the structure of the clusters formed in the process. In particular, the aggregates' fractal dimension was determined by static light scattering. The results indicate that the aggregates are more compact than expected for diffusive aggregation. A reversible aggregation mechanism is proposed on the basis of the competition between osmotic and elastic contributions arising from the soft character of the particles. Aggregation proceeds in an energy minimum of restricted depth, giving rise to the formation of more compact clusters than expected. Finally, the process reversibility is tested, confirming the secondary minimum controlled aggregation.

Los microgeles son geles coloidales que se emplean por el inflado y/o desinflado que manifiestan ante ciertas condiciones externas. En este trabajo, se describe experimentalmente el inflado y el desinflado de particulas de gel. El pH del medio, su concentracion ionica y el esfuerzo osmotico externo son las variables externas que se emplean para provocar el cambio de fase en las particulas. Todas las transiciones ocurren por encima del punto critico del sistema. Adicionalmente, se presentan algunas de las aplicaciones en donde se emplea la transicion provocada por la variable externa concreta.

The influence of the swelling of charged microgel particles on their motion under an external electric field has been studied. The selected experimental observable was the electrophoretic mobility of the particles, which was measured as a function of the pH since its value controls the electrical charge of the particles. The mobility-pH curve presents a maximum and a minimum as a consequence of the competition between charge-density and friction coefficient variations during swelling. Ohshima's theory for polyelectrolyte-coated particles was employed, describing qualitatively the experimental results. Quantitative discrepancies suggest that charge renormalization should be considered.

The objective of this work is to test the very recent Levin's theory for charged colloids (Levin, Y.; Barbosa, M. C.; Tamashiro, M. N. Europhys. Lett. 1998, 41 (2), 123). To accomplish this, the fraction of condensed ions has been experimentally obtained from electrophoretic mobility data and surface charge values. Latexes with different functionalities, surface charges, and sizes were selected in order to support the universality of the ion condensation effect. The good agreement between charge and fraction of condensed ions confirms Levin's theory and makes ion condensation a clear candidate for explaining the observed insensitivity of the electrophoretic mobility to surface charge variations, for sufficiently large charge.

The swelling of cationic microgel particles has been studied experimentally, in the weak screening regime. The solution pH was selected as the external variable triggering the swelling, which was followed by dynamic light scattering. The particle charge was determined by conductometric and potentiometric titrations, leading to a good correlation between the charge of the microgel network and its size. This leads to the conclusion that the swelling is mainly charge controlled. The Flory−Huggins thermodynamic theory for gels, including a term accounting for the counterion distribution within the microgel, has been used to interpret the experimental data. The osmotic term associated with the counterions explains fairly well the observed behavior, and additional contributions due to the internal microgel microscopic structure are not necessary, as Pincus et al. have suggested.

The present paper deals with the electrokinetic characterization and colloidal stability of different TiO2/water interfaces under the presence of several electrolytes. The electrophoretic mobility of the TiO2 particles was measured versus NaCl, NaNO3, Na2SO4 and CaCl2 concentrations, at pHs 4, 5.5 and 10.5. ζ Potential calculations have been carried out with the aid of O’Brien and Whites’ conversion theory. The medium pH and the valence of the ions have proven to be of great influence in our suspension electrokinetic behaviour. The colloidal stability has been studied by means of the Eilers and Korff index. The different flocculant character of the ionic species has been established, which is of great importance in the solar photocatalytic proccess where our titanium dioxide is employed.

As research on the rational design of targeted drug delivery vehicles continue to expand, cell movement is a model system to emulate when designing synthetic systems. Cell migration is a central phenomenon that supports morphogenesis, regeneration, and wound healing. Cell migration can also physiologically contribute pathologically through cancer metastasis to healthy tissues and organs. Cells migrate as a result of dynamic changes in cell adhesion, cytoskeletal organization, and mechanotransduction, and the machinery within their cytoskeletons and plasma membranes allow them to carry out this migratory functions. Ultra-low crosslinked (ULC) microgel particles are synthetic deformable polymeric particles, which can be thought to model the viscoelasticity of the cytoplasm of cells in certain ways. Past research from our group has demonstrated that ULCs coated with specific domain-recognition motifs can induce clotting in vitro for wound healing. Particle adhesion, deformability, and stiffness were factors that contributed to the particles’ ability to close the wound, and ULC migration within the wound site also contributed significantly to the wound healing. These studies have lead to the desire to understand if synthetic particles migrate by similar biophysical principles as cells. In this work, we intend to demonstrate the physical environment required to induce ULC movement on a surface. We begin by demonstrating how ULC particle spreading impacts adhesion to a surface by measuring the bond interaction between ULC poly (N-isopropylacrylamide) [NIPAM] microgel particles, coated with biomolecules (i.e. short DNA sequences and antibodies), and glass surfaces coated with complementary sequences. This study is expected to provide a road map for tuning the adhesion properties of deformable particles on surfaces.

The propulsion system governing the motion of a kayak is studied, comparing it with the observed motion of the Central American reptile Basiliscus Basiliscus. The physical background under which both systems move is described. Special attention is paid to the way the applied forces are transmitted yielding the motion, as well as to the flow patterns that are generated. A brief description of the mechanics of kayaking is also given.

We study ionic microgel suspensions composed of swollen particles for various single-particle stiffnesses. We measure the osmotic pressure $\pi$ of these suspensions and show that it is dominated by the contribution of free ions in solution. As this ionic osmotic pressure depends on the volume fraction of the suspension $\phi$, we can determine $\phi$ from $\pi$, even at volume fractions so high that the microgel particles are compressed. We find that the width of the fluid-solid phase coexistence, measured using $\phi$, is larger than its hard-sphere value for the stiffer microgels that we study and progressively decreases for softer microgels. For sufficiently soft microgels, the suspensions are fluid-like, irrespective of volume fraction. By calculating the dependence on $\phi$ of the mean volume of a microgel particle, we show that the behavior of the phase-coexistence width correlates with whether or not the microgel particles are compressed at the volume fractions corresponding to fluid-solid coexistence., Comment: 5 pages, 3 figures

We present a small-angle x-ray scattering study of crystals formed by temperature-sensitive, swollen microgel particles consisting of poly($N$-isopropylacrylamide) copolymerized with acrylic acid and 5 mol % of a cross-linker. As for hard spheres, the random hexagonal close-packed structure is predominant during crystal growth and slowly transforms toward the face-centered-cubic structure. However, a transient phase of body-centered-cubic crystal is observed in an intermediate range of effective volume fractions. We estimate that the studied suspensions are close to a transition from face-centered-cubic to body-centered-cubic structure that can be understood by the tendency of the system to maximize the excluded volume and minimize the contact area between the particles.

We stabilize nematic droplets with handles against surface-tension-driven instabilities using a yield-stress material as outer fluid and study the complex nematic textures and defect structures that result from the competition between topological constraints and the elasticity of the nematic liquid crystal. We uncover a surprisingly persistent twisted configuration of the nematic director inside the droplets when tangential anchoring is established at their boundaries, which we explain after considering the influence of saddle-splay on the elastic free energy. For toroidal droplets, we find that the saddle-splay energy screens the twisting energy resulting in a spontaneous breaking of mirror symmetry; the chiral twisted state persists for aspect ratios as large as ~20. For droplets with additional handles, we observe in experiments and computer simulations that there are two additional -1 surface defects per handle; these are located in regions with local saddle geometry to minimize the nematic distortions and hence the corresponding elastic free energy., Comment: 23 pages, 4 figures, PNAS (2013)

A novel approach for the synthesis of silicone capsules using double W/O/W emulsions as templates is introduced. The low viscosity of the silicone precursors enables the use of microfluidic techniques to accurately control the size and morphology of the double emulsion droplets, which after cross-linking result in the desired monodisperse silicone capsules. Their shell thickness can be finely tuned, which in turn allows control over their permeability and mechanical properties; the latter are particularly important in a variety of practical applications where the capsules are subjected to large external forces. The potential of these capsules for controlled release is also demonstrated using a model hydrophilic substance.