19 research outputs found

    Jamming and overpacking fuzzy microgels: Deformation, interpenetration, and compression

    Get PDF
    Tuning the solubility of fuzzy polymer microgels by external triggers, such as temperature or pH, provides a unique mechanism for controlling the porosity and size of colloidal particles on the nanoscale. As a consequence, these smart microgel particles are being considered for applications ranging from viscosity modifiers and sensing to drug delivery and as models for the glass and the jamming transition. Despite their widespread use, little is known about how these soft particles adapt their shape and size under strong mechanical compression. We use a combination of precise labeling protocols and two-color superresolution microscopy to unravel the behavior of tracer microgels inside densely packed soft solids. We find that interpenetration and shape deformation are dominant until, in the highly overpacked state, this mechanism saturates and the only remaining way to further densify the system is by isotropic compression

    Superresolution microscopy of individual and densely packed pNIPAM microgels

    Get PDF
    Responsive microgels are among the most studied polymeric systems of the last decades. The N-isopropylacrylamide (NIPAM) monomer can be readily cross-linked during synthesis to obtain polymer microgel particles with a size that can be controlled in the range 100-1000nm. The vast interest stems from the fact that the polymer is thermosensitive with a lower-critical solution temperature of approximately 32°C, which is close to physiological conditions. The well-defined shape and size of a colloid provides control over the microstructural length scales and response times while the polymeric nature offers physico-chemical control parameters that can be sensitive to external stimuli. Please click Additional Files below to see the full abstract

    Field-directed assembly of responsive colloids

    Get PDF
    Field-directed self-assembly (DSA) has recently moved into the focus of the soft-matter and nanotechnology community. It employs the basic principles of self-assembly through carefully chosen building blocks, but the underlying self-assembly process is then aided or modulated using external fields. Here we demonstrate how we can apply a combination of responsive nanoparticles and external electromagnetic fields in order to modulate the intrinsic interparticle interactions and tune the subtle balance between thermal motion and the action of interparticle forces, and thus generate novel self-assembled structures. We will show in particular how we can use field-driven self-assembly to induce phase transitions, cycle through various equilibrium and non-equilibrium phases, and study the micro-structural changes and the underlying kinetic mechanisms of these phase transitions in-situ and in real time. Moreover, we will demonstrate the effect of particle anisotropy in field-driven assembly. References [1] J. J. Crassous, A. M. Mihut, E. Wernersson, P. Pfleiderer, J. Vermant, P. Linse, and P. Schurtenberger, Nature Communications 5 (2014) 5516. [2] P. S. Mohanty, P. Bagheri, S. Nöjd, A. Yethiraj and P. Schurtenberger, Phys. Rev. X 5 (2015) 011030

    Structure, Dynamics and Phase Behaviour of Charged Soft Colloidal Dispersions

    No full text
    Soft and deformable ionic microgels such as poly(N-isopropylacrylamide-co-acrylic acid), PNIPAM-co-AA, microgels have shown to be a versatile alternative to the already well-established hard sphere model systems. Their tuneable interaction potential makes them well suited as model systems to study the phase behaviour found for particles interacting via a soft isotropic potential. This stems from the microgels ability to respond to changes in the environment, such as changes in temperature, pH or particle concentration. Additionally, subjecting these particles to an alternating electric field induces a dipolar contribution to the interaction potential, which strongly depends on the amplitude and frequency of the applied field. This additional, tuneable and directional attraction allows us to explore and deepen our understanding of systems interacting via an even more complex and anisotropic potential.At low number densities particles are shown to be aligning into strings along the direction of the field. These strings assemble into crystal structures as the field strength is further increased. At concentrations above freezing the parent face-centred cubic, FCC, structure is found to melt and diffusively transforms into a BCT crystal via nucleation and growth but in the reverse direction, the BCT phase transforms cooperatively into a metastable body-centred orthorhombic, BCO, phase, which only relaxes back to the parent FCC phase as the temperature is increased. The kinetics is consequently either diffusive or martensitic depending on the path and is believed to be due to the interpenetrable nature of the microgel particles. In order to learn more about the origin of this puzzling path dependence, we studied the shape and size of the particles as a function of packing fraction and field strength by performing a combination of scattering experiments, both in the absence and presence of the electric field. We found that the particle size is dramatically altered by a small increase in particle concentration to reach a plateau value at intermediate concentration. In the over-packed state the particle size is again seen to decrease due to shell overlap. The applied electric field however was shown to only slightly alter the particle size, thus confirming the interpenetration of particles in field-induced structures. As a last step we performed dielectric spectroscopy measurements to obtain information about the polarisation mechanisms present in the system.In the future the collected information will be used to derive a theoretical model that will provide us with the intrinsic and field-induced interaction potential at the relevant concentrations and field-strengths. This potential will be compared to obtained data in the absence and presence of the alternating electric field

    Microgels at ultrahigh densities : a small angle neutron scattering study

    No full text
    Populärvetenskaplig sammanfattning Microgelpartiklar är runda partiklar som har förmågan att ändra storlek beroende på olika yttre påverkan. I detta projekt har vi tittat på partiklar som blir upp till 10 gånger mindre när temperaturen höjs från 15˚C till 39˚C. Detta beror på att det nätverk som partiklarna är uppbyggda börjar ogilla lösningsmedlet när temperaturen höjs och hellre vill vara nära sig själva vilket medför att de drar ihop sig. Eftersom microgeler kan ändra storlek under kontrollerade former vill man titta på hur de beter sig om man ökar antalet partiklar. Detta kan man sedan jämföra med mer komplicerade system så som proteiner och hur de beter sig i närheten av varandra. För att titta på detta har vi använt oss av neutroner och en metod som gör det möjligt att titta på enbart ett fåtal microgelpartiklar i en lösning med väldigt många. Neutroner stålas på provet, microgelpartiklarna, och beroende på hur stora partiklarna i provet är kommer neutronerna efter kollisionen med partiklarna att böja av åt olika håll. Det går sedan att räkna hur många neutroner som har böjt av i olika vinklar och utifrån det få reda på hur stora partiklarna i provet är och vilken form de har. Det är av olika anledningar komplicerat att räkna ut storleken på partiklar i prover med hög koncentration. För att komma runt det har vi använt en metod som går ut på att proverna blandas på ett visst sätt så att information om enskilda microgelpartklar är det ända som mäts. På så vis kan storleken mätas utan att mängden partiklar stör mätningen. Det visade sig att metoden fungerade bra och information om de enskilda partiklarna kunde erhållas. En storleksminskning med ökad temperatur påvisades vilket var väntat då partiklarna drar ihop sig när temperaturen höjs. Det visades även att storleken inte påverkades mycket när antalet partiklar ökades. Detta bevisar att partiklarna gå in i varandra eftersom deras yttersta skal är väldigt fluffigt.Microgel particles have been widely investigated for their ability to undergo a volume phase transition due to different stimuli such as solvent quality, pH or salt addition. Their ability to reversibly change their size and thus the volume fraction makes them suitable to perform thorough studies on intra and inter‐particle interactions and various phase transitions. In this research work we show how the particle interactions were affected with changing temperature, and thereby volume fraction, in highly concentrated suspensions. For this purpose, the zero average contrast method (ZAC) has been employed, which allows canceling the effects of interparticle interactions in the scattering data, thus giving information about the form factor of the particles even at very high volume fractions. The results show that at low temperatures, i.e. in the fully expanded state, the microgel size appears almost independent of volume fraction, with a small decrease of the outer fuzzy shell only at concentrations far above close packing. With increasing temperature we find a shift in the form factor to higher q, which was expected due to a decrease in particle size: particles collapse when water starts to act as a poorer solvent at temperatures above the LCST (Lower Critical Solution Temperature). This temperature‐induced collapse of the particle size is found to be independent of the effective volume fraction eff even at ultrahigh densities up to eff = 1.7, i.e., far above close packing. These results clearly demonstrate that the temperature‐responsive behavior of the microgels is not altered by the interpenetration of the particles at high densities, and that the effective volume fraction of the microgels is a function of the temperature only

    Electric field driven self-assembly of ionic microgels

    Get PDF
    We study, using fluorescent confocal laser scanning microscopy, the directed self-assembly of cross-linked ionic microgels under the influence of an applied alternating electric field at different effective packing fractions ϕeff in real space. We present a detailed description of the contribution of the electric field to the soft interparticle potential, and its influence on the phase diagram as a function of ϕeff and field strength E at a constant frequency of 100 kHz. In our previous work [Mohanty et al., Soft Matter, 2012, 8, 10819], we demonstrated the existence of field-induced structural transitions both at low and high ϕeff. In this work, we revisit the phase behavior at low and intermediate ϕeff with a focus on both structure and dynamics. We demonstrate the existence of various field induced transitions such as an isotropic fluid to string phase to body centered tetragonal (BCT) crystal phase at low concentrations and a reversible field-induced crystal (face centered cubic, FCC) to crystal (BCT) transition at intermediate concentrations. We also investigate the kinetics of the crystal–crystal transition and demonstrate that this occurs through an intermediate melting process. These results are discussed in the light of previous studies of dipolar hard and charged colloids

    Multiple Path-Dependent Routes for Phase-Transition Kinetics in Thermoresponsive and Field-Responsive Ultrasoft Colloids

    Get PDF
    The nature of solid-solid phase transformations has been a long-standing question spanning the fields of metallurgy and condensed-matter physics, with applications from metallic alloys and ceramics to modern shape-memory materials. In spite of the importance of solid-to-solid transformations in many areas of materials science and condensed-matter physics and the numerous experimental and theoretical studies, a deep understanding of the microstructural changes and the underlying kinetic mechanisms is still missing. In this work, we establish a versatile model system composed of micron-scale ionic microgel colloids, where we not only probe the single-particle kinetics in real space and real time but also tune the phase transition in a multiple-parameter space. In the presence of an imposed electric field, a face-centered cubic (FCC) crystal transforms diffusively into a body-centered tetragonal (BCT) crystal via nucleation and growth. In the reverse direction, however, the BCT phase transforms cooperatively into a long-lived metastable body-centered orthorhombic phase, which only relaxes back to the equilibrium FCC when annealed at higher temperatures. The kinetics is thus either diffusive or martensitic depending on the path, and we believe that these two path-dependent transitions provide the first real-space, particle-level insights of diffusive and martensitic transformations, respectively, in a single system

    Crystal to crystal transformation in soft ionic microgels: Kinetics and the role of local mechanical susceptibilities

    No full text
    We report confocal microscopy experiments of electric-field-driven crystal-crystal transformations in microgel colloids with varying particle softness. The transformation kinetics are well described by phenomenological nucleation and growth theory. Using a spatial projection formalism, we determine local susceptibilities to affine shear deformation and nonaffine defect formation in the parent crystal. Curiously, the softer particles have lower shear strain susceptibility, while nonaffine susceptibility and phenomenological growth coefficients depend nonmonotonically on particle softness.publishe

    Mucoadhesion : mucin-polymer molecular interactions

    No full text
    Mucoadhesion, adhesion of a material to a mucous membrane or a mucus-covered surface, has been employed in drug delivery to prolong contact with adsorption sites and consequently a likely improvement of drug absorption. Mucoadhesion in the oral cavity also provides additional effects on tactile mouthfeel and extended flavor delivery, which impact consumer perception. The mechanisms behind mucoadhesion have not been well understood and there are contradictory literature results on the ranking of mucoadhesive properties of different polymers based on what in-vitro methods that are used. We herein examine the molecular interactions of different polymers with mucin from bovine submaxillary glands at pH 6.6 by using 1H NMR (Nuclear Magnetic Resonance) that provides atomically resolved information on conformational mobility of the mucin. Studying different types of polymers with different chemical structures and degrees of polymerization (DP), we can via the NMR linewidths and the signal intensities distinguish if the polymers interact with specific segments of the mucin or if they have a universal effect on the mobility of all the molecular segments of the mucin. The specific interaction sites on the mucin for positively charged polymer poly(ethyleneimine) are shown to be different from those for negatively and neutrally charged polymers. In addition, the interactions are also driven by the DP, the concentration of the polymers, and the dehydration. Deepened understanding of molecular effects of the different polymers on the mucin can therefore have strong impact on the development of mucoadhesive products in pharmaceutical and food applications. Finally, we raise awareness of the interpretation of rheological data in terms of mucoadhesion

    Deswelling behaviour of ionic microgel particles from low to ultra-high densities

    No full text
    The swelling of ionic microgel particles is investigated at a wide range of concentrations using a combination of light, X-ray and neutron scattering techniques. We employ a zero-average contrast approach for small-angle neutron scattering experiments, which enables a direct determination of the form factor at high concentrations. The observed particle size initially decreases strongly with the particle concentration in the dilute regime but approaches a constant value at intermediate concentrations. This is followed by a further deswelling at high concentrations above particle overlap. Theory and experiments point at a pivotal contribution of dangling polymer ends to the strong variation in size of ionic microgels, which presents itself mainly through the hydrodynamics properties of the system
    corecore