confocal-imaging-confined-quiescent-flowing-colloid-polymer

Confocal Imaging of Confined Quiescent and Flowing Colloid-polymer Mixtures

The behavior of confined colloidal suspensions with attractive interparticle interactions is critical to the rational design of materials for directed ...

We show that colloidal particles with attractive interactions induced by a nonadsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction: With a shorter range, clusters are tenuous and branched; with a longer range, they are more compact.

Fluids of clusters in attractive colloids.

The microfluidic assembly of colloid-filled hydrogel granules of varying shape and composition is described. First, drops are formed by shearing a concentrated colloidal microsphere-acrylamide suspension in a continuous oil phase using a sheath-flow device. Both homogeneous and Janus (hemispherically distinct) spheres and disks are produced by confining the assembled drops in microchannels of varying geometry. Next, photopolymerization is carried out shortly after drop breakup to preserve their morphology. Representative wet and dried granules are characterized using fluorescence and scanning electron microscopy, respectively. Our approach offers a facile route for assembling colloid-filled hydrogel granules with controlled shape and composition.

Microfluidic assembly of homogeneous and Janus colloid-filled hydrogel granules.

Die Microfluidic-Versammlung des Kolloid gef&uuml;llten Hydrogel-Granulat von unterschiedlicher Form und Zusammensetzung wird beschrieben. Zun&auml;chst bilden sich Tropfen durch Scherung eine konzentrierte kolloidale Mikrosph&auml;ren-Acrylamid-Suspension in einer kontinuierlichen &Ouml;lphase mit einem Mantel-Flow-Ger&auml;t. Beide homogene und Janus (hemisph&auml;risch unterschiedliche) Kugeln und Festplatten werden von beschr&auml;nken die versammelten Tropfen in Mikrokan&auml;len unterschiedlicher Geometrie erzeugt. Als n&auml;chstes erfolgt Photopolymerisation kurz nach Drop Auseinanderbrechen ihrer Morphologie zu bewahren. Repr&auml;sentative nass und getrockneten K&ouml;rnchen zeichnen mit Fluoreszenz und Raster-Elektronenmikroskopie, beziehungsweise. Unser Ansatz bietet eine einfache Route f&uuml;r Montage Kolloid gef&uuml;llten Hydrogel-Granulat mit kontrollierter Form und Zusammensetzung.

Mikrofluidische Zusammenbau von homogenen und Janus Kolloid gef&uuml;llten Hydrogel-Granulat.

We use confocal microscopy to visualize individual particles near the colloidal glass transition. We identify the most slowly-relaxing particles and show that they form spatially correlated clusters that percolate across the sample. In supercooled fluids, the largest cluster spans the system on short time scales but breaks up on longer time scales. In contrast, in glasses, a percolating cluster exists on all accessible time scales. Using molecular dynamics simulation, we show that these clusters make the dominant contribution to the bulk elasticity of the sample.

Contribution of slow clusters to the bulk elasticity near the colloidal glass transition.

We investigate evaporative lithography as a route for patterning colloidal films. Films are dried beneath a mask that induces periodic variations between regions of free and hindered evaporation. Direct imaging reveals that particles segregate laterally within the film, as fluid and entrained particles migrate towards regions of higher evaporative flux. The films exhibit remarkable pattern formation that can be regulated by tuning the initial suspension composition, separation distance between the mask and underlying film, and mask geometry.

Patterning colloidal films via evaporative lithography.

Wir untersuchen Verdunstungs-Lithografie als eine Route f&uuml;r die Musterung kolloidale Filme. Filme werden unter einer Maske getrocknet, die periodische Ver&auml;nderungen zwischen Regionen frei und gehinderte Verdunstung bedingt. Direkte Bildgebung zeigt, dass Partikel seitlich im Film trennen, fl&uuml;ssige und mitgerissen Teilchen in Richtung Regionen h&ouml;here Verdunstungs-Fluss zu migrieren. Die Filme zeigen bemerkenswerte Musterbildung, die durch das Optimieren der anf&auml;nglichen Aussetzung Komposition, Schutzabstand zwischen Maske und zugrunde liegenden Film und Maske Geometrie geregelt werden kann.

Musterung kolloidale Filme &uuml;ber Verdunstungs-Lithografie.

We investigate the structure and flow behavior of colloidal gels in microchannels using confocal microscopy. Silica particles are first coated with a cationic polyelectrolyte and then flocculated by the addition of an anionic polyelectrolyte. In the quiescent state, the suspension is an isotropic and homogeneous gel. Under shear flow, the suspension contains dense clusters that yield at intercluster boundaries, resulting in network breakup at high shear rates. These structural changes coincide with a transition from pluglike flow at low pressures to fluidlike flow at high pressures.

Structure of colloidal gels during microchannel flow.

Wir untersuchen die Struktur und Verhalten der kolloidale Gele in Mikrokan&auml;len mit confocal Mikroskopie flie&szlig;en. Kiesels&auml;ure-Teilchen sind zun&auml;chst mit einer kationischen Polyelectrolyte beschichtet und dann durch die Zugabe von einem anionischen Polyelectrolyte flocculated. In den Ruhezustand ist die Aussetzung einer isotropen und homogenen Gel. Unter Scherung flie&szlig;en enth&auml;lt die Aussetzung Dichte Clustern, die Ausbeute an intercluster Grenzen, was zu Netzwerk-Trennung zu hohe Scherung Preisen. Diese strukturellen Ver&auml;nderungen zeitgleich mit einem &Uuml;bergang von pluglike flie&szlig;en bei niedrigen Dr&uuml;cken zum fluidlike Flow bei hohen dr&uuml;cken.

Struktur der kolloidale Gele w&auml;hrend Microchannel-Fluss.

Evaporative lithography offers a promising new route for patterning a broad array of soft materials. In this approach, a mask is placed above a drying film to create regions of free and hindered evaporation, which drive fluid convection and entrained particles to regions of highest evaporative flux. We show that binary colloidal films exhibit remarkable pattern formation when subjected to a periodic evaporative landscape during drying.

Evaporative lithographic patterning of binary colloidal films.

Verdunstungs-Lithografie bietet einen viel versprechender neuen Weg f&uuml;r die Musterung einer breiten Palette von weichen Materialien. Bei diesem Ansatz wird eine Maske platziert, oberhalb einer trocknenden Film Regionen von freien und gehinderte Verdampfung zu erstellen, die Fl&uuml;ssigkeit Konvektion und mitgerissen Teilchen zu Regionen der h&ouml;chsten Verdunstungs-Fluss zu fahren. Wir zeigen, dass bin&auml;re kolloidale Filme bemerkenswerte Strukturbildung zeigen bei einer periodischen Verdunstungs-Landschaft w&auml;hrend der Trocknung unterzogen.

Evaporative lithographischen Musterung bin&auml;re kolloidale Filme.

We investigate the structure of colloidal gels flowing through constrictions in microchannels using confocal microscopy. As the gel traverses the constricted region, both the average velocity and particle density increase downstream. While the average flow profile is smoothly varying, stagnation zones develop at the constriction entry, leading to markedly nonuniform local flow profiles. Dense clusters undergo shear-induced yielding at intercluster boundaries, which enhances the structural heterogeneity of the suspension at the constriction outlet.

Structural evolution of colloidal gels during constricted microchannel flow.

Wir untersuchen die Struktur der kolloidale Gele flie&szlig;t durch Einschn&uuml;rungen in Mikrokan&auml;len mit confocal Mikroskopie. Das Gel die engen Region durchl&auml;uft, erh&ouml;hen die durchschnittliche Geschwindigkeit und die Teilchendichte stromabw&auml;rts. W&auml;hrend das durchschnittliche Durchfluss-Profil problemlos unterschiedliche ist, entwickeln Stagnation Zonen am Eingang Verengung, f&uuml;hrt zu deutlich ungleichm&auml;&szlig;ige lokale Str&ouml;mung-Profile. Dichte Clustern unterzogen werden induziert durch Scherung intercluster Grenzen, woraus die verbessert der strukturellen Heterogenit&auml;t der Aufh&auml;ngung die Verengung Steckdose an.

Strukturelle Entwicklung der kolloidale Gele w&auml;hrend engen Microchannel-Fluss.

Bacterial biofilms are structured multicellular communities involved in a broad range of infections. Knowing how free-swimming bacteria adapt their motility mechanisms near surfaces is crucial for understanding the transition between planktonic and biofilm phenotypes. By translating microscopy movies into searchable databases of bacterial behavior, we identified fundamental type IV pili-driven mechanisms for Pseudomonas aeruginosa surface motility involved in distinct foraging strategies. Bacteria stood upright and "walked" with trajectories optimized for two-dimensional surface exploration. Vertical orientation facilitated surface detachment and could influence biofilm morphology.

Bacteria use type IV pili to walk upright and detach from surfaces.

Bacterial biofilms are structured multicellular communities that are responsible for a broad range of infections. Knowing how free-swimming bacteria adapt their motility mechanisms near a surface is crucial for understanding the transition from the planktonic to the biofilm phenotype. By translating microscopy movies into searchable databases of bacterial behavior and developing image-based search engines, we were able to identify fundamental appendage-specific mechanisms for the surface motility of Pseudomonas aeruginosa. Type IV pili mediate two surface motility mechanisms: horizontally oriented crawling, by which the bacterium moves lengthwise with high directional persistence, and vertically oriented walking, by which the bacterium moves with low directional persistence and high instantaneous velocity, allowing it to rapidly explore microenvironments. The flagellum mediates two additional motility mechanisms: near-surface swimming and surface-anchored spinning, which often precedes detachment from a surface. Flagella and pili interact cooperatively in a launch sequence whereby bacteria change orientation from horizontal to vertical and then detach. Vertical orientation facilitates detachment from surfaces and thereby influences biofilm morphology.

Flagella and pili-mediated near-surface single-cell motility mechanisms in P. aeruginosa.

Bacteria optimize the use of their motility appendages to move efficiently on a wide range of surfaces prior to forming multicellular bacterial biofilms. The "twitching" motility mode employed by many bacterial species for surface exploration uses type-IV pili (TFP) as linear actuators to enable directional crawling. In addition to linear motion, however, motility requires turns and changes of direction. Moreover, the motility mechanism must be adaptable to the continually changing surface conditions encountered during biofilm formation. Here, we develop a novel two-point tracking algorithm to dissect twitching motility in this context. We show that TFP-mediated crawling in Pseudomonas aeruginosa consistently alternates between two distinct actions: a translation of constant velocity and a combined translation-rotation that is approximately 20× faster in instantaneous velocity. Orientational distributions of these actions suggest that the former is due to pulling by multiple TFP, whereas the latter is due to release by single TFP. The release action leads to a fast "slingshot" motion that can turn the cell body efficiently by oversteering. Furthermore, the large velocity of the slingshot motion enables bacteria to move efficiently through environments that contain shear-thinning viscoelastic fluids, such as the extracellular polymeric substances (EPS) that bacteria secrete on surfaces during biofilm formation.

Bacteria use type-IV pili to slingshot on surfaces.

Quantifying collective behavior in mammalian cells.

Using a model colloid-polymer suspension, we show that confinement induces solidification in attractive colloidal suspensions via a fundamentally different route from that active in hard sphere colloidal suspensions. For a range of polymer concentrations, the suspensions undergo a phase transition from a colloidal fluid of clusters to a colloidal gel as confinement increases while polymer and particle concentration are held constant. In both fluid- and solidlike attractive suspensions, effects of confinement on the structure and dynamics appear at much larger thicknesses than for hard-sphere suspensions. The solidification does not originate from structuring of the colloids by the walls. Instead, by analyzing cluster size distributions in the fluid phase and particle dynamics in the gel phase as a function of confinement, we find that the strength of the effective interparticle attraction increases as the samples are confined. We show that the increase in the effective attraction can be understood as a consequence of the increasing importance of excluded volume due to the walls to the free energy of the polymer as confinement is increased.

Confinement-induced solidification of colloid-polymer depletion mixtures.

We review physically-motivated studies of bacterial near-surface motility driven by flagella and type IV pili (TfP) in the context of biofilm formation. We describe the motility mechanisms that individual bacteria deploying flagella and TfP use to move on and near surfaces, and discuss how the interactions of motility appendages with fluid and surfaces promote motility, attachment and dispersal of bacteria on surfaces prior to biofilm formation.

Physics of bacterial near-surface motility using flagella and type IV pili: implications for biofilm formation.

During embryonic development and in metastatic cancers, cells detach from the epithelium and migrate with persistent directionality. Directional cell migration is also crucial for the regeneration and maintenance of the epithelium and impaired directional migration is linked to chronic inflammatory diseases. Despite its significance, the mechanisms controlling epithelial cell migration remain poorly understood. Villin is an epithelial-cell-specific actin modifying protein that regulates epithelial cell plasticity and motility. In motile cells villin is associated with the highly branched and the unbranched actin filaments of lamellipodia and filopodia, respectively. In this study we demonstrate for the first time that villin regulates directionally persistent epithelial cell migration. Functional characterization of wild-type and mutant villin proteins revealed that the ability of villin to self-associate and bundle actin as well as its direct interaction with phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] regulates villin-induced filopodial assembly and directional cell migration. Our findings suggest that convergence of different signaling cascades could spatially restrict villin activity to areas of high PtdIns(4,5)P(2) and F-actin concentration to assemble filopodia. Furthermore, our data reveal the ability of villin to undergo actin- and PtdIns(4,5)P(2)-induced self-association, which may be particularly suited to coalesce and reorganize actin bundles within the filopodia.

Regulation of directional cell migration by membrane-induced actin bundling.

The diffusive dynamics of dilute dispersions of nanoparticles of diameter 200-400 nm were studied in microfabricated arrays of nanoposts using differential dynamic microscopy and single particle tracking. Posts of diameter 500 nm and height 10 μm were spaced by 1.2-10 μm on a square lattice. As the spacing between posts was decreased, the dynamics of the nanoparticles slowed. Moreover, the dynamics at all length scales were best represented by a stretched exponential rather than a simple exponential. Both the relative diffusivity and the stretching exponent decreased linearly with increased confinement and, equivalently, with decreased void volume. The slowing of the overall diffusive dynamics and the broadening distribution of nanoparticle displacements with increased confinement are consistent with the onset of dynamic heterogeneity and the approach to vitrification.

Diffusive dynamics of nanoparticles in arrays of nanoposts.

Nanoparticles transported through highly confined porous media exhibit faster breakthrough than small molecule tracers. Despite important technological applications in advanced materials, human health, energy, and environment, the microscale mechanisms leading to early breakthrough have not been identified. Here, we measure dispersion of nanoparticles at the single-particle scale in regular arrays of nanoposts and show that for highly confined flows of dilute suspensions of nanoparticles the longitudinal and transverse velocities exhibit distinct scaling behaviors. The distributions of transverse particle velocities become narrower and more non-Gaussian when the particles are strongly confined. As a result, the transverse dispersion of highly confined nanoparticles at low Péclet numbers is significantly less important than longitudinal dispersion, leading to early breakthrough. This finding suggests a fundamental mechanism by which to control dispersion and thereby improve efficacy of nanoparticles applied for advanced polymer nanocomposites, drug delivery, hydrocarbon production, and environmental remediation.

Article	Year
Fluids of clusters in attractive colloids. Physical review letters\| PubMed ID: 16486659	2006
Microfluidic assembly of homogeneous and Janus colloid-filled hydrogel granules. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 17014093	2006
Contribution of slow clusters to the bulk elasticity near the colloidal glass transition. Physical review letters\| PubMed ID: 17280428	2006
Patterning colloidal films via evaporative lithography. Physical review letters\| PubMed ID: 17501317	2007
Structure of colloidal gels during microchannel flow. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 18582141	2008
Evaporative lithographic patterning of binary colloidal films. Philosophical transactions. Series A, Mathematical, physical, and engineering sciences\| PubMed ID: 19933133	2009
Structural evolution of colloidal gels during constricted microchannel flow. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 20369847	2010
Bacteria use type IV pili to walk upright and detach from surfaces. Science (New York, N.Y.)\| PubMed ID: 20929769	2010
Flagella and pili-mediated near-surface single-cell motility mechanisms in P. aeruginosa. Biophysical journal\| PubMed ID: 21463573	2011
Bacteria use type-IV pili to slingshot on surfaces. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 21768344	2011
Quantifying collective behavior in mammalian cells. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 22570502	2012
Confinement-induced solidification of colloid-polymer depletion mixtures. Physical review letters\| PubMed ID: 23030210	2012
Physics of bacterial near-surface motility using flagella and type IV pili: implications for biofilm formation. Research in microbiology\| PubMed ID: 23103335	2012
Regulation of directional cell migration by membrane-induced actin bundling. Journal of cell science\| PubMed ID: 23132923	2013
Diffusive dynamics of nanoparticles in arrays of nanoposts. ACS nano\| PubMed ID: 23672180	2013
Transport and Dispersion of Nanoparticles in Periodic Nanopost Arrays. ACS nano\| PubMed ID: 24738873	2014

Jacinta C. Conrad

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