Capillary breakup extensional rheometry of semi-dilute polymer solutions

Christian Clasen 한국유변학회 2010 Korea-Australia rheology journal Vol.22 No.4

In this paper the thinning and breakup of filaments of semi-dilute solutions of polystyrene in diethyl phthalate is investigated with a capillary breakup extensional rheometer. The solutions show a long initial visco-capillary balance that can be fitted with 0-D Newtonian and power law models. The onset of the elasto-capillary balance is molecular weight dependent and the coil stretch transition of the polymer is shifted close to the breaking point and to small filament diameters. However, the transition is even for concentrations up to c/c* ~ 30 observable due to a strong extension thinning in the Caber experiment. It is shown that the transition to the elasto-capillary balance takes place at local elasto-capillary number Ec* = 4.7 that incorporates the extensional relaxation time lE and the extension thinning viscosity hE,app.

Self-propelling micro-disks

Reddy, Naveen Krishna,Clasen, Christian 한국유변학회 2014 Korea-Australia rheology journal Vol.26 No.1

In this paper we introduce a simple and scalable method to produce micrometer sized 'Janus' disks whose rim is coated half with platinum/palladium and half with gold. The disks pinned upright to the air/liquid interface exhibit self-propulsion of ${\sim}100{\mu}m/s$ when submerged in $H_2O_2$ solution, due to the catalytic growth of oxygen bubbles at the disks upper (platinum/palladium-coated) rim. The disks exhibit two different travel trajectories, linear and rotary, depending on the bubble growth position, and are propelled via two different mechanisms, the bubble growth and the bubble burst. The displacement speed due to the bubble burst is three orders of magnitude larger than from the bubble growth process, whereas displacement distances are of similar order of magnitude for both processes.

Orthogonal superposition rheometry of colloidal gels: time-shear rate superposition

Sung, Sang Hoon,Kim, Sunhyung,Hendricks, Jan,Clasen, Christian,Ahn, Kyung Hyun The Royal Society of Chemistry 2018 Soft matter Vol.14 No.42

<P>We explore the relaxation behavior of model colloidal gels under steady shear flow by means of orthogonal superposition rheometry. Fumed silica and carbon black dispersions in Newtonian matrices are used as a model system. As shear rate increases, the frequency dependent orthogonal moduli of the gels shift along the frequency axis without changing their shape, which finally can be superimposed to yield a single master curve. This indicates that the shear rate tunes a master clock for overall relaxation modes in the sheared colloidal gels to produce a “time-shear rate superposition (TSS)”, as temperature does in polymeric liquids to produce a time-temperature superposition (TTS). The horizontal shift factor required at each shear rate to obtain the master curve is found to be directly proportional to the suspension viscosity for all the cases. From this result, we suggest that the suspension viscosity determines the overall relaxation time of the particles in the flowing colloidal gel.</P>

Depletion Stabilization in Nanoparticle–Polymer Suspensions: Multi-Length-Scale Analysis of Microstructure

Kim, Sunhyung,Hyun, Kyu,Moon, Joo Yong,Clasen, Christian,Ahn, Kyung Hyun American Chemical Society 2015 Langmuir Vol.31 No.6

<P>We study the mechanism of depletion stabilization and the resultant microstructure of aqueous suspensions of nanosized silica and poly(vinyl alcohol) (PVA). Rheology, small-angle light scattering (SALS), and small-angle X-ray scattering (SAXS) techniques enable us to analyze the microstructure at broad length scale from single particle size to the size of a cluster of aggregated particles. As PVA concentration increases, the microstructure evolves from bridging flocculation, steric stabilization, depletion flocculation to depletion stabilization. To our surprise, when depletion stabilization occurs, the suspension shows the stabilization at the cluster length scale, while maintaining fractal aggregates at the particle length scale. This sharply contrasts previously reported studies on the depletion stabilization of microsized particle and polymer suspensions, which exhibits the stabilization at the particle length scale. On the basis of the evaluation of depletion interaction, we propose that the depletion energy barrier exists between clusters rather than particles due to the comparable size of silica particle and the radius gyration of PVA.</P>

Differential Expression of Genes Associated with Endomembrane Transport and Cell Wall Metabolism during Ripening of ‘Chimarrita’ and ‘Granada’ Peach

Camila Pegoraro,Roberta Manica-Berto,F?bio Clasen Chaves,Carolina Terra Borges,Jader Job Franco,Cesar Valmor Rombaldi,Jorge Adolfo Silva 한국원예학회 2010 Horticulture, Environment, and Biotechnology Vol.51 No.3

Expression of genes involved in endomembrane transport and cell wall metabolism, particularly in synthesis and degradation of cell wall macromolecules were compared during the ripening of a white melting flesh cultivar, Chimarrita and a yellow non-melting flesh cultivar, Granada in order to evaluate their association with fruit softening. In both, most genes were up-regulated as ripening evolved. In ‘Chimarrita’, overall higher transcript accumulation occurred at 95 days after anthesis, while in ‘Granada’ higher relative transcription was detected at 115 days after anthesis. Although transcript accumulation was correlated with fruit softening, the process does not seem to be totally dependent on the set of studied genes, since the peak of RNA abundance occurred when flesh firmness had already decreased.

Drying of a Charge-Stabilized Colloidal Suspension in Situ Monitored by <i>Vertical</i> Small-Angle X-ray Scattering

Kim, Sunhyung,Hyun, Kyu,Kim, Yun Soo,Struth, Bernd,Clasen, Christian,Ahn, Kyung Hyun American Chemical Society 2013 Langmuir Vol.29 No.32

<P>We report a first application of <I>vertical</I> small-angle X-ray scattering to investigate the drying process of a colloidal suspension by overcoming gravity related restrictions. From the observation of the drying behavior of charge-stabilized colloidal silica in situ, we find the solidification of the colloidal particles exhibits an initial ordering, followed by a sudden aggregation when they overcome an electrostatic energy barrier. The aggregation can be driven not only by capillary pressure but also by thermal motion of the particles. The dominating contribution is determined by the magnitude of the energy barrier at the transition, which significantly decreases during drying due to an increased ionic strength.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/langd5/2013/langd5.2013.29.issue-32/la401897n/production/images/medium/la-2013-01897n_0009.gif'></P>

Structural Development of Nanoparticle Dispersion during Drying in Polymer Nanocomposite Films

Kim, Sunhyung,Hyun, Kyu,Struth, Bernd,Ahn, Kyung Hyun,Clasen, Christian American Chemical Society 2016 Macromolecules Vol.49 No.23

<P>Dispersions of nanoparticles (NPs) in a polymer matrix are a key element to set various properties of solution-cast polymer nanocomposite films. While the dispersion state of NPs in nanocomposite films has been extensively studied over the past decades, the structural development during drying and the relation of liquid and solid structure still remains poorly understood. In the present work, we study how NP dispersions develop during drying in polymer nanocomposite films, particularly focusing on the role of particle polymer interaction in the structural development. Aqueous dispersions of nanosilica and poly(vinyl alcohol) are used as model NP/polymer mixtures, where the particle polymer interaction can systemically be varied via the pH. A novel vertical small-angle X-ray scattering technique enables us to directly observe the development of the NP's dispersion state during drying. At a high pH 10, where silica particles have a poor affinity to PVA, SAXS intensity evolution shows phase separation during drying, resulting in the formation of dense aggregates of bare particles in the solid film. On the other hand, at a low pH 3, where silica particles have a good affinity with PVA, the SAXS data indicate a gradual densification of the NPs while maintaining a constant interparticle distance, which is originating from adsorbed PVA. The resulting solid film after drying exhibits an improved dispersion of NPs. The evaluation of the interparticle interaction suggests that the adsorbed polymer plays generally a key role in the uniform distribution of NPs in solid films, as it sterically stabilizes NPs over short ranges during all drying stages whereas depletion attraction dominates at longer ranges.</P>