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Albert P Philipse

Publications and source records attributed to Albert P Philipse.

12 recordsLinked to original sources
PubMed2006

In situ imaging of field-induced hexagonal columns in magnetite ferrofluids.

Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids in 2D. The columns are distorted by lens-shaped voids, due to the weak interchain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions that eventually dominate the fluctuation-induced attractions between dipole chains that initiate the columnar transition.

Journal Article↗
PubMed2006

Polymer-in-a-silica-crust membranes: macroporous materials with tunable surface functionality.

We report on alkaline hydrolysis of tetraethoxysilane (Stöber synthesis) inside a macroporous polymer matrix resulting in a homogeneous coverage of silica onto the polymer surface. The encapsulation of the polymer struts by a continuous silica crust allows further functionalization with hydrophilic and hydrophobic silylating agents. The porous silica polymeric hybrid material combines the morphological control and mechanical flexibility of the polymeric matrix with the convenient surface modifications developed for glass and amorphous silica. This concept is applied to macroporous membranes where alteration in surface functionality allows tuning of hydrophobicity (contact angle and liquid entry pressure), streaming potential, and adsorption capacity of double-stranded DNA.

Adsorption↗
PubMed2006

Quantitative real-space analysis of self-assembled structures of magnetic dipolar colloids.

We present the first real-space analysis on a single-particle level of the dipolar chains and branched clusters self-assembling in magnetic fluids in zero field. Spatial correlations and chain-length distributions directly obtained from tracked particle positions in vitrified films of synthetic magnetic (Fe3O4) dispersions provide a quantitative test for simulations and theory of dipolar fluids. A pertinent example is the cluster-size distribution that can be analyzed with a one-dimensional aggregation model to yield a dipolar attraction energy that agrees well with the dipole moment found from independent magnetization measurements.

Journal Article↗
PubMed2005

Monodisperse DNA restriction fragments I. Synthesis and characterization.

We present a convenient and low-cost method to prepare milligram amounts of completely monodisperse DNA restriction fragments in a physico-chemical laboratory setting to study (in part II) the effect of limited flexibility on the concentration dependent sedimentation velocity. Four fragments of 200, 400, 800, and 1600 bp were designed to span a range of 1-11 persistence lengths. The fragments were synthesized by cloning fragments of controlled lengths obtained by PCR into bacterial plasmid DNA. The constructs were amplified in large-scale bacterial cultures from which the fragments were obtained by a modified alkaline lysis procedure and subsequent digestion with EcoRV. A method is presented to isolate the DNA from the digestion mixture using horizontal agarose-slab gels and agarose columns in a home-built preparative gel electrophoresis set-up. We show that a combination of optical absorbance readings, ethidium bromide fluorescence, and hyperchromicity measurements allows assessment of both the purity of the DNA solutions and the fraction of double-stranded DNA.

Cloning, Molecular↗
PubMed2005

Monodisperse DNA restriction fragments II. Sedimentation velocity and equilibrium experiments.

We report sedimentation velocity and equilibrium measurements performed with an analytical ultracentrifuge to elucidate the effects of limited flexibility on the transport properties of semiflexible, monodisperse, double-stranded, blunt-ended DNA restriction fragments. We study a homologous series of fragments with 400, 800, and 1600 base pairs (3 to 11 persistence lengths), which are specifically designed and synthesized for this purpose (Part I). The molecular weights following from the sedimentation measurements agree well with the values expected on the basis of the number of base pairs. The sedimentation coefficients at infinite dilution are in good agreement with theoretical predictions for wormlike cylinders. The first order in volume fraction (varphi) coefficient K of the varphi-dependent sedimentation coefficient s(varphi)=1-Kvarphi decreases from 1178 for the shortest fragment to 882 for the longest fragment. These values are much larger than predicted for uncharged rigid rods, indicating the presence of associates with an enhanced aspect ratio and excluded volume. The precise match of the molecular weights obtained from exponential sedimentation-diffusion equilibrium distributions with weights calculated from the number of base pairs shows that any association is reversible and disappears at sufficiently low DNA concentration.

Colloids↗
PubMed2005

Thermal motion of magnetic iron nanoparticles in a frozen solvent.

The thermal rotation of iron nanoparticles dispersed in cyclohexane was studied by measuring the dynamic magnetic susceptibility above and below the freezing point of the solvent. Above the freezing point, the orientation of the magnetic dipoles changes mainly by reorientation of the entire particle. Below the freezing point, complete arrest of particle motion was expected, such that the magnetic dipoles would only be able to reorient themselves inside the nanoparticles (Neel relaxation). However, we find that thermal motion continues well below the temperature at which the bulk of the solvent is frozen. We ascribe this to local lowering of the freezing point, due to the presence of polymers in the close vicinity of the colloids. Furthermore, because strong dipole-dipole interactions result in the formation of dipolar chains, we have systematically studied the effect of particle size on dynamics in a frozen solvent. For the larger particles, our data indicate that local wiggling of the individual particles in a chain may become the dominating mode of thermal motion.

Journal Article↗
PubMed2004

Direct imaging of zero-field dipolar structures in colloidal dispersions of synthetic magnetite.

Magnetite (Fe3O4) forms the basis of most dispersions studied in the field of magnetic fluids and magnetic colloids. Despite extensive theory and simulations on chain formation in dipolar fluids in zero field, such structures have not yet been imaged in laboratory-made magnetite dispersions. Here, we present the first direct observation of dipolar chain formation in zero field in a ferrofluid containing the largest synthetic single-domain magnetite particles studied so far. To our knowledge, this is the only ferrofluid system available at present that allows quantifying chain length and ring-size distributions of dipolar structures as a function of concentration and particle size.

Journal Article↗
PubMed2004

Evidence for a macroscopic electric field in the sedimentation profiles of charged colloids.

The determination of molecular masses from barometric sedimentation profiles, a main topic in ultracentrifugal analysis, is thought to be quantitatively correct for non-interacting particles. Whereas this expectation is justified for uncharged colloids or macromolecules at low volume fractions, early ultracentrifugation studies on charged particles had already indicated that the obtained masses might be much too low. More recently, expanded sedimentation profiles have been observed for charged particles, sometimes inflated by orders of magnitude relative to the barometric prediction, which highlights a shortcoming in our understanding of centrifugation of even very dilute charged species. Theory and simulations, anticipated by various authors, now propose that strongly non-barometric sedimentation profiles might be caused by an internal macroscopic electric field that, even for non-interacting particles, significantly decreases the buoyant particle mass. The existence of this field and its intriguing consequences still lack experimental verification. Here we report ultracentrifugation experiments on charged colloidal silica spheres, showing both the existence of such a macroscopic electric field and its drastic effects on the sedimentation profiles of very dilute dispersions at low ionic strength.

Journal Article↗
PubMed2003

On the validity of Stokes-Einstein-Debye relations for rotational diffusion in colloidal suspensions.

According to the Stokes-Einstein-Debye (SED) relation, the rotational diffusion coefficient of a colloidal tracer sphere scales with the inverse of the solvent viscosity. Here we investigate the generalization of the SED relation to tracer diffusion in suspensions of neutral and charged colloidal host spheres. Rotational diffusion coefficients are measured with dynamic light scattering and phosphorescence spectroscopy, and calculated including two- and three-particle hydrodynamic interactions. We find that rotational tracer diffusion is always faster than predicted by the SED relation, except for large tracer/host size ratios lambda. In the case of neutral particles this observation is rationalized by introducing an apparent lambda-dependent slip boundary coefficient. For charged spheres at low ionic strength, large deviations from SED scaling are found due to the strongly hindered host sphere dynamics. Finally, we present some first experiments on tracer sphere diffusion in suspensions of host rods, showing that hydrodynamic hindrance by rods is much stronger than by spheres. We conclude by pointing to some interesting unresolved issues for future research.

Journal Article↗
PubMed2002

Preparation and properties of starch-based colloidal microgels.

Novel starch microgels were prepared by emulsion cross-linking and characterized with respect to shape, volume, and mass density. Starch microgels appear to be negatively charged ( approximately -50 mV), with a particle size varying as a function of the type of cross-linker (ca. 0.25-10 microm). Environmental scanning electron microscopy observations show a dependence of the particle swelling on the cross-linking density. Viscosimetry reveals that starch microgels behave as charged polymers, where the reduced viscosity increases with dilution (anomalous viscosity behavior) for sufficiently low kappaalpha (ca. kappaalpha <3), the ratio of the particle radius (a), and the Debye length (kappa(-1)). Analogous results are obtained for reference-charged rigid silica spheres, which approach the hard sphere limit for increasing ionic strengths. The shape of the microgels appears to play a minor role in the anomalous viscosity behavior, which is more likely dominated by electrostatic effects.

Journal Article↗
PubMedDate not supplied

Morphology and kinetics of phase separating transparent xanthan-colloid mixtures.

We present a study on the morphology and kinetics of depletion-induced phase separation in aqueous xanthan-colloid mixtures with light microscopy and small angle light scattering (SALS), using fluorinated colloids with a refractive index close to that of water to prevent complications of multiple scattering. Microscopy with the direction of observation perpendicular to gravity enabled us to observe the development of the microstructure during the entire phase separation process including the formation of a macroscopic interface. Bicontinuous structures typical of a spinodal decomposition mechanism were observed at early times. These structures coarsened in time until hydrodynamic flow resulted in lane formation. Close to the binodal, a nucleation-and-growth mechanism was observed with formation of droplets. The coarsening kinetics were studied in more detail with SALS and turbidity measurements. Above polysaccharide concentrations at which entanglements become dominant, a slower coarsening and macroscopic phase separation were found because of the high continuous phase viscosity.

Carbohydrate Conformation↗