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Nigel Clarke

Publications and source records attributed to Nigel Clarke.

12 recordsLinked to original sources
PubMed2006

Multiscale model of miscible polymer blends in porous media: from flow fields to concentration fluctuations.

We have developed a multiscale approach for simulating the concentration fluctuations in a miscible blend subject to complex flow dynamics. We first simulate the hydrodynamics of a fluid as it flows through porous media. In particular, we monitor the velocity gradients as a function of time for a fluid "particle" as it follows a tortuous path through the system. Next, we evolve the structure factor of the spatial concentration fluctuations subject to this flow environment. The velocity gradients experienced by this fluid particle can result in elongation and rotation of the concentration fluctuations. In this manner, we couple the macroscopic flow fields in porous media with the microscopic concentration fluctuations in the polymer blend. We find a close correlation between the tortuous pathways, the velocity gradients in the fluid, and the perturbance of the structure factor from it quiescent state. Furthermore, we find that the concentrations tend to elongate towards the flow direction or at an acute angle with the flow direction.

Journal Article↗
PubMed2006

Anion binding inhibition of the formation of a helical organogel.

A chiral tris(urea) organogelator gels dmso-water and methanol-water mixtures at low weight percent. The formation of the helical gel fibres is partially inhibited by addition of chloride, which is bound by the gelator, resulting in fully crystalline material characterised by X-ray crystallography.

Journal Article↗
PubMed2006

Stress-guided self-assembly in Dutcher films.

Dutcher films consist of a layer of liquid sandwiched between two solid capping layers and can spontaneously self-assemble to form corrugated surfaces. The interplay between the attractive van der Waals forces across the film, and the elastic forces due to the deformation of the capping layers, produces well-defined periodic undulations. We show how computer simulations can capture both the formation of undulations in Dutcher films and the correct periodicity. Furthermore, we simulate Dutcher films which are either compressed or stretched, resulting in the promotion or suppression of undulation growth. In this manner, applied deformations can be shown to guide the self-assembly process in Dutcher films and result in the formation of highly oriented surface corrugations over large distances.

Journal Article↗
PubMed2005

Reaction-induced phase separation of pseudo-interpenetrating polymer networks in polydisperse polymer blends: a simulation study.

We develop a minimal model for the process of reaction-induced phase separation in a polydisperse polymer blend. During the reaction, one component undergoes polymerization, leading to phase separation via spinodal decomposition. The effect that changing the final degree of polymerization has on the phase-separation process is studied. Finally an elastic energy term is included mimicking the cross-linking process and the generation of a semi-interpenetrating polymer network. We show that the scaling of the dominant lengthscale with time varies according to the reaction conditions.

Journal Article↗
PubMed2005

Creating structures in polymer blends via a dissolution and phase-separation process.

We show how three-dimensional structures can be formed in polymer blends from pre-existing structures. "Tape" of one polymer is inserted into a matrix of an alternative polymer to form an array of parallelepipeds. We subject this regular structure to partial dissolution in the one-phase region, before quenching the system into the two-phase region. The interplay between dissolution and phase separation can result in complex hierarchic structures. In particular, arrays of microchannels of one polymer species can be formed inside the other polymer.

Journal Article↗
PubMed2005

Anisotropic suppression of phase separation in polymer solutions by oscillatory shear.

We consider the effects of subjecting a polymer solution to a simultaneous oscillatory shear flow and temperature jump into the two-phase region of the phase diagram. We predict that if the oscillatory shear stresses are significant enough then the flow suppresses phase separation in the flow direction, leading to the possibility of creating strongly aligned structures. We construct a quantitative dynamic phase diagram in the amplitude-frequency plane showing the conditions for the growth or decay of concentration fluctuations. Further, we discuss the time dependence of periodic structure factors in the flow-vorticity and flow-gradient planes. It is also shown that significantly enhanced scattering occurs at large scattering vectors regardless of whether the unbounded growth of fluctuations at smaller scattering vectors is suppressed.

Journal Article↗
PubMed2003

Recombinants of intrachromosomal transposition of subtelomeres in chromosomes 1 and 2: a cause of minute terminal chromosomal imbalances.

Two cases of submicroscopic recombinants of intrachromosomal transposition of telomeres, one each from chromosome 1 and 2 are described. Meiotic crossing-over would generate the recombinants from these reciprocal rearrangements. In both cases, which were detected by FISH with subtelomeric probes, there is a minute deletion of the qter region and a second presence of the pter subtelomeric region on the respective qter, i.e., a duplication of 1pter or 2pter respectively. The deletion on 2qter (case 2) was confirmed by microsatellite inheritance and was of paternal origin, but in case 1 there was no detectable 1q deletion other than of the subtelomeric probe, and parental origin could not be determined. The present case 2 with del(2qter)/dup(2pter) shares many features with reported cases of simple deletion (2qter) but did not have features of Albright hereditary osteodystrophy, which are seen in half of such deletion patients. The clinical features present in case 1 were similar to those of the previously reported case of a submicroscopic 1qter deletion but also to cases with microscopically visible 1qter deletions, presumably because of gene enrichment in subtelomeric regions. Recombinants of such intrachromosomal subtelomere transpositions detected by subtelomeric probes may comprise up to 10% of submicroscopic pter or qter deletion cases. Other cases of this unusual mechanism may be detected with more common use of subtelomeric probes. It is suggested the bouquet associations of telomeres in early meiosis may facilitate such unusual rearrangements.

Adolescent↗
PubMed2002

Target morphologies via a two-step dissolution-quench process in polymer blends.

A novel process for obtaining controlled morphologies in polymer blends is modeled numerically. Particles of one type of polymer are allowed to dissolve in a matrix of a dissimilar polymer. Prior to complete dissolution the blend is quenched into the two-phase region, such that phase separation takes place. The combination of the incomplete dissolution and the wavelength selection process associated with phase separation results in particles that during the "intermediate" stages have a core that is significantly richer in the matrix material.

Journal Article↗
PubMed2002

Effect of oscillatory shear on polymer solutions.

We consider the effects of oscillatory shear flow on small concentration fluctuations in polymer solutions, which we model as highly asymmetric entangled polymer blends. A simple model that enables us to predict the scattering patterns in the flow-vorticity plane is presented. It is shown that peaks in the patterns occur in the flow direction at a finite wave vector, their position strongly depends on the angular frequency of oscillatory shear flow, whereas the intensity of these peaks is rather controlled by the amplitude of the flow. The model reproduces characteristic double-winged anisotropic scattering patterns called "butterfly," which are stable and thus represent concentration fluctuations rather than phase separation.

Journal Article↗
PubMed2002

Automated 3-D echocardiography analysis compared with manual delineations and SPECT MUGA.

A major barrier for using 3-D echocardiography for quantitative analysis of heart function in routine clinical practice is the absence of accurate and robust segmentation and tracking methods necessary to make the analysis automatic. In this paper, we present an automated three-dimensional (3-D) echocardiographic acquisition and image-processing methodology for assessment of left ventricular (LV) function. We combine global image information provided by a novel multiscale fuzzy-clustering segmentation algorithm, with local boundaries obtained with phase-based acoustic feature detection. We then use the segmentation results to fit and track the LV endocardial surface using a 3-D continuous transformation. To our knowledge, this is the first report of a completely automated method. The protocol is evaluated in a small clinical case study (nine patients). We compare ejection fractions (EFs) computed with the new approach to those obtained using the standard clinical technique, single-photon emission computed tomography multigated acquisition. Errors on six datasets were found to be within six percentage points. A further two, with poor image quality, improved upon EFs from manually delineated contours, and the last failed due to artifacts in the data. Volume-time curves were derived and the results compared to those from manual segmentation. Improvement over an earlier published version of the method is noted.

Algorithms↗
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