Anja Vananroye

Effect of confinement on the steady-state behavior of single droplets during shear flow

The effect of geometrical confinement on the deformation and orientation of single droplets during steady-state shear flow is investigated microscopically in a counterrotating device. The model system consists of poly(dimethyl siloxane) droplets of varying sizes and viscosities in a poly(isobutylene) matrix. The experimental results are first compared with the predictions of the model by Maffettone and Minale [J. Non-Newtonian Fluid Mech. 78, 227–241 (1998)] for bulk flow. For all viscosity ratios, deviations from the Maffettone and Minale model start to occur at a droplet diameter to gap spacing ratio of the order of 0.4. The droplet deformation increases and the droplets orient more towards the flow direction as a consequence of confinement. At low viscosity ratios, the deviations remain small, whereas at high viscosity ratios, larger deviations from bulk behavior are observed. The observations are also compared with the theory of Shapira and Haber [Int. J. Multiphase Flow 16, 305–321 (1990)] which incl...

Generalized behavior of the breakup of viscous drops in confinements

The breakup of confined drops in shear flow between parallel plates is investigated as a function of viscosity ratio and confinement ratio. Using a boundary-integral method for numerical simulations and a counter-rotating experimental device, critical capillary numbers in shear flow are obtained. It is observed that different viscosity ratios yield different trends with increasing confinement ratio: a low viscosity ratio drop shows an increase in critical capillary number, at a viscosity ratio of unity no major trend is seen, and the critical capillary number for a high viscosity ratio drop decreases significantly. A generalized explanation for all viscosity ratios is that confinement affects the orientation of the drop with respect to the direction of the local strain field. At moderate confinement ratios, the drop orients more toward the strain direction, where it experiences a stronger flow and hence, the critical capillary number is decreased. As the drop gets more confined, it aligns more in the flow...

Effect of confinement and viscosity ratio on the dynamics of single droplets during transient shear flow

The deformation and orientation of droplets during transient shear flow is studied in a counterrotating device using microscopy. The effect of the degree of confinement and viscosity ratio is systematically investigated. The system consists of polydimethylsiloxane droplets of varying sizes and viscosities dispersed in a polyisobutylene matrix. The observations are compared with the predictions of an adapted version of the Maffettone and Minale model [Maffettone, and Minale, J. Non-Newtonian Fluid Mech. 78, 227–241 (1998)] which includes confinement effects [Minale, Rheol. Acta 47, 667–675 (2008)]. For flow start-up at low capillary numbers, the deformation of confined droplets and their orientation towards the flow direction are increased with respect to the unconfined situation for all viscosity ratios under investigation. The confined model results for start-up and the experimental data at low capillary numbers are in good agreement both showing similar monotonous transients. At high degrees of confinem...

Nanoscale Study of Polymer Dynamics

The thermal motion of polymer chains in a crowded environment is anisotropic and highly confined. Whereas theoretical and experimental progress has been made, typically only indirect evidence of polymer dynamics is obtained either from scattering or mechanical response. Toward a complete understanding of the complicated polymer dynamics in crowded media such as biological cells, it is of great importance to unravel the role of heterogeneity and molecular individualism. In the present work, we investigate the dynamics of synthetic polymers and the tube-like motion of individual chains using time-resolved fluorescence microscopy. A single fluorescently labeled polymer molecule is observed in a sea of unlabeled polymers, giving access to not only the dynamics of the probe chain itself but also to that of the surrounding network. We demonstrate that it is possible to extract the characteristic time constants and length scales in one experiment, providing a detailed understanding of polymer dynamics at the single chain level. The quantitative agreement with bulk rheology measurements is promising for using local probes to study heterogeneity in complex, crowded systems.

Interfacial rheology of sterically stabilized colloids at liquid interfaces and its effect on the stability of Pickering emulsions

Particle-laden interfaces can be used to stabilize a variety of high-interface systems, from foams over emulsions to polymer blends. The relation between the particle interactions, the structure and rheology of the interface, and the stability of the system remains unclear. In the present work, we experimentally investigate how micron-sized, near-hard-sphere-like particles affect the mechanical properties of liquid interfaces. In particular, by comparing dried and undried samples, we investigate the effect of aggregation state on the properties of the particle-laden liquid interface and its relation to the stability of the corresponding Pickering emulsions. Partially aggregated suspensions give rise to a soft-solid-like response under shear, whereas for stable PMMA particulate layers a liquid-like behavior is observed. For interfacial creep-recovery measurements, we present an empirical method to correct for the combined effect of the subphase drag and the compliance of the double-wall ring geometry, which makes a significant contribution to the apparent elasticity of weak interfaces. We further demonstrate that both undried and dried PMMA particles can stabilize emulsions for months, dispelling the notion that particle aggregation, in bulk or at the interface, is required to create stable Pickering emulsions. Our results indicate that shear rheology is a sensitive probe of colloidal interactions but is not necessarily a predictor of the stability of interfaces, e.g., in quiescent Pickering emulsions, as in the latter the response to dilatational deformations can be of prime importance.

Biofilm-induced changes to the composite surface

OBJECTIVES Composites may undergo biodegradation in the oral cavity. The objective was to investigate the effect of single- and multi-species biofilms on the surface roughness and topography of two composites. METHODS Disk-shaped specimens of a paste-like, Bis-GMA-free (Gradia Direct Anterior, GC), and a flowable, Bis-GMA-based composite (Tetric EvoFlow, Ivoclar-Vivadent) were prepared. After ethylene-oxide sterilization (38°C), specimens (n=3) were incubated with Streptococcus mutans or mixed bacterial culture (Streptococcus mutans, Streptococcus sanguinis, Actinomyces naeslundii and Fusobacterium nucleatum). As negative controls, unexposed specimens and specimens exposed to sterile medium (BHI) were used. Specimens exposed to acidified BHI medium (pH=5) and enzymatic solution of cholesterol esterase served as positive control. Following 6-week incubation, the attached biofilms were collected for real-time PCR assessment, after which the surface roughness and topography of the specimens were analyzed with atomic force microscopy. Surface hydrophilicity/hydrophobicity was determined by contact angle measurements. Biofilm structure was analyzed with scanning electron microscopy. RESULTS Even though multi-species biofilms were thicker, with more cells attached, they did not significantly affect the surface roughness of the composites. On the other hand, S. mutans alone significantly increased the roughness of Tetric by 40.3%, while its effect on Gradia was lower (12%). The total amount of attached bacteria, however, did not differ between the composites. CONCLUSIONS S. mutans can increase the surface roughness of composites, depending on their composition. This ability of S. mutans is, however, mitigated in co-culture with other species. In particular, bacterial esterases seem to be responsible for the increased composite surface roughness upon biofilms exposure. CLINICAL SIGNIFICANCE Cariogenic bacteria can degrade composites, thereby increasing the surface roughness. Increased roughness and subsequent improved bacterial accumulation may facilitate the development of secondary caries around composites, which is the most common reason for the restoration failure.

On the Pressure Dependency of the Bagley Correction

Abstract The effect of pressure on the viscosity of polymer melts is an often forgotten parameter due to the inherent difficulty to measure this quantity. Different experimental approaches have already been undertaken in literature in the past. A popular methodology to measure the pressure dependence of the viscosity is to use a capillary rheometer equipped with a counter pressure chamber in which the exit pressure can be controlled. In order to process the data, one of the key elements is the Bagley correction that is required to determine the correct entrance pressure at a specific shear rate. In all analysis approaches presented in literature on data at controlled exit pressure, the Bagley correction was always determined at atmospheric exit pressure, disgarding possible effects of an enhanced exit pressure. In this paper, a new analytical approach is presented that for the first time allows for a direct assessment of the entrance pressures obtained when capillary measurements are performed with controlled counter pressures. It is demonstrated, using polycarbonate, that the entrance pressure correction needed to obtain correct viscosity values under pressure is significantly different than the one needed to correct measurements performed at atmospheric exit pressure.