Numerical Simulations of Polymers at the Nanoscale

<p>In this thesis we study a variety of nanoscale phenomena in certain polymer systems using a combination of numerical simulation methods and mathematical modelling. The problems considered are: (a) the mixing behaviour of polymeric fluids in micro- and nanofluidic devices, (b) capillary absorption of polymer droplets into narrow capillaries, and (c) modelling the phase separation and self-assembly behaviour in polymer systems with freely deforming boundaries. These problems are significant in nanotechnological applications of polymer-based systems. First, the mixing behaviour of a polymeric melt over two parallely patternedslip surfaces is considered. Using molecular dynamics (MD) simulations, it is shown that mixing is enhanced when the polymer chain size is smaller than the wavelength of the chemical pattern of the surfaces. An off-set in the upper and lowerwall patterns improved themixing in the centre of the channel. Application of a sinusoidally varying body force in addition to the patterned-slip conditions is shown to enhance mixing further, compared to a constant body force case, with some limitations. Simulation findings for the constant body force cases are in qualitative agreement with the continuum theory of Pereira [1]. However, in the case of a sinusoidally varying body force our simulations do not agree with the continuum theory. We explain the reasons for the discrepancy between the two and point out the deficiencies in the continuum theory in predicting the correct behaviour. Second, the capillary phenomena of polymer droplets in narrow capillaries is studied using MD simulations. It is demonstrated that droplets composed of longer chains require wider tubes for absorption and this result is in agreement with our continuum modelling. The observed capillary dynamics deviate significantly from the standard Lucas-Washburn description thus questioning its validity at the nanoscale. The metastable states during the capillary absorption in some cases cannot be explained using the existing models of capillary dynamics. Lastly, the phase separation process in polymer blends between both confined and unconfined boundaries is studied using Smoothed Particle Hydrodynamics (SPH). The SPH technique has the advantage of not using a grid to discretize the spatial domain, which makes it appealing when dealing with problems where the spatial domain can change with time. The applicability of the SPH method in describing phase separation in these systems is demonstrated. In particular, its ability to model freely deforming polymer blends is shown.</p>

Numerical Simulations of Polymers at the Nanoscale

<p>In this thesis we study a variety of nanoscale phenomena in certain polymer systems using a combination of numerical simulation methods and mathematical modelling. The problems considered are: (a) the mixing behaviour of polymeric fluids in micro- and nanofluidic devices, (b) capillary absorption of polymer droplets into narrow capillaries, and (c) modelling the phase separation and self-assembly behaviour in polymer systems with freely deforming boundaries. These problems are significant in nanotechnological applications of polymer-based systems. First, the mixing behaviour of a polymeric melt over two parallely patternedslip surfaces is considered. Using molecular dynamics (MD) simulations, it is shown that mixing is enhanced when the polymer chain size is smaller than the wavelength of the chemical pattern of the surfaces. An off-set in the upper and lowerwall patterns improved themixing in the centre of the channel. Application of a sinusoidally varying body force in addition to the patterned-slip conditions is shown to enhance mixing further, compared to a constant body force case, with some limitations. Simulation findings for the constant body force cases are in qualitative agreement with the continuum theory of Pereira [1]. However, in the case of a sinusoidally varying body force our simulations do not agree with the continuum theory. We explain the reasons for the discrepancy between the two and point out the deficiencies in the continuum theory in predicting the correct behaviour. Second, the capillary phenomena of polymer droplets in narrow capillaries is studied using MD simulations. It is demonstrated that droplets composed of longer chains require wider tubes for absorption and this result is in agreement with our continuum modelling. The observed capillary dynamics deviate significantly from the standard Lucas-Washburn description thus questioning its validity at the nanoscale. The metastable states during the capillary absorption in some cases cannot be explained using the existing models of capillary dynamics. Lastly, the phase separation process in polymer blends between both confined and unconfined boundaries is studied using Smoothed Particle Hydrodynamics (SPH). The SPH technique has the advantage of not using a grid to discretize the spatial domain, which makes it appealing when dealing with problems where the spatial domain can change with time. The applicability of the SPH method in describing phase separation in these systems is demonstrated. In particular, its ability to model freely deforming polymer blends is shown.</p>

Non-linear frequency-dependence of neurovascular coupling in the cerebellar cortex implies vasodilation-vasoconstriction competition

Neurovascular coupling (NVC) is the process associating local cerebral blood flow (CBF) to neuronal activity (NA). Although NVC provides the basis for the blood-oxygen-level-dependent (BOLD) effect used in functional MRI (fMRI), the relationship between NVC and NA is still unclear. Since recent studies reported cerebellar non-linearities in BOLD signals during motor tasks execution, we investigated the NVC/NA relationship using a range of input frequencies in acute mouse cerebellar slices of vermis and hemisphere. The capillary diameter increased in response to mossy fiber activation in the 6-300Hz range, with a marked inflection around 50Hz (vermis) and 100Hz (hemisphere). The corresponding NA was recorded using high-density multi-electrode arrays and correlated to capillary dynamics through a computational model dissecting the main components of granular layer activity. Here, NVC is known to involve a balance between the NMDAR-NO pathway driving vasodilation and the mGluRs-20HETE pathway driving vasoconstriction. Simulations showed that the NMDAR-mediated component of NA was sufficient to explain the time-course of the capillary dilation but not its non-linear frequency-dependence, suggesting that the mGluRs-20HETE pathway plays a role at intermediate frequencies. These parallel control pathways imply a vasodilation-vasoconstriction competition hypothesis that could adapt local hemodynamics at the microscale bearing implications for fMRI signals interpretation.

Capillary imbibition of non-Newtonian fluids in a microfluidic channel: analysis and experiments

We have presented an experimental analysis on the investigations of capillary filling dynamics of inelastic non-Newtonian fluids in the regime of surface tension dominated flows. We use the Ostwald–de Waele power-law model to describe the rheology of the non-Newtonian fluids. Our analysis primarily focuses on the experimental observations and revisits the theoretical understanding of the capillary dynamics from the perspective of filling kinematics at the interfacial scale. Notably, theoretical predictions of the filling length into the capillary largely endorse our experimental results. We study the effects of the shear-thinning nature of the fluid on the underlying filling phenomenon in the capillary-driven regime through a quantitative analysis. We further show that the dynamics of contact line motion in this regime plays an essential role in advancing the fluid front in the capillary. Our experimental results on the filling in a horizontal capillary re-establish the applicability of the Washburn analysis in predicting the filling characteristics of non-Newtonian fluids in a vertical capillary during early stage of filling (Digilov 2008 Langmuir 24 , 13 663–13 667 ( doi:10.1021/la801807j )). Finally, through a scaling analysis, we suggest that the late stage of filling by the shear-thinning fluids closely follows the variation x ~ t . Such a regime can be called the modified Washburn regime (Washburn 1921 Phys. Rev. 17 , 273–283 ( doi:10.1103/PhysRev.17.273 )).

Magneto-capillary dynamics of amphiphilic Janus particles at curved liquid interfaces

Static homogeneous fields drive motions of magnetic particles along curved liquid interfaces.

Heat exchange between a bouncing drop and a superhydrophobic substrate

The ability to enhance or limit heat transfer between a surface and impacting drops is important in applications ranging from industrial spray cooling to the thermal regulation of animals in cold rain. When these surfaces are micro/nanotextured and hydrophobic, or superhydrophobic, an impacting drop can spread and recoil over trapped air pockets so quickly that it can completely bounce off the surface. It is expected that this short contact time limits heat transfer; however, the amount of heat exchanged and precise role of various parameters, such as the drop size, are unknown. Here, we demonstrate that the amount of heat exchanged between a millimeter-sized water drop and a superhydrophobic surface will be orders of magnitude less when the drop bounces than when it sticks. Through a combination of experiments and theory, we show that the heat transfer process on superhydrophobic surfaces is independent of the trapped gas. Instead, we find that, for a given spreading factor, the small fraction of heat transferred is controlled by two dimensionless groupings of physical parameters: one that relates the thermal properties of the drop and bulk substrate and the other that characterizes the relative thermal, inertial, and capillary dynamics of the drop.