P. Sunthar

Optimization of a Brownian-dynamics algorithm for semidilute polymer solutions

Simulating the static and dynamic properties of semidilute polymer solutions with Brownian dynamics (BD) requires the computation of a large system of polymer chains coupled to one another through excluded-volume and hydrodynamic interactions. In the presence of periodic boundary conditions, long-ranged hydrodynamic interactions are frequently summed with the Ewald summation technique. By performing detailed simulations that shed light on the influence of several tuning parameters involved both in the Ewald summation method, and in the efficient treatment of Brownian forces, we develop a BD algorithm in which the computational cost scales as O(N(1.8)), where N is the number of monomers in the simulation box. We show that Beenakkers original implementation of the Ewald sum, which is only valid for systems without bead overlap, can be modified so that θ solutions can be simulated by switching off excluded-volume interactions. A comparison of the predictions of the radius of gyration, the end-to-end vector, and the self-diffusion coefficient by BD, at a range of concentrations, with the hybrid lattice Boltzmann-molecular dynamics (LB-MD) method shows excellent agreement between the two methods. In contrast to the situation for dilute solutions, the LB-MD method is shown to be significantly more computationally efficient than the current implementation of BD for simulating semidilute solutions. We argue, however, that further optimizations should be possible.

Influence of micro-mixing on the size of liposomes self-assembled from miscible liquid phases.

Ethanol injection and variations of it are a class of methods where two miscible phases-one of which contains dissolved lipids-are mixed together leading to the self-assembly of lipid molecules to form liposomes. This method has been suggested, among other applications, for in situ synthesis of liposomes as drug delivery capsules. However, the mechanism that leads to a specific size selection of the liposomes in solution based self-assembly in general, and in flow-focussing microfluidic devices in particular, has so far not been established. Here we report two aspects of this problem. A simple and easily fabricated device for the synthesis of monodisperse unilamellar liposomes in a co-axial flow-focussing microfluidic geometry is presented. We also show that the size of liposomes is dependent on the extent of micro-convective mixing of the two miscible phases. Here, a viscosity stratification induced hydrodynamic instability leads to a gentle micro-mixing which results in larger liposome size than when the streams are mixed turbulently. The results are in sharp contrast to a purely diffusive mixing in macroscopic laminar flow that was believed to occur under these conditions. Further precise quantification of the mixing characteristics should provide the insights to develop a general theory for size selection for the class of ethanol injection methods. This will also lay grounds for obtaining empirical evidence that will enable better control of liposome sizes and for designing drug encapsulation and delivery devices.

Universal solvent quality crossover of the zero shear rate viscosity of semidilute DNA solutions

The scaling behavior of the zero shear rate viscosity of semidilute unentangled Deoxyribonucleic acid (DNA) solutions, in the double crossover regime driven by temperature and concentration, is mapped out by systematic experiments. The viscosity is shown to have a power law dependence on the scaled concentration c/c*, with an effective exponent that depends on the solvent quality parameter z. The determination of the form of this universal crossover scaling function requires the estimation of the θ-temperature of dilute DNA solutions in the presence of excess salt, and the determination of the solvent quality parameter at any given molecular weight and temperature. The θ-temperature is determined to be Tθ ≈ 15 °C using static light scattering, and the solvent quality parameter has been determined by dynamic light scattering.

Viscosity radius of polymers in dilute solutions: Universal behavior from DNA rheology and brownian dynamics simulations

The swelling of the viscosity radius, αη, and the universal viscosity ratio, UηR, have been determined experimentally for linear DNA molecules in dilute solutions with excess salt, and numerically by Brownian dynamics simulations, as a function of the solvent quality. In the latter instance, asymptotic parameter free predictions have been obtained by extrapolating simulation data for finite chains to the long chain limit. Experiments and simulations show a universal crossover for αη and UηR from θ to good solvents in line with earlier observations on synthetic polymer− solvent systems. The significant difference between the swelling of the dynamic viscosity radius from the observed swelling of the static radius of gyration is shown to arise from the presence of hydrodynamic interactions in the nondraining limit. Simulated values of αη and UηR are in good agreement with experimental measurements in synthetic polymer solutions reported previously and with the measurements in linear DNA solutions reported here.

Efficient lattice Boltzmann algorithm for Brownian suspensions

A lattice Boltzmann (LB)-based hybrid method is developed to simulate suspensions of Brownian particles. The method uses conventional LB discretization (without fluid- level fluctuations) for suspending fluid, and treats Brownian particles as point masses with a stochastic thermal noise. LB equations are used to compute the velocity perturbations induced by the particle motion. It is shown that this method correctly reproduces the short-time and long-time diffusive behaviour of a Brownian particle. Unlike the earlier hybrid methods that use thermal fluctuations in the fluid, this method correctly reproduces the temperature of the particle and does not require an empirical rescaling of the bare friction coefficient to obtain the correct diffusive behaviour. It is observed that the present method is at least twice as fast as the earlier method. This method is best suited for flows of polymers and Brownian suspensions in microfluidic devices.

Multiscale simulation of dilute DNA in a roll-knife coating flow

The dynamics of dilute solutions of DNA flowing in a scaled down roll-knife coating flow are investigated on multiple scales. The flow is generated between a rotating roll and a stationary glass knife, and extension of fluorescently stained DNA molecules is measured at the minimum gap at low roll speeds. The macroscopic flow is computed by solving the continuum equations of motion with the finite element method; the microscale behavior of DNA molecules is predicted by Brownian dynamics combined with successive fine-graining. The simulations predict that the DNA should stretch almost to full extension near the roll surface in the region of minimum gap; this does not agree with experimental measurements. The assumption that the flow is nearly homogeneous on the length scale of the polymer molecules, commonly used in processing flows as well as Brownian dynamics simulations of simple flows, fails near free surfaces, and is the likely cause of the discrepancy. Evidence from the literature suggests that simila...

Spontaneous formation of single component liposomes from a solution

The diameter of lipid vesicles is generally known to be determined by parameters external to the system, such as fluid shear, electric fields, co-surfactants, etc. We present a mechanism by which a system consisting of a single component lipid can spontaneously assemble from a solution phase to form monodisperse unilamellar vesicles of well-defined diameters dictated only by thermodynamic parameters intrinsic to the system. Here, the lipids self-assemble as vesicles when an aqueous phase diffusively replaces the original solvent in a macroscopically stationary (or quiescent) manner. We demonstrate this using phosphatidyl choline lipid-ethanol-water systems, where the average diameter of the liposomes is shown to be intrinsic, in reasonable agreement with the Helfrichs model of the vesicle free energy. The size depends only on the temperature and the lipid type, eliminating dependence on kinetic effects or external forcing normally observed. The method provides the first pure system to study the self-assembly of vesicle-forming surfactants; and with a natural thermodynamic length scale, it may have an implication for the vesicle size selection under pre-biotic conditions.

Shear thinning in dilute and semidilute solutions of polystyrene and DNA

The viscosity of dilute and semidilute unentangled deoxyribonucleic acid (DNA) solutions, in steady simple shear flow, has been measured across a range of temperatures and concentrations. For polystyrene solutions, measurements of viscosity have been carried out in the semidilute unentangled regime, while results of prior experimental measurements in the dilute regime have been used for the purpose of data analysis, and for comparison with the behavior of DNA solutions. Interpretation of the shear rate dependence of viscosity in terms of suitably defined nondimensional variables is shown to lead to master plots, independent of temperature and concentration, in each of the two concentration regimes. In the case of semidilute unentangled solutions, defining the Weissenberg number in terms of a concentration dependent large scale relaxation time is found not to lead to data collapse across different concentrations. On the other hand, the use of an alternative relaxation time, with the concentration dependence of a single correlation blob, suggests the existence of universal shear thinning behavior at large shear rates.The viscosity of dilute and semidilute unentangled deoxyribonucleic acid (DNA) solutions, in steady simple shear flow, has been measured across a range of temperatures and concentrations. For polystyrene solutions, measurements of viscosity have been carried out in the semidilute unentangled regime, while results of prior experimental measurements in the dilute regime have been used for the purpose of data analysis, and for comparison with the behavior of DNA solutions. Interpretation of the shear rate dependence of viscosity in terms of suitably defined nondimensional variables is shown to lead to master plots, independent of temperature and concentration, in each of the two concentration regimes. In the case of semidilute unentangled solutions, defining the Weissenberg number in terms of a concentration dependent large scale relaxation time is found not to lead to data collapse across different concentrations. On the other hand, the use of an alternative relaxation time, with the concentration dependenc...

Rapid single-step formation of liposomes by flow assisted stationary phase interdiffusion

Laboratory preparation of unilamellar liposomes often involves multiple steps carried out over several hours to achieve a monodisperse size distribution. Here, we present a methodology based on a recently introduced lipid self-assembly principle-stationary phase interdiffusion (SPI)-to prepare large unilamellar vesicles (LUVs) of a monodisperse population in a short period of about 10 min. The stationary interface between a lipid-ethanol phase and an aqueous phase is created by a density difference induced convective flow in a horizontal capillary. The average size of the liposomes, as expected from the SPI principle, is modulated only by the temperature and the type of lipids. Lipid concentration, ethanol content, pH of the aqueous phase, and the time duration of the experiment have little influence on the mean diameter of the vesicles. This simple methodology can be easily carried out with a capillary and a micro-needled syringe and provides a rapid production tool for researchers requiring reproducible liposome suspensions. Refined natural lipids, based on soy and egg lecithin mixtures, yield LUVs in the range 100-200 nm, suitable for drug delivery applications.