Slow sedimentation and deformability of charged lipid vesicles

The study of vesicles in suspension is important to understand the complicated dynamics exhibited by cells in vivo and in vitro. We developed a computer simulation based on the boundary-integral method to model the three dimensional gravity-driven sedimentation of charged vesicles towards a flat surface. The membrane mechanical behavior was modeled using the Helfrich Hamiltonian and near incompressibility of the membrane was enforced via a model which accounts for the thermal fluctuations of the membrane. The simulations were verified and compared to experimental data obtained using suspended vesicles labelled with a fluorescent probe, which allows visualization using fluorescence microscopy and confers the membrane with a negative surface charge. The electrostatic interaction between the vesicle and the surface was modeled using the linear Derjaguin approximation for a low ionic concentration solution. The sedimentation rate as a function of the distance of the vesicle to the surface was determined both experimentally and from the computer simulations. The gap between the vesicle and the surface, as well as the shape of the vesicle at equilibrium were also studied. It was determined that inclusion of the electrostatic interaction is fundamental to accurately predict the sedimentation rate as the vesicle approaches the surface and the size of the gap at equilibrium, we also observed that the presence of charge in the membrane increases its rigidity

Design and construction of tank-chassis and lifting structure for centrifugal pump HL260 M powered by a Diesel Engine

This article deals with the design, simulation and construction of a fuel storage tank-chassis and a lifting system coupled as a single unit to a Cummins QSK19 engine driven HL260m pump that can guarantee an operating autonomy of up to 12 continuous hours and can be transported to different locations by means of lifting systems. For the mechanical design the recommendations of the American Institute of Steel Construction (AISC) and the application of the failure criteria for Von Mises ductile materials or Maximum Energy Distortion were used. For the dimensioning of the storage tank, the average consumption stipulated by the manufacturer was used and the simulations were performed with SolidWorks®. A functional and safe system that can be used in on-site applications was achieved

Effects of Non-Linear Interfacial Mechanics on the Transient Response of a Drop to an External Flow Field

The time-dependent response of a drop suspended in an axisymmetric extensional flow is studied. The dynamics of such multiphase systems is of interest in areas that include the petroleum, pharmaceutical, biomedical and food industries. The focus is on the role that a surfactant monolayer on the drop interface plays during the deformation and retraction processes. The steady-state characteristics and the bulk flow are analyzed for validation. The transient processes are simulated using an axisymmetric boundary integral method. At each time step the shape of the drop is determined based on the velocity of points at the interface and surfactant is redistributed accordingly. The response time of drop deformation in an extensional flow was seen to depend monotonically on the initial surfactant coverage on the interface. That is, higher surfactant coverage leads to increasing values of response time. It was seen that Marangoni stresses developing on the interface due to surface tension gradients slow the response of the drop. Retraction of the drop when the external flow ceases is caused by an imbalance between the traction along the drop's interface and the shear stresses. Neglecting the initial stages, the retraction process can be closely approximated by an exponential decay. It was shown that the retraction time depends non-monotonically on the surfactant coverage. Small amounts of surfactant slow down the retraction process compared to that of the clean drop for coverage less than 50%. Further increasing the surfactant concentration accelerates the retraction process and as the coverage approaches 100%, the value of the retraction time approaches that of the clean drop. These dynamics emerge from the competition between the convective flux that creates the surfactant gradients and the large Marangoni stresses that develop to oppose them during the extension process. At low to intermediate surfactant concentrations, large surfactant gradients developed during the deformation process persist until equilibrium is reached because diffusion time is much longer than the retraction rate. At higher initial surfactant concentrations, lower surfactant gradients develop during deformation and these quickly vanish during the initial stages of retraction leading to a nearly uniform surface tension distribution along the interface

Effects of Non-Linear Interfacial Mechanics on the Transient Response of a Drop to an External Flow Field

The time-dependent response of a drop suspended in an axisymmetric extensional flow is studied. The dynamics of such multiphase systems is of interest in areas that include the petroleum, pharmaceutical, biomedical and food industries. The focus is on the role that a surfactant monolayer on the drop interface plays during the deformation and retraction processes. The steady-state characteristics and the bulk flow are analyzed for validation. The transient processes are simulated using an axisymmetric boundary integral method. At each time step the shape of the drop is determined based on the velocity of points at the interface and surfactant is redistributed accordingly. The response time of drop deformation in an extensional flow was seen to depend monotonically on the initial surfactant coverage on the interface. That is, higher surfactant coverage leads to increasing values of response time. It was seen that Marangoni stresses developing on the interface due to surface tension gradients slow the response of the drop. Retraction of the drop when the external flow ceases is caused by an imbalance between the traction along the drop's interface and the shear stresses. Neglecting the initial stages, the retraction process can be closely approximated by an exponential decay. It was shown that the retraction time depends non-monotonically on the surfactant coverage. Small amounts of surfactant slow down the retraction process compared to that of the clean drop for coverage less than 50%. Further increasing the surfactant concentration accelerates the retraction process and as the coverage approaches 100%, the value of the retraction time approaches that of the clean drop. These dynamics emerge from the competition between the convective flux that creates the surfactant gradients and the large Marangoni stresses that develop to oppose them during the extension process. At low to intermediate surfactant concentrations, large surfactant gradients developed during the deformation process persist until equilibrium is reached because diffusion time is much longer than the retraction rate. At higher initial surfactant concentrations, lower surfactant gradients develop during deformation and these quickly vanish during the initial stages of retraction leading to a nearly uniform surface tension distribution along the interface

CFD Modeling of Hydrocyclones—A Study of Efficiency of Hydrodynamic Reservoirs

The dynamics of hydrocyclones is complex, because it is a multiphase flow problem that involves interaction between a discrete phase and multiple continuum phases. The performance of hydrocyclones is evaluated by using Computational Fluid Dynamics (CFD), and it is characterized by the pressure drop, split water ratio, and particle collection efficiency. In this paper, a computational model to improve and evaluate hydrocyclone performance is proposed. Four known computational turbulence models (renormalization group (RNG) k- ε , Reynolds stress model (RSM), and large-eddy simulation (LES)) are implemented, and the accuracy of each for predicting the hydrocyclone behavior is assessed. Four hydrocyclone configurations were analyzed using the RSM model. By analyzing the streamlines resulting from those simulations, it was found that the formation of some vortices and saddle points affect the separation efficiency. Furthermore, the effects of inlet width, cone length, and vortex finder diameter were found to be significant. The cut-size diameter was decreased by 33% compared to the Hsieh experimental hydrocyclone. An increase in the pressure drop leads to high values of cut-size and classification sharpness. If the pressure drop increases to twice its original value, the cut-size and the sharpness of classification are reduced to less than 63% and 55% of their initial values, respectively

Multibody Simulation of an Electric Go-Kart: Influence of Powertrain Weight Distribution on Dynamic Performance

A multibody model of an electric go-kart was developed in Msc-Adams Car software to simulate the vehicle’s dynamic performance. In contrast to an ICE kart, its electric counterpart bares an extra weight load accounted for the batteries and other powertrain components. The model is inspired on a prototype vehicle developed at Universidad de los Andes. The prototype was built on top of an ICE frame where a PMAC motor, controller, battery pack and the subsequent powertrain components were installed. A petrol-based Go-kart weight distribution was defined as baseline and several variants of the electric adaptation with different weight distributions were constructed. The main objective of the model is to evaluate different configurations and identify the ones that can give performance advantages. Step steer simulations ran at 40 km/h (64 mph) were analyzed to assess the dynamic performance of the vehicle for different configuration of the battery bank placement. For most iterations of powertrain location, considerable differences in dynamic response were obtained and the handling balance was identified as Understeer contrary to a priori thoughs. Understeer gradient, weight distribution for both axles, trajectory among other results of interest were observed in the simulations. The model allowed to showcase the effect of redistribution of weight on the dynamic behavior in this specific application. Among the main consequences lies the fact that battery distribution can affect the lifting of the internal rear tire and the detriment in turning effectiveness

Energy Efficiency Comparison of Hydraulic Accumulators and Ultracapacitors

Energy regeneration systems are a key factor for improving energy efficiency in electrohydraulic machinery. This paper is focused on the study of electric energy storage systems (EESS) and hydraulic energy storage systems (HESS) for energy regeneration applications. Two test benches were designed and implemented to compare the performance of the systems under similar operating conditions. The electrical system was configured with a set of ultracapacitors, and the hydraulic system used a hydraulic accumulator. Both systems were designed to have the same energy storage capacity. Charge and discharge cycle experiments were performed for the two systems in order to compare their power density, energy density, cost, and efficiency. According to the experimentally obtained results, the power density in the hydraulic accumulator was 21.7% higher when compared with the ultracapacitors. Moreover, the cost/power ($/Watt) ratio in the hydraulic accumulator was 2.9 times smaller than a set of ultracapacitors of the same energy storage capacity. On the other hand, the energy density in the set of ultracapacitors was 9.4 times higher, and the cost/energy ($/kWh) ratio was 2.9 times smaller when compared with the hydraulic accumulator. Under the tested conditions, the estimated overall energy efficiency for the hydraulic accumulator was 87.7%, and the overall energy efficiency for the ultracapacitor was 78.7%

Sedimentation rate as a function of the distance to the surface.

<p>Experimental data (symbols) and computer simulations (lines) are shown. The light solid line corresponds to a simulation which incorporates the electrostatic interaction between the vesicle and the surface, the dark solid line corresponds to a simulation which does not take into account this interaction.</p

Schematic of vesicle subjected to gravity induced sedimentation.

<p>Sedimentation is driven by the density difference .</p