Session T36: Micro/Nano scale Flows: Mixing and Separation

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We use a combination of molecular dynamics simulations and computational fluid dynamics to discern the effect of enthalpic and entropic contribution towards polyelectrolyte translocation in a finite length nanopore. We consider the electrophoretic transport of a single chain charged polymer through a nanopore under the influence of an applied electric field. The confinement and the fluid-solid interaction at the surface of the polyelectrolyte results in a relative slip of the fluid molecules and ions at the nanopore-fluid interface and polyelectrolyte-fluid interface, respectively. Moreover, the translocating charged interface leads to development of an electric double layer which evolves with the position of the polyelectrolyte in the pore and tweaks the induced potential both in the axial and transverse direction due to ion concentration polarization and ion partitioning effect. Thus, the resultant dynamics is a complex concoction of the fluid-solid hydrodynamic interactions at polyelectrolyte and pore wall, the electrostatic interactions in the pore and the structural/entropic interactions of the polyelectrolyte. We observe that polyelectrolyte which otherwise fails enter the nanopore by virtue induced potential, successfully translocate due to slip and enhanced applied potential. An increase in length of polyelectrolyte also enhances the charge and counter-ion accumulation increasing likelihood of successful translocation.   

Numerical study of particle migration and equilibrium positions in inertial particle microfluidics: effect of channel cross-section

Inertial particle microfluidics (IPMF) is a technique used to separate cells on the microscale, such as circulating tumour cells (CTCs) for disease diagnosis, by focusing cells to a lateral equilibrium position which depends on cell properties. IPMF is a passive technique which, aside from a pump to generate a flow inside the device, solely relies on the intrinsic hydrodynamic forces between the channel, fluid, and particles. Two key forces in IPMF are the wall lift force and the shear gradient lift force, which are influenced by a number of parameters, such as the Reynolds number, particle confinement (particle size relative to the channel size), and velocity profile. The velocity profile is determined by the shape of the channel's cross-section, leading to variations in the number and location of equilibrium positions of the particles. 

In our study, we employ a 3D immersed-boundary-lattice-Boltzmann method to numerically analyze the dynamics of both soft and rigid particles, including their migration paths and equilibrium positions. 

By simulating particle migration in channels with different cross-sectional shapes, we demonstrate that changes in the channel geometry significantly impact the behaviour of particles by modifying the particle trajectory and the location and number of equilibrium positions, while keeping the Reynolds number and particle size constant. The results will enable improved designs in order to more finely manipulate particles in microfluidic channels for applications such as cell cytometry and separation.    

Numerical study of particle separation from heterogeneous suspensions in inertial microfluidics

Sorting particles in heterogeneous suspensions has applications from disease diagnosis to water filtration. A number of separation methods exist, however, there is a need for label-free methods that passively separate particles with high throughput and without dilution. Inertial microfluidics is an emerging technology that passively manipulates particles by exploiting inertial forces which are relevant at Reynolds number of the order of 10-100. Due to these forces, particles migrate to equilibrium positions within a channel cross-section based on their properties, and can therefore be separated. Inertial microfluidic devices are used in real-world applications using dilute suspensions. However, there is significant scope to identify new applications with dense suspensions (up to 45%). In particular, particle size and softness heterogeneity effects are not well understood. We use an in-house lattice-Boltzmann-immersed-boundary-finite-element code to uncover underlying physical mechanisms. We investigate concentration effects on the separation of dense heterogeneous suspensions (5-45%). We also investigate size and softness heterogeneity effects. We compare heterogeneous and homogeneous suspension behaviour. We show that particles of different size and softness migrate to different locations, and these locations are concentration dependent. Our results provide evidence that inertial microfluidic devices can be used for separation in dense heterogeneous suspensions.   

Numerical investigation of heterogeneous soft particle pairs in inertial microfluidics

Inertial particle microfluidics is a relatively new technology that exploits inertial effects to manipulate and control particle dynamics. Under inertial flow conditions, particle-particle interactions can lead to the formation of pairs and trains of particles. We analyse the behaviour of two particles, modelled as capsules, with different softness in channel flow under moderate inertia. We employ an immersed-boundary-lattice-Boltzmann-finite-element solver to account for the fluid mechanics, the capsule dynamics, and their coupling. We find that the initial configuration and the heterogeneity of particle softness are important. Pairs are more likely to form when the leading particle is stiffer than the lagging particle. The lateral equilibrium position of the particles in the pair is determined by both the leading and lagging particle. However, the axial spacing in a stable pair is dominated by the softness of the lagging particle. Our study has important implications for real-world applications. Since pair formation is undesired for particle separation, it is generally important to know under which circumstances particles with different properties are able or unable to form a pair. Applications relying on precise inter-particle spacing, such as cytometry, might benefit from our findings as well.   

Diffusiophroesis-controlled separation of a colloid-electrolyte suspension under gravity and solvent evaporation

Unidirectional drying a colloidal suspension has been used widely for manufacturing microstructured materials, such as ceramics, electrodes, and photonic crystals. Recent experiments and simulations demonstrated significant impacts of gravity on the phase separation process. However, in the process, the role of colloid transport induced by an electrolyte concentration gradient, a mechanism known as diffusiophoresis, is unexplored to date when coupled with gravity. In this work, we utilize direct numerical simulations and develop a macrotransport theory to analyze the advective-diffusive transport of an electrolyte-colloid suspension in a unidirectional drying cell under gravity and diffusiophoresis. We report two new significant findings. First, our simulations and theory demonstrate new scalings for the growth of the colloidal layer, where the layer produced with solute-repelled diffusiophoretic colloids could be an order of magnitude thicker than with non-diffusiophoretic colloids. Second, our results show that the enhancement in the growth of the colloidal layer due to diffusiophoresis can be achieved not just on Earth under normal gravity but also in space under microgravity. Our results enable tailoring the separation of colloid-electrolyte suspensions by tuning the interactions between the solvent, electrolyte, and colloids under Earth's and microgravity, which is central to ground-based and in-space applications.   

Interactions of fibers with pillars: toward a sorting device

Flowing suspensions of elongated particles are encountered in many biological and industrial systems. The motion of the particles results from the complex interplay between the surrounding flow, internal elastic forces, as well as potential interactions with obstacles and walls. In this work, we conduct experiments and simulations to study the transport of fibers interacting with pillars in a microfluidic channel. In the case of rigid fibers, we identify four dominant behaviors depending on their position and orientation as they enter the channel. The geometrical and mechanical properties of the fibers also affect their trajectory, especially if they hit the pillars. Long and stiff fibers are indeed found to be much more laterally displaced by the pillars than short and flexible ones which rather tend to flow back along their initial streamline with no significant deviation. Based on these findings, we make suggestions on how to optimize a microfluidic device to sort fibers by length and/or flexibility.   

Interactions of fibers with pillars: from one pillar to pillar arrays

The transport dynamics of flexible fibers in complex environments like porous media underpin numerous industrial and biomedical processes. The complex dynamics of these flexible fibers stem from the presence of obstacles that disturb the fluid flow, their inherent deformability, and potential interactions with obstacles. In our work, we combine experimental and numerical methods first to investigate the dynamics of a flexible fiber interacting with an individual triangle pillar. Owing to their flexibility, fibers tend to deform and align with streamlines, causing negligible lateral displacement pre- and post-pillar. In contrast, the fibers exhibit much richer dynamics in circular pillar arrays, with different lateral drift angles for varying fiber lengths. This separation effect, based on fiber lengths, relates to the flow field distribution and various fiber dynamics in the array. Our research identifies an effective method for length-based flexible fiber sorting in microfluidic applications.   

Experimental and numerical investigations of liquid transfer between inclined surfaces

The liquid transfer process is an intricate phenomenon influenced by the combined effects of viscous, inertial, and surface forces. There have been a number of previous experimental and numerical studies investigating the mechanisms of liquid transfer; however, the effect of contact angle hysteresis has been overlooked. This is essential for accurately reproducing the complex contact line motions that play a significant role in controlling the precise volume deposited during liquid transfer. In our study, we investigate the effects of a range of control parameters such as viscosity, velocity, and surface wettability on liquid transfer via both experiment and numerical simulations based on the phase-field model and incorporating contact angle hysteresis. We manipulate the surface tilting angle to examine its key role in governing the liquid transfer between non-parallel surfaces. The results give new insight into optimizing Roll-to-Roll printing system, and other liquid transfer applications.