Session U34: Micro/Nano Particles: General

Automated monitoring and positioning of single microparticles via ultrasound-driven microbubble streaming.

We present a novel technique that uses non-invasive and non-destructive ultrasound-driven microbubble streaming for automated positioning or isolating of single microparticles or rare biological cells in microchannels. We introduce a microfluidic bead sorter with an accuracy of up to tens of micrometers in single-particle positioning, which does not require additional structures to precondition the flow for particle sorting (e.g., pre-focusing of particles by using sheath flows). By using the controlled microbubble streaming (activation and deactivation of the piezo transducer at the resonant frequency of the microbubbles) as a sorting operator, a variety of small particles with arbitrary initial positions can be precisely positioned across the width of the microchannel in a live mode, which cannot be readily achieved in applications using just an external on-off force. Lab-VIEW is used to simultaneously control image acquisition, image analysis, particle tracking, and positioning by a custom-designed feedback system in a live mode. The particles can be distinguished by their facial characters or the intensity of their emitted lights. The final position of the particle can be defined manually by an operator or by the movement of a mouse pointer in a live mode. This method is universally applicable in all areas of microfluidics, for particles of different sizes, shapes, densities, and compressibility.   

Optimal statistical estimators for diffusivity in particle-based simulations of fluids

Although particle-based simulations are used to study a broad range of micro- and nano-scale flow phenomena, a task as seemingly simple as constructing an error bar to place on a result obtained from a particle-based simulation can be fraught with subtleties. In this talk, by performing simulations on a variety of systems of interest to micro- and nano-scale fluid mechanics, we demonstrate that traditional approaches for quantifying uncertainty in transport calculations, including and especially for diffusivity, can fail (at times dramatically) to capture the true variance of this quantity. We argue that commonplace techniques for computing standard errors in transport measurements (typically based upon ordinary-least-squares estimators) are prone to overconfident predictions. Motivated by tools used to resolve a similar problem in econometrics, we present a simple and computationally efficient scheme that can significantly alleviate these issues, yielding more confidence in particle-based simulations of fluid transport phenomena.   

Viscoelastic lift forces on non-spherical particles in pressure-driven flows: theory and experiment

When particles are in a pressure-driven flow of a non-Newtonian fluid, the particles can acquire lift forces due to the imbalance of normal stresses on the particle surface. This phenomenon has been well-studied for spherical particles, but the role of particle shape is still in its early stages.  In this work, we develop a theory to describe the rigid body motion of a non-spherical particle in a polymeric fluid.  The theory is based on a retarded expansion in the Deborah number (i.e., second order fluid model), for the case when the particle is in a quadratic (i.e., pressure-driven) flow.  We find that for particles in a circular tube flow, spherical particles move to the center of the tube faster than prolate and oblate particles of the same volume, due to the unique orientation dynamics of the spheroids in the polymeric fluid.  We also find that prolate particles move slower than oblate particles of the same aspect ratio.  These trends are verified by performing microfluidic experiments where we visualize polystyrene particles of various shapes moving through circular capillaries in a Boger fluid with weak viscoelasticity (De~O(10^-2)) and vanishing inertia (Re~O(10^-4)).  The work here gives crucial understanding of how viscoelastic lift forces are altered by particle shape.   

Particle focusing in a small-amplitude wavy channel

Both inertial lift forces and oscillatory straining effects have been suggested as mechanisms for particle focusing in wavy channels. To determine the dominant mechanism, we predict the focusing locations of rigid neutrally buoyant particles in a small-amplitude wavy channel. We decompose the undisturbed channel flow into a main-order Poiseuille flow and secondary eddies induced by the waviness. We calculate the perturbation of the particle on the undisturbed flow and the resulting lateral lift force exerted on the particle with the method of matched asymptotic expansions. We obtain a main-order lift force determined from the Poiseuille flow and a first-order lift force due to the waviness of the channel. Unlike the main-order lift force which is only a function of the lateral position of the particle, the first-order lift force also varies sinusoidally along the channel. By employing the Maxey-Riley equation, we identify the lift force as the predominant mechanism for particle focusing. The balance between the main- and first-order lift forces determines the focusing locations, which do not significantly deviate from those in a straight channel. We validate the predictions experimentally at Reynolds numbers ranging from 10 to 250.   

Sedimentation of rigid and flexible sheets in a viscous fluid

Most of our everyday experience of objects settling periodically happens in inertia-dominated systems. Examples include a fluttering motion of a leaf falling in air or of a coin falling in water or a spinning motion of maple seeds.

In this experimental study, we first focus on rigid circular sheets bent symmetrically in the shape of U, falling in a viscous liquid (Re<0.02). We show that even such a symmetric and inertia-less system does not have a steady state, but exhibits periodic reorientation.

Our second focus is the fluid-structure interaction of settling flexible sheets, inspired by the process of segregating flakes of 2D materials by centrifugation [1]. We observe that flexible sheets often quickly adopt a U-bent configuration, but as they reorient, the curvature distribution changes. These transients are slow and no steady state is reached within the traversed length of 30 diameters.

We highlight the similarities and differences between flexible and rigid sheets. The behaviours are similar when the sheets are initialised in upright-U-orientations, but if they are initialised upside-down, the rigid sheet rolls, while the flexible sheet bends through and, rather surprisingly, changes the axis of bending in the process.

[1]Khan et al., Carbon 50, 2 (2012)   

Sedimentation of a Möbius strip

Sedimentation of particles is a classical problem of fluid mechanics. While it is well understood that the particle shape uniquely determines its motion, the precise trajectory and orientational dynamics are only known for a limited set of particle shapes. Here we study the sedimentation of a Möbius strip in a viscous fluid under gravity numerically. We approximate the Möbius strip by a rigid collection of spheres, employing the Rotne-Prager-Yamakawa interactions, and solve for its dynamics [1].

We find that the sedimenting Möbius strips follow chiral trajectories, in line with a recent theoretical suggestion [2]. Depending on the initial orientation of the Möbius strip, we report several classes of trajectories and present a proposal for the phase diagram of the system.

[1] D. Palanisamy and W. K. den Otter, J. Chem. Phys. 148, 194112 (2018)

[2] T. A. Witten and H. Diamant, Rep. Prog. Phys. 83 116601 (2020)   

Demonstrating use of continuous flow microfluidics to assemble colloidal particles on porous substrates

Self-assembly of colloidal particles for ‘bottom-up’ fabrication of various patterns and structures is critical for a range of applications including, but not limited to, energy migration, material science, biomimetics, and biosensing. Multiple self-assembly techniques, such as substrate templating — via topological or chemical patterning — and solvent evaporation were discussed in our previous papers and have been developed for the deposition of patterned self-assembled structures, such as bands of colloidal particles, on various substrates. While the templating techniques are limited due to the requirement of pattern-specific, prior substrate engineering to fabricate the desired structure, solvent evaporation requires longer assembly times and precise control over environmental conditions. In this paper, a template-free process, which is facilitated by continuous solvent drainage through porous substrates, is demonstrated for the self-assembly of colloidal particles into high-aspect ratio (>10³, length to height) structures, such as linear arrays or grid structures. Colloidal particles were assembled both on polymeric and metallic porous membranes, with assembly times up to ~ 10^- 2 seconds per unit structure.

    

Height Prediction in Particle Images Using Deep Learning

Particle images are used to study flow fields by observing changes in tracer particle location over time. Typically, one camera is used to obtain 2D information. However, particles can be at different height. Knowing the particle height completes the particle movement vector in region observed. Without using a second camera, the current work provides a novel approach to extract height by approximating inverse Lommel function using a convolution neural network (CNN) to learn the relationship between particle shape and a dimensionless defocused distance as a regression problem. With the camera parameters, the true particle height can be deduced. The trained CNN predicts particle height with an R² as high as 0.983 on synthetically generated datasets. Knowing the particle height enables the user to observe complicated 3D motion in fluids. It also benefits the measurement of fluid properties, such as the diffusion coefficient since the displacement of particles is involved and is predicted accurately by considering all three dimensions. Another use of the technique is in the measurement of femtonewton-scale forces in the rapid electronic patterning. The utility of the measurement technique will be demonstrated by measuring the velocity field of a microfluidic electrothermal vortex.   

Colloid Quincke-electrorotation near a boundary

The Quincke effect is an electrohydrodynamic instability that gives rise to a torque on a dielectric particle in a uniform DC electric field. In an unbounded medium, a spherical particle with conductivity and permittivity mismatch was found to undergo a Lorenz chaotic rotation (Peters et al, Chaos, 2005). Pradillo et al, Soft Matter (2019) reported a new dynamics regime, where a colloid, which initially rests on the bottom electrode, lifts off and levitates in the space of two electrodes. We analyze the Quincke rotation in this hovering state.

 We consider a spherical particle rotating near a plane electrode in a uniform electric field. The effect of the electrode is considered using the method of images. We analytically calculate the electric torque on the particle and formulate the equations describing the evolution of the rotation rate and the electric multipoles. We investigate the colloid dynamics numerically by solving the full set of evolution equations and analytically by means of asymptotic solutions for large separations from the boundary.

    

Complex Dynamics of Inextensible Elastic Sheets in Shear Flow

Two-dimensional materials like graphene and polymeric films, due to their superior mechanical properties, are widely applied in many applications, where the dynamics in flow are still poorly understood. We present a numerical study of thin polymer films to investigate dynamics under simple shear. Elastic sheets are modeled with out-of-plane bending and negligible in-plane stretching, and the fluid motion is computed by the regularized Stokeslets. The presence of a free surface can be incorporated through use of an image system. We consider sheets with circular or rectangular rest shapes, freely suspended in shear flow. We observed both shapes undergo a quasi-periodic flapping motion, with part of the edges, when facing the compressive axis, flapping up and down alternatively. When close to the free surface, the rectangle only flaps away from surface and the disk forms a stable taco shape while rotating, like a 3D tank-treading motion. The bias in flapping direction here is caused by the broken symmetry introduced by the surface. The sheets slowly drift away from the free surface due to force dipole of the image system. During drifting, the bias in flapping gradually vanishes. The simulation results agree with experimental observations from the group of Lorenzo Botto.

 

    

Author not Attending

Neutrally buoyant, force-free particles in Stokes flow have a strong tendency to passively follow streamlines. When encountering an obstacle, steric requirements force a particle to deviate from its initial streamline, but if the flow geometry is fore-aft symmetric reversibility necessitates the particle's return to the initial streamline. However, in scenarios that lack this symmetry particles may be irreversibly displaced. We investigate when such permanent displacement can be expected, using prototypical vortical flow fields as well as transport flows to assess the relevant conditions for such particle manipulation at zero Reynolds number. As theory predicts that the particle motion relative to the fluid is governed by gradients and curvatures of the flow field, counterintuitive particle trajectories can result if the flow is chosen judiciously. This may open up new possibilities for particle manipulation and a deeper understanding of deterministic lateral displacement.   

Dimensionless analysis on inertial microfluidic particle migration across laminar streams from Newtonian to non-Newtonian fluid

Microscale sample separation based on inertial microfluidic particle migration across laminar streams from Newtonian to non-Newtonian fluid has many advantages including separation resolution, throughput, and simplicity. Precise control of operational conditions is highly required for the separation; however, no concrete theoretical investigation has been conducted. Here, we propose a dimensionless analysis on the inertial particle migration to determine the separation conditions. We adopted a co-flow of Newtonian and non-Newtonian fluids to utilize both inertial and elastic lift forces. Along with Reynolds, Weissenberg, and elastic numbers, we introduce a new dimensionless number composed of viscosity, particle diameter, and width of the Newtonian fluid stream to predict whether the particle migrates from Newtonian to non-Newtonian fluid stream. We experimentally validated the proposed analysis method using polystyrene microparticles with diameter of 2 and 3 μm in a co-flow of water and polyethylene oxide solution at varying concentration.