An official website of the United States government
We present a three-dimensional computational study of the impact of external magnetic fields on the dynamics of superparamagnetic ferrofluid droplets and rheology of dilute ferrofluid emulsions in planar extensional flows. Specifically, we show how the intensity and direction of uniform magnetic fields affect the planar extensional rheology of ferrofluid emulsions by changing the shape and magnetization of the constituent ferrofluid droplets in suspension. We find that the two traditional extensional viscosities associated with the normal stresses of the bulk emulsion in extension either remain constant or increase with the field intensity; the only exception occurs when the field direction is perpendicular to the extension plane, where increasing the field intensity keeps the planar extensional viscosity constant and modestly decreases the second extensional viscosity. We also find that the droplet tilts in the flow when the external field is not aligned with one of the flow main directions, which changes the recirculation pattern and flow topology inside the droplet. At the microscopic level, the droplet experiences a magnetic torque because of a small misalignment between its magnetization and the external field direction. At the macroscopic level, the bulk emulsion experiences a field-induced internal torque that leads to a nonsymmetric stress tensor with unexpected shear components in extension. To account for this unconventional stress-strain response, we introduce new extensional material functions such as shear and rotational viscosity coefficients that unveil novel rheological signatures of ferrofluid emulsions in planar extensional flows. This study offers new insights into applications based on the field-assisted manipulation of ferrofluid droplets and sheds light on the potential of ferrofluid emulsions as a model system for chiral fluids with internal rotational degrees of freedom that can be activated and controlled by coupling static magnetic fields with hydrodynamic flows.
more »
« less
3D manipulation and dynamics of soft materials in 3D flows
Flow-based manipulation of particles is an essential tool for studying soft materials, but prior work has nearly exclusively relied on using two-dimensional (2D) flows generated in planar microfluidic geometries. In this work, we demonstrate 3D trapping and manipulation of freely suspended particles, droplets, and giant unilamellar vesicles in 3D flow fields using automated flow control. Three-dimensional flow fields including uniaxial extension and biaxial extension are generated in 3D-printed fluidic devices combined with active feedback control for particle manipulation in 3D. Flow fields are characterized using particle tracking velocimetry complemented by finite-element simulations for all flow geometries. Single colloidal particles (3.4 μm diameter) are confined in low viscosity solvent (1.0 mPa s) near the stagnation points of uniaxial and biaxial extensional flow for long times (≥10 min) using active feedback control. Trap stiffness is experimentally determined by analyzing the power spectral density of particle position fluctuations. We further demonstrate precise manipulation of colloidal particles along user-defined trajectories in three dimensions using automated flow control. Newtonian liquid droplets and GUVs are trapped and deformed in precisely controlled uniaxial and biaxial extensional flows, which is a new demonstration for 3D flow fields. Overall, this work extends flow-based manipulation of particles and droplets to three dimensions, thereby enabling quantitative analysis of colloids and soft materials in complex nonequilibrium flows.
more »
« less
- Award ID(s):
- 2030537
- PAR ID:
- 10446561
- Date Published:
- Journal Name:
- Journal of Rheology
- Volume:
- 67
- Issue:
- 4
- ISSN:
- 0148-6055
- Page Range / eLocation ID:
- 877
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Capillary suspensions are three-phase mixtures containing a solid particulate phase, a continuous liquid phase, and a second immiscible liquid forming capillary bridges between particles. Capillary suspensions are encountered in a wide array of applications including 3D printing, porous materials, and food formulations, but despite recent progress, the micromechanics of particle clusters in flow is not fully understood. In this work, we study the dynamics of meniscus-bound particle clusters in planar extensional flow using a Stokes trap, which is an automated flow control technique that allows for precise manipulation of freely suspended particles or particle clusters in flow. Focusing on the case of a two-particle doublet, we use a combination of experiments and analytical modeling to understand how particle clusters rearrange, deform, and ultimately break up in extensional flow. The time required for cluster breakup is quantified as a function of capillary number Ca and meniscus volume V. Importantly, a critical capillary number Cacrit for cluster breakup is determined using a combination of experiments and modeling. Cluster relaxation experiments are also performed by deforming particle clusters in flow, followed by flow cessation prior to breakup and observing cluster relaxation dynamics under zero-flow conditions. In all cases, experiments are complemented by an analytical model that accounts for capillary forces, lubrication forces, hydrodynamic drag forces, and hydrodynamic interactions acting on the particles. Results from the analytical models are found to be in good agreement with experiments. Overall, this work provides a new quantitative understanding of the deformation dynamics of capillary clusters in extensional flow.more » « less
-
Smart structures with actuation function are desired for aerospace applications, including morphing airfoils, deployable structures and more. While shape memory alloys and piezoelectric ceramics and polymers are currently a popular smart material options for such applications, magnetoelastomers (MEs) can be uniquely actuated with application of non-contact magnetic field. Magnetoelastomers (MEs), composite materials made of magnetic particles and soft, non-magnetic matrix, can potentially contribute to such smart structures as a light-weight, smart material option with large strain change, fast response time (milliseconds) and anisotropic actuation properties. Other than aerospace applications, MEs, as soft actuators, have been investigated for flexible electronics, soft robotics, and biomedical applications. Anisotropic actuation properties of MEs can be controlled with particle organization within the elastomer. To provide this control, parametric studies on fabrication of MEs need to be performed. This study presents experimental work on nanoparticle organization within MEs using uniaxial, biaxial and triaxial magnetic fields and on the structure-property relationships of MEs. Iron oxide nanoparticles were used as a model nanofillers, and their surfaces were treated with silane coupling agent to improve dispersion and suspension within a polydimethylsiloxane (PDMS) elastomer. The fabricated MEs were inspected using microCT, and their anisotropic susceptibilities are being measured.more » « less
-
Inkjet-based three-dimensional (3D) printing is widely used for fast and efficient non-contact manufacturing, yet it suffers from several drawbacks, such as coarse resolution, lack of adhesion, manufacturing inconsistency, and uncertain final part mechanical properties. These undesirable effects are related to complex flow phenomena in colloidal droplets in inkjet 3D printing, particularly the internal flows and droplet deformations during the deposition and drying processes. These challenges are due to the colloidal suspension droplets being kept in the liquid state during printing. To overcome these disadvantages, this paper presents a novel freezing-sublimation-based inkjet 3D printing concept that freezes the colloidal droplets upon impact followed by sublimation, eliminating the undesirable particle transport and fluid motions during deposition. A series of experiments were conducted to characterize the colloidal droplet behaviors during the impinging/freezing and sublimation processes and evaluate the effects of the freezing process on droplet impinging dynamics as well as the final deposition patterns through sublimation. It was demonstrated that the deposition patterns obtained from this new method are much more uniform than the conventional evaporation-based deposition method. Both qualitative and quantitative methods were applied to analyze the colloidal droplet profiles during the printing process (impinging, freezing, and sublimation), as well as the final deposition patterns. The study shows promising results of using this new method, providing a foundation for the development of the novel freezing-sublimation-based inkjet 3D printing technique.more » « less
-
Viscous streaming is an efficient rectification mechanism to exploit flow inertia at small scales for fluid and particle manipulation. It typically entails a fluid vibrating around an immersed solid feature that, by concentrating stresses, modulates the emergence of steady flows of useful topology. Motivated by its relevance in biological and artificial settings characterized by soft materials, recent studies have theoretically elucidated, in two dimensions, the impact of body elasticity on streaming flows. Here, we generalize those findings to three dimensions, via the minimal case of an immersed soft sphere. We first improve existing solutions for the rigid-sphere limit, by considering previously unaccounted terms. We then enable body compliance, exposing a three-dimensional, elastic streaming process available even in Stokes flows. Such effect, consistent with two-dimensional analyses but analytically distinct, is validated against direct numerical simulations and shown to translate to bodies of complex geometry and topology, paving the way for advanced forms of flow control.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1122/8.0000600
