R-3 Repository :: Browsing by Author "Du, Di"

Browsing by Author "Du, Di"

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Dynamics of paramagnetic squares in uniform magnetic fields
(Elsevier, 2016) Du, Di; He, Peng; Zeng, Yongchao; Biswal, Sibani Lisa
The magnetic forces between paramagnetic squares cannot be calculated using a classic dipolar model because the magnetic field distribution is not uniform within square particles. Here, we present the calculation of magnetic forces and torques on paramagnetic squares in a uniform 2-D magnetic field using a Laplace's equation solver. With these calculations, we simulate the variations in equilibrium configurations as a function of number of interacting squares. For example, a single square orients with its diagonal directed to the external field while a system of multiple squares will assemble into chain-like structures with their edges directed to the external field. Unlike chains of spherical magnetic particles, that easily stagger themselves to aggregate, chains consisting of magnetic squares are unable to aggregate due to interchain repulsion.
Interfacial energetics of two-dimensional colloidal clusters generated with a tunable anharmonic interaction potential
(American Physical Society, 2018) Hilou, Elaa; Du, Di; Kuei, Steve; Biswal, Sibani Lisa
Interfacial characteristics are critical to various properties of two-dimensional (2D) materials such as band alignment at a heterojunction and nucleation kinetics in a 2D crystal. Despite the desire to harness these enhanced interfacial properties for engineering new materials, unexpected phase transitions and defects, unique to the 2D morphology, have left a number of open questions. In particular, the effects of configurational anisotropy, which are difficult to isolate experimentally, and their influence on interfacial properties are not well understood. In this work, we begin to probe this structure-thermodynamic relationship, using a rotating magnetic field to generate an anharmonic interaction potential in a 2D system of paramagnetic particles. At low magnetic field strengths, weakly interacting colloidal particles form non-close-packed, fluidlike droplets, whereas, at higher field strengths, crystallites with hexagonal ordering are observed. We examine spatial and interfacial properties of these 2D colloidal clusters by measuring the local bond orientation order parameter and interfacial stiffness as a function of the interaction strength. To our knowledge, this is the first study to measure the tunable interfacial stiffness of a 2D colloidal cluster by controlling particle interactions using external fields.
Micro-mutual-dipolar model for rapid calculation of forces between paramagnetic colloids
(American Physical Society, 2014) Du, Di; Biswal, Sibani Lisa
Typically, the force between paramagnetic particles in a uniform magnetic field is calculated using either dipole-based models or the Maxwell stress tensor combined with Laplace's equation for magnetostatics. Dipole-based models are fast but involve many assumptions, leading to inaccuracies in determining forces for clusters of particles. The Maxwell stress tensor yields an exact force calculation, but solving Laplace's equation is very time consuming. Here, we present a more elaborate dipole-based model: the micro-mutual-dipolar model. Our model has a time complexity that is similar to that of other dipole-based models but is much more accurate especially when used to calculate the force of small aggregates. Using this model, we calculate the force between two paramagnetic spheres in a uniform magnetic field and a circular rotational magnetic field and compare our results with those of other models. The forces for three-particle and ten-particle systems dispersed in two-dimensional (2D) space are examined using the same model. We also apply this model to calculate the force between two paramagnetic disks dispersed in 2D space. The micro-mutual-dipolar model is demonstrated to be useful for force calculations in dynamic simulations of small clusters of particles for which both accuracy and efficiency are desirable.
Modified Mason number for charged paramagnetic colloidal suspensions
(American Physical Society, 2016) Du, Di; Hilou, Elaa; Biswal, Sibani Lisa
The dynamics of magnetorheological fluids have typically been described by the Mason number, a governing parameter defined as the ratio between viscous and magnetic forces in the fluid. For most experimental suspensions of magnetic particles, surface forces, such as steric and electrostatic interactions, can significantly influence the dynamics. Here we propose a theory of a modified Mason number that accounts for surface forces and show that this modified Mason number is a function of interparticle distance. We demonstrate that this modified Mason number is accurate in describing the dynamics of a rotating pair of paramagnetic colloids of identical or mismatched sizes in either high or low salt solutions. The modified Mason number is confirmed to be pseudoconstant for particle pairs and particle chains undergoing a stable-metastable transition during rotation. The interparticle distance term can be calculated using theory or can be measured experimentally. This modified Mason number is more applicable to magnetorheological systems where surface forces are not negligible.
Nonlinear multimode buckling dynamics examined with semiflexible paramagnetic filaments
(American Physical Society, 2018) Zhao, Jingjing; Du, Di; Biswal, Sibani Lisa
We present the contractile buckling dynamics of superparamagnetic filaments using experimental, theoretical, and simulation approaches. Under the influence of an orthogonal magnetic field, flexible magnetic filaments exhibit higher-order buckling dynamics that can be identified as occurring in three stages: initiation, development, and decay. Unlike initiation and decay stages where the balance between magnetic interactions and elastic forces is dominant, in the development stage, the influence of hydrodynamic drag results in transient buckling dynamics that is nonlinear along the filament contour. The inhomogeneous temporal evolution of the buckling wavelength is analyzed and the contractions under various conditions are compared.
Novel Dynamics and Structures Using Paramagnetic Colloids with Rotating Magnetic Fields
(2015-04-21) Du, Di; Biswal, Sibani Lisa; Chapman, Walter G; Pasquali, Matteo; Natelson, Douglas
Micron-sized colloids have long been used as model systems to study the dynamic and thermodynamic behavior of atomic systems. This is attributed to the fact that their dynamics are driven by thermal energy and they are large enough to be visualized using optical microscopy. Moreover, the interactions between particles can be tuned by surface functionalization or application of external fields. In this thesis, I will introduce the use of various rotating magnetic fields on a system of confined paramagnetic colloids to model different physical phenomena in two dimensions (2-D), whose dynamics are not easily observed at a single molecule length scale. The dynamics of a particle pair under a classic rotating magnetic field is first described with a modified Mason number, to describe the relationship between magnetic, viscous, and electrostatic interactions governing the rotational dyanmics. Next, I will describe a novel method to induce an isotropic attractive interaction between paramagnetic colloids when the frequency of the rotating field is sufficiently high. The pair interaction potential is comprised of a long-range attractive interaction induced by the external magnetic field and an electrostatic Yukawa-type repulsive interaction from the charged surfaces of the particles. This interaction potential is described by a theoretical model, which is verified by experimental measurement. Three-body effects are also measured using a three-particle system, which is the leading term of many-body effect. By solving the Laplace’s equation for magnetostatics, this three-body effect is proved to be negligible for particles far from the edges of a many-particle cluster. This validates the assumption of pair additivity in the interaction potential used in a Monte Carlo simulation. The tunable isotropic interaction provides an ideal platform to study the phase behavior of 2-D atomic systems. The melting thermodynamics and dynamics are studied in detail using simulation and experiment respectively. Thermodynamics properties, such as radial distribution function, translational order parameter, bond-orientational order parameter and Lindemann parameter of different phases are measured to show that melting transition for this system is first-order as opposed to the KTHNY theory, which states melting in 2-D should be a two-stage second-order transition. Phase coexistences are observed for the first time in 2-D system with long-range attraction, which further confirms the first-order nature of the transition. The simultaneous dislocation unbinding and disclination unbinding observed in experiment explains the inconsistency against the prediction given by the KTHNY theory. The phase diagram of this system is also constructed, which is shown to be very similar to that of atomic systems. Another novel aspect described in this thesis is the development of a novel method to achieve microscale swimming. A constant offset can be added to the rotating magnetic field if the temporal symmetry needs to be broken, and the resulting field is referred to as an eccentric rotating magnetic field. Under such a field, paramagnetic particles with mismatched sizes are shown to be able to swim in a directed manner. Swimmers consisting of multiple particles are able to fragment their arms. Stochastic forces can change the type of the fragmentation from the surrounding fluid, leading to decreased or increased swimming speed for different swimmers.
Numerical calculation of interaction forces between paramagnetic colloids in two-dimensional systems
(American Physical Society, 2014) Du, Di; Toffoletto, Frank; Biswal, Sibani Lisa
Typically the force between paramagnetic particles in a uniform magnetic field is described using the dipolar model, which is inaccurate when particles are in close proximity to each other. Instead, the exact force between paramagnetic particles can be determined by solving a three-dimensional Laplace's equation for magnetostatics under specified boundary conditions and calculating the Maxwell stress tensor. The analytical solution to this multi-boundary-condition Laplace's equation can be obtained by using a solid harmonics expansion in conjunction with the Hobson formula. However, for a multibody system, finite truncation of the Hobson formula does not lead to convergence of the expansion at all points, which makes the approximation physically unrealistic. Here we present a numerical method for solving this Laplaceﾒs equation for magnetostatics. This method uses a smoothed representation to replace all the boundary conditions. A two-step propagation is used to dramatically accelerate the calculation without losing accuracy. Using this method, we calculate the force between two paramagnetic particles in a uniform and a rotational external field and compare our results with other models. Furthermore, the many-body effects for three-particle, ten-particle, and 24-particle systems are examined using the same method. We also calculate the interaction between particles with different magnetic susceptibilities and particle diameters. The Laplaceﾒs equation solver method described in this article that is used to determine the force between paramagnetic particles is shown to be very useful for dynamic simulations for both two-particle systems and a large cluster of particles.
Reconfigurable paramagnetic microswimmers: Brownian motion affects non-reciprocal actuation
(Royal Society of Chemistry, 2018) Du, Di; Hilou, Elaa; Biswal, Sibani Lisa
Swimming at low Reynolds number is typically dominated by a large viscous drag, therefore microscale swimmers require non-reciprocal body deformation to generate locomotion. Purcell described a simple mechanical swimmer at the microscale consisting of three rigid components connected together with two hinges. Here we present a simple microswimmer consisting of two rigid paramagnetic particles with different sizes. When placed in an eccentric magnetic field, this simple microswimmer exhibits non-reciprocal body motion and its swimming locomotion can be directed in a controllable manner. Additional components can be added to create a multibody microswimmer, whereby the particles act cooperatively and translate in a given direction. For some multibody swimmers, the stochastic thermal forces fragment the arm, which therefore modifies the swimming strokes and changes the locomotive speed. This work offers insight into directing the motion of active systems with novel time-varying magnetic fields. It also reveals that Brownian motion not only affects the locomotion of reciprocal swimmers that are subject to the Scallop theorem, but also affects that of non-reciprocal swimmers.

Browsing by Author "Du, Di"

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