Browsing by Author "Hilou, Elaa"

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    Characterizing the spatiotemporal evolution of paramagnetic colloids in time-varying magnetic fields with Minkowski functionals
    (Royal Society of Chemistry, 2020) Hilou, Elaa; Joshi, Kedar; Biswal, Sibani Lisa
    Phase separation processes are widely utilized to assemble complex fluids into novel materials. These separation processes can be thermodynamically driven due to changes in concentration, pressure, or temperature. Phase separation can also be induced with external stimuli, such as magnetic fields, resulting in novel nonequilibrium systems. However, how external stimuli influence the transition pathways between phases has not been explored in detail. Here, we describe the phase separation dynamics of superparamagnetic colloids in time-varying magnetic fields. An initially homogeneous colloidal suspension can transition from a continuous colloidal phase with voids to discrete colloidal clusters, through a bicontinuous phase formed via spinodal decomposition. The type of transition depends on the particle concentration and magnitude of the applied magnetic field. The spatiotemporal evolution of the microstructure during the nucleation and growth period is quantified by analyzing the morphology using Minkowski functionals. The characteristic length of the colloidal systems was determined to correlate with system variables such as magnetic field strength, particle concentration, and time in a power-law scaling relationship. Understanding the interplay between particle concentration and applied magnetic field allows for better control of the phases observed in these magnetically tunable colloidal systems.
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    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.
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    Investigation of a system with a tunable anharmonic interaction potential using paramagnetic colloids
    (2019-04-19) Hilou, Elaa; Biswal, Sibani L
    Colloidal physics dictates properties of many small-scale and large-scale systems with applications ranging from drug delivery to catalysis. Colloidal systems can also be used as emulsion stabilizers or used in liquid crystal displays. Scientists and engineers have studied colloids to model the behavior of macroscopic systems at the atomic level. Colloids are uniquely suited for this application because the associated length scales are large enough to observe under an optical microscope, yet small enough so that their dynamics are driven by thermal motion. In this work, we manipulate magnetically induced and negatively charged colloidal particles in which we can control their interactions by applying a tunable magnetic field. Magnetically tunable particles provide us the ability to control the structure and phase behavior of the colloidal dispersions. Tunability allows for precise manipulation of particle interactions and thus, particle assembly into colloidal agglomerates that exhibit both fluid-like and crystal-like properties. These collections of particles nucleate, coalesce, and grow over time, which are properties of a model system for non-equilibrium and quasi-equilibrium behaviors. At quasi-equilibrium, these systems do not exhibit great changes in morphology, but can still coarsen over time. A non-equilibrium state is characterized by dynamics such as nucleation, decomposition, and coalescence. The system changes at a scale large enough to affect the energetics and morphological properties of the system. This dissertation is divided into two parts, one that focuses on the characterization of what we call colloidal clusters which are finite-sized aggregates that form in a sample with low particle concentration. These aggregates are individual islands of particles with adequate spacing such that we are able to examine them individually. The second half of this thesis describes the kinetics that take place when a colloidal dispersion undergoes quenching, causing behavior analogous to that of phase separating systems. We quantify the stability and kinetics of the system by measuring thermodynamic properties and morphological features as a function of three main parameters: time, t , field strength, B, and particle concentration. We characterize the phase behavior by considering both the bulk and interfacial properties of colloidal aggregates. We also find a scaling relationship between the three parameters to predict the aggregation kinetics governing systems that undergo quenching caused by long-range interactions.
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    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.
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    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.