publications

2023

1. Coiling of semiflexible paramagnetic colloidal chains

   Aldo Spatafora-Salazar* ,  Steve Kuei* ,  Lucas HP Cunha , and 1 more author

   Soft Matter, 2023

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   Semiflexible filaments deform into a variety of configurations that dictate different phenomena manifesting at low Reynolds number. Harnessing the elasticity of these filaments to perform transport-related processes at the microfluidic scale requires structures that can be directly manipulated to attain controllable geometric features during their deformation. The configuration of semiflexible chains assembled from paramagnetic colloids can be readily controlled upon the application of external time-varying magnetic fields. In circularly rotating magnetic fields, these chains undergo coiling dynamics in which their ends close into loops that wrap inward, analogous to the curling of long nylon filaments under shear. The coiling is promising for the precise loading and targeted transport of small materials, however effective implementation requires an understanding of the role that field parameters and chain properties play on the coiling features. Here, we investigate the formation of coils in semiflexible paramagnetic chains using numerical simulations. We demonstrate that the size and shape of the initial coils are governed by the Mason and elastoviscous numbers, related to the field parameters and the chain bending stiffness. The size of the initial coil follows a nonmonotonic behavior with Mason number from which two regions are identified: (1) an elasticity-dependent nonlinear regime in which the coil size decreases with increasing field strength and for which loop shape tends to be circular, and (2) an elasticity-independent linear regime where the size increases with field strength and the shape become more elliptical. From the time scales associated to these regimes, we identify distinct coiling mechanisms for each case that relate the coiling dynamics to two other configurational dynamics of paramagnetic chains: wagging and folding behaviors.

2022

1. A small-volume microcapillary rheometer

   Paul F Salipante ,  Steve Kuei ,  and  Steven D Hudson

   Rheologica acta, 2022

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   We demonstrate a capillary device used to measure the shear rate-dependent viscosity of microliter scale volumes. Liquid samples are driven pneumatically through a microcapillary and partially fill a larger glass capillary. The glass capillary is mounted on an optical linear sensor to track the air-liquid meniscus in real time and trigger the reversal of flow direction by switching a pneumatic valve. Each transit provides a volumetric flow rate measurement, which is used with the pressure drop to determine viscosity as a function of shear rate. A given sample of at least 50 µL can be measured over at least 2 to 3 decades in shear rate, in the range of 10 to , and be essentially fully recovered. Validation by comparison to reference measurements is performed using samples of Newtonian and non-Newtonian fluid, with viscosity ranging from 1 to 100 mPa s. The range of operation and uncertainty arising from instrumentation, meniscus effects, and inertial effects are discussed. The performance of this rheometer is advantageous, especially for use and reuse of small volumes.

2018

1. Interfacial energetics of two-dimensional colloidal clusters generated with a tunable anharmonic interaction potential

   Elaa Hilou ,  Di Du ,  Steve Kuei , and 1 more author

   Physical Review Materials, 2018

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   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.

2017

1. From strings to coils: Rotational dynamics of DNA-linked colloidal chains

   Steve Kuei ,  Burke Garza ,  and  Sibani Lisa Biswal

   Physical Review Fluids, 2017

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   We investigate the dynamical behavior of deformable filaments experimentally using a tunable model system consisting of linked paramagnetic colloidal particles, where the persistence length lp, the contour length lc, and the strength and frequency of the external driving force are controlled. We find that upon forcing by an external magnetic field, a variety of structural and conformational regimes exist. Depending on the competition of forces and torques on the chain, we see classic rigid rotator behavior, as well as dynamically rich wagging, coiling, and folding behavior. Through a combination of experiments, computational models, and theoretical calculations, we are able to observe, classify, and predict these dynamics as a function of the dimensionless Mason and magnetoelastic numbers.

2015

1. Dynamics and topology of a flexible chain: knots in steady shear flow

   Steve Kuei ,  Agnieszka M Słowicka ,  Maria L Ekiel-Jeżewska , and 2 more authors

   New Journal of Physics, 2015

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   We use numerical simulations of a bead–spring model chain to investigate the evolution of the conformations of long and flexible elastic fibers in a steady shear flow. In particular, for rather open initial configurations, and by varying a dimensionless elastic parameter, we identify two distinct conformational modes with different final size, shape, and orientation. Through further analysis we identify slipknots in the chain. Finally, we provide examples of initial configurations of an ’open’ trefoil knot that the flow unknots and then knots again, sometimes repeating several times.

2013

1. Mechanical and functional properties of epothilone-stabilized microtubules

   Dezhi Yu ,  Veronica Pessino ,  Steve Kuei , and 1 more author

   Cytoskeleton, 2013

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   Using a suite of biophysical tools, we assess the mechanical, structural, and functional properties of microtubules (MTs) stabilized by the chemotherapeutic compounds epothilone-A, epothilone-B, and taxol in vitro. We demonstrate that MTs stabilized by epothilone-A or epothilone-B are competent to bind tau proteins and support kinesin translocation. Kinesin speed is sensitive not only to the type of small molecule stabilizer used but also to the presence of the essential MT-associated protein tau. Epothilone-stabilized MTs are substantially less stiff than taxol-stabilized MTs. The addition of tau proteins to MTs stabilized by either epothilone compound or taxol further reduces stiffness. Taken together, these results suggest that small molecule stabilizers do not simply stabilize a “native” MT structure, but rather they modulate the structure, function, and mechanics of the MTs they bind. This may have important consequences to the therapeutic use of these agents in cancer chemotherapies.

2012

1. Spectral analysis methods for the robust measurement of the flexural rigidity of biopolymers

   David Valdman ,  Paul J Atzberger ,  Dezhi Yu , and 2 more authors

   Biophysical journal, 2012

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   The mechanical properties of biopolymers can be determined from a statistical analysis of the ensemble of shapes they exhibit when subjected to thermal forces. In practice, extracting information from fluorescence microscopy images can be challenging due to low signal/noise ratios and other artifacts. To address these issues, we develop a suite of tools for image processing and spectral data analysis that is based on a biopolymer contour representation expressed in a spectral basis of orthogonal polynomials. We determine biopolymer shape and stiffness using global fitting routines that optimize a utility function measuring the amount of fluorescence intensity overlapped by such contours. This approach allows for filtering of high-frequency noise and interpolation over sporadic gaps in fluorescence. We use benchmarking to demonstrate the validity of our methods, by analyzing an ensemble of simulated images generated using a simulated biopolymer with known stiffness and subjected to various types of image noise. We then use these methods to determine the persistence lengths of taxol-stabilized microtubules. We find that single microtubules are well described by the wormlike chain polymer model, and that ensembles of chemically identical microtubules show significant heterogeneity in bending stiffness, which cannot be attributed to sampling or fitting errors. We expect these approaches to be useful in the study of biopolymer mechanics and the effects of associated regulatory molecules.

1956

1. 

   TEST Investigations on the Theory of the Brownian Movement

   Albert Einstein

   1956

   Bib

   @book{einstein1956investigations,
     title = {TEST Investigations on the Theory of the Brownian Movement},
     author = {Einstein, Albert},
     year = {1956},
     publisher = {Courier Corporation},
   }