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Vanipalli SA, Iacovella CR, Sung KE, Mukhija D, Millunchick JM, Burns MA, Glotzer SC and Solomon MJ
Fluidic Assembly and Packing of Microspheres in Confined Channels
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We study fluidic assembly and packing of spherical particles in rectilinear microchannels that are terminated by a flow constriction. First, we introduce a method for active assembly of particles in the confined microchannels by triggering a local constriction in the fluid channel using a partially closed membrane valve. This microfluidic valve allows active, on-demand particle assembly as opposed to previous passive assembly methods based on terminal channels and weirs. Second, we study the three-dimensional assembly and packing of particles against a weir in confined rectilinear microchannels. The packings result in achiral particle chains with alternating (zigzag) structure. This structure is characterized by a single, repeated bond angle whose components projected into the frame of the channel are quantified by confocal microscopy and image processing. Brownian dynamics simulation of the packing comprehensively delineates the range of bond angles possible in narrow, rectilinear microchannels as well as the complex dependence of these angles on the relative dimensions of the channel and particles. The simulations of the three-dimensional packings are accurately modeled by a compact theory based on trigonometric relationships. The experimentally measured bond angles show excellent agreement with the simulations, thereby validating the functional dependence of the achiral packing bond angles on channel dimensions. This functional relationship is immediately useful for the design of anisotropic particles by microfluidic synthesis.
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Carney RP, DeVries GA, Dubois C, Kim H, Kim JY, Singh C, Ghorai PK, Tracy JB, Stiles RL, Murray RW, Glotzer SC, Stellacci F
Size Limitations for the Formation of Ordered Striped Nanoparticles
Journal of American Chemical Society (In Press)
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Singh C, Ghorai PK, Horsch MA, Jackson AM, Larson RG, Stellacci F, Glotzer SC
Entropy-Mediated Patterning of Surfactant-Coated Nanoparticles and Surfaces
Physical Review Letters 99, 226106 (2007)
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We perform atomistic and mesoscale simulations to explain the origin of experimentally observed
stripelike patterns formed by immiscible ligands coadsorbed on the surfaces of gold and silver nano-
particles. We show that when the conformational entropy gained via this morphology is sufficient,
microphase-separated stripelike patterns form. When the entropic gain is not sufficient, we instead predict bulk phase-separated Janus particles. We also show corroborating experimental results that confirm our simulational predictions that stripes form on flat surfaces as well as on curved nanoparticle surfaces.
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Ghorai, PK and Glotzer, SC
Molecular Dynamics Simulation Study of Self-Assembled Monolayers of Alkanethiol Surfactants on Spherical Gold Nanoparticles
J. Phys. Chem. C, 111, 15857-15862, 2007
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Atomistic molecular dynamics (MD) simulations of self-assembled alkanethiol monolayers are performed to
investigate the ligand shell organization of homoligand surfactants on spherical gold nanoparticle surfaces as
a function of temperature, nanoparticle size, and ligand tail length. At high temperature, we show that the
ligands orient randomly with respect to the surface normal with a small tilt angle. As the temperature decreases,
the molecules order and adopt a larger tilt angle. The effects of alkanethiol tail length and nanoparticle size
on the tilt structure are also significant. At low temperature, we find the equilibrium conformation of alkanethiols
obeys the crystallographic model, whereas at high temperature the continuous model is valid. The dependence
of tilt angle on different parameters and comparison with self-assembled monolayers on flat surfaces are also
discussed.