Session L07: Electrostatic Manipulation of Fluids and Soft Matter II

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It is well-known that friction produces static charging which has intensely interwoven the histories of tribology and static electrification. However, the underlying mechanisms of this phenomenon continue to remain under investigation as triboelectrification is often unpredictable.[1] One method of studying this is by peeling adhesive tape since it replicates the separation of two contacting surfaces. Within this talk, we discuss relationships between distinct patterns from peeled tape and quantitative voltage data acquired during the peeling process. We contextualize these findings within industry and nature and describe new avenues for this field.

[1] D. Lacks, Angew. Chem. (Int. Ed.) 51, 6822-6823 (2012)   

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Electrically charged particles suspended in gas exist in numerous planetary environments. Salient examples include volcanic plumes, airborne Saharian sand, dust devils on Mars, and the dune fields of Titan. Yet the microphysical processes that lead to electrification are not understood, such as the lifetime of charge on electrified particles or the processes that lead to their electrostatic neutrality. Our laboratory measurements aim to answer these questions through acoustic levitation to ensure that charge is not lost through physical contact. Two hemispherical transducers produce an acoustic standing wave that lofts particles of millimetric scale. An ionizer then charges these particles, and the remaining charge is measured over days or weeks by moving the particle through a Faraday cup. Previous measurements were limited to low-density, porous particles. Now, we have constructed an acoustic levitation device capable of suspending dense materials such as copper. Additionally, in environments of varying humidity and pressure, we investigate the mechanisms leading to airborne particle charge loss, and it’s relation to the material hydrophobicity.   

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We report a hydrogel formation process from 2-nm emissive, low molecular-weight metal-organic cages at low concentrations (>15 mg/mL) based on counterion-mediated attraction, π-π/hydrophobic interactions as well as σ-π interactions. Experiments and all-atom simulations confirm that with additional of small electrolytes, the cages in aqueous solution first self-assemble into 2D nanosheets via counterion-mediated attraction due to their unique molecular structure and charge distribution, as well as σ-π interactions. The stiff nanosheets are difficult to bend into 3-D hollow, spherical blackberry type structures, as observed in many other macroions systems. Instead, they stay in solution and their very large excluded volumes lead to gelation at low (~1.5 wt%) MOC concentrations, with additional helps from hydrophobic and partially π-π interactions similar to the gelation of graphene oxides.   

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Although tension-propagation theory indicates that the field-driven translocation time of a polyelectrolyte across a nanopore depends upon its initial conformation, most simulations of this process use equilibrium conformations. During capture, a polymer deforms and orients along the converging field lines. Our Langevin Dynamic (LD) simulations suggest that a long semi-flexible polymer stretches while approaching the pore but then shrinks when it reaches the pore. Rod-like chains, on the other hand, slightly stretch throughout the entire process. In this talk, we compare LD results to those obtained when Hydrodynamic Interactions (HI) and explicit salt are added. The HI are modelled using a Lattice-Boltzmann algorithm and the electrostatic interactions employ the P3M method. We examine how conformations evolve during capture and how much they deviate from equilibrium when translocation start. We also investigate the impact of these "captured conformations" on the translocation process itself. Finally, we investigate the possibility of using time-dependent fields to help end monomers find the nanopore, the last step before translocation. Our simulations are done using ESPResSo.   

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Transport phenomena at the nanoscale are dramatically influenced by confinement effects and the presence of interfaces. Reversible formation of vapor bubbles may occur inside hydrophobic nanopores as a consequence of water confinement [1]. In electrolyte solutions, this is accompanied by a drop in the channel conductance, resulting in a hydrophobic gate for ionic current which is relevant in several biological channels [2].
We studied hydrophobic gating in a model nanopore system with a NaCl solution via Restrained Molecular Dynamics (RMD) simulations [3]. We computed the free energy map and the diffusion coefficients as a function of the number of water molecules inside the nanopore and the axial coordinate of both ions.
Based on the data from RMD simulations, we developed a coarse grained model for the dynamics of the water inside the pore and computed the conductivity of both ions as a function of the number of water molecules inside the pore. The model can be used to simulate the combined effect of multiple channels on timescales not easily accessible for molecular dynamics simulations.

[1] A.Tinti et al., PNAS (2017)
[2] P. Aryal et al., J. Mol. Bio. (2015)
[3] L. Maragliano and E. Vanden-Eijnden, Chem. Phys. Lett. (2006)   

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The induced polarization charge appears to diverge as curved dielectrics approach and form close contacts. Resolving the diverging charge density slows down the convergence rate in numerical calculations of electrostatic energy. We show that this divergence depends on both the gap distance and the dielectric permittivity logarithmically. We propose a parameterized form of the electrostatic energy, which generalizes the known logarithmic singularity for conductors to dielectrics. Isolating this singular contribution allows for the calculation of exact cohesive energy for ensembles of dielectric particles in close contact. Explicit calculations are presented to illustrate how the contact singularity affects the cohesive energy of aggregates for a range of permittivity values.   

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The Boulder group recently showed that the liquid crystal RM734 possesses a ferroelectric nematic (N_F) phase [1]. In the search for additional ferroelectric nematic liquid crystals, we studied the liquid crystal DIO, which was previously reported to be ferroelectric-like [2]. In an antiparallel-rubbed liquid crystal cell, polarized light microscopy of DIO in the polar phase reveals two twisted states of the director and polarization field that have opposite handedness. These chiral states can be converted from one to the other with an applied electric field. This phenomenology is similar to that observed in RM734 in the same type of cell. The ferroelectric polarization of DIO obtained by analyzing the switching current generated during reversal of an in-plane field was found to be similar in magnitude to that of RM734. These observations confirm that the previously reported polar phase is indeed ferroelectric, as tentatively suggested by Kikuchi et al. [2].
[1] X. Chen et al. PNAS 2020
[2] H.Nishikawa et al. AdvMat 2017
Correspondence Author: joseph.maclennan@colorado.edu   

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The recently described ferroelectric nematic (N_F) phase [1] offers a variety of opportunities for novel electro-optic devices and effects. Its significant spontaneous polarization, as large as 6 µC/cm², enables field-induced nematic director reorientation in applied fields as small as ~1 V/cm. Here we report an N_F twist structure with two possible chiralities stabilized by polar anchoring at the cell surfaces that forms spontaneously in anti-parallel rubbed cells. The cells exhibit a polar electro-optic response, with applied voltage selecting between two twist states of the director field with opposite handedness. The director reorientation dynamics have been modeled using numerical simulations and the electrooptic response computed using Jones Matrix and Berreman formalism.
[1] “First-Principles Demonstration of Ferroelectricity in a Thermotropic Nematic Liquid Crystal: Spontaneous Polar Domain Formation and Spectacular Electro-Optics”, X. Chen et al. PNAS 2020.
* Author for Correspondence: noel.clark@colorado.edu   

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Clay minerals are natural 2D siblings of graphite stacks and graphene oxide nanolayers.
Very recently it was demonstrated that nanolayered clay mineral particle stacks can adsorb carbon dioxide with exceptionally high volumetric capacity. This is due to the very high effective interfacial area of clay mineral stacks, combined with targeted nanoscale functionalization of those interlayers
Clay minerals in aqueous suspension can delaminate and spontaneously form aligned 2D colloidal nematics. This talk will further discuss how nanoscale functionalization of exfoliated clay nanolayered colloids may lead to high-tech applications far beyond the traditional low-tech ones. This is particularly intereting given that natrual clay minerals are abundant, low-cost, non-toxic, environmentally friendly and sustainable 2D materails that in several contexts partly could replace or reduce the use of plastics.   

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Charged, chiral molecules are ubiquitous in biology. Prominent examples include many amino acids that constitute proteins and lipids that constitute cell membranes. Assemblies of such molecules are expected to be dictated by an interplay between electrostatic and chiral interactions. We combine in situ small-/wide-angle X-ray scattering (SAXS/WAXS), cryo-transmission electron microscopy (TEM) and atomic force microscopy (AFM) to analyze the self-assembly of charged chiral amphiphiles consisting of an amino acid (lysine) head group coupled to alkyl tails (C_n=12,14,16). By tuning the solution ionic conditions, we modulate the ionization tendency of the head groups as well as the range of electrostatic interactions. Our principal finding is that molecules assemble into helical ribbons or nanotubes when the electrostatic interactions are weak, but long ranged. By contrast, scroll-like cochleate are formed when the electrostatic interactions are short-ranged. The role of alkyl tail lengths that modulate the intermolecular van der Waals' interactions will also be discussed. Overall, our study shows how electrostatic interactions control the polymorphism in mesoscopic chiral shapes.   

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Polyelectrolyte (PE) brushes are charged polymers grafted on solid-liquid interfaces, which render special surface functionalities in many applications such as colloidal stabilization, wear protection in biological systems, and drug delivery. In this work, we introduce new scaling laws for the brush height H as a function of the grafting density σ, salt concentration c_s, and charge fraction φ, by employing a cell model to provide a detailed analysis of the electrostatic interactions between the charged monomers and mobile ions based on the Poisson-Boltzmann theory. Our predictions are consistent with the classical scaling theory for a charged “salted brush”, H~(σ/c_s)^1/3, within the Debye-Hückel limit and conditions of overlapping electrical double layers (EDLs) that only apply to weak PEs. Further, we obtain new scaling laws that are expected for both weak and strong PE brushes: 1) H~(σ/c_s)^0.11 beyond the Debye-Hückel limit which is applicable for low salt and high grafting density, and 2) H~(σ/c_s)^1/5 for non-overlapping EDLs, which is applicable for high salt and low grafting density. We also provide predictions and insights on the abrupt shrinkage of PE brushes in the presence of multivalent ions, which has been demonstrated in experiments.   

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We have shown previously that the second nematic phase (N_F) of the thermotropic liquid crystal RM374 is ferroelectric [1]. We have also recently confirmed ferroelectricity in DIO, a nematic previously reported to be ferroelectric-like [2]. Here we study the phase behavior, miscibility, and electro-optics of binary mixtures of RM734 and DIO. As reported in the literature, the N_F phase of RM734 ranges from 133°C to 70°C and the N_F phase of DIO from 68.8°C to 34°C. As expected, the N_F temperature range in the mixtures is extended to lower temperatures, with field-induced molecular reorientation in response to a 10 V/mm, in-plane applied field observed at temperatures as low as 28°C in a 50:50 mixture. The temperature range of the phase between the N and N_F phases in DIO is reduced in the mixtures and disappears at higher RM734 concentration.
[1] “First-Principles Demonstration of Ferroelectricity in a Thermotropic Nematic Liquid Crystal: Spontaneous Polar Domain Formation and Spectacular Electro-Optics”, X. Chen et al. PNAS 2020.
[2] “A Fluid Liquid-Crystal Material with Highly Polar Order”, H. Nishikawa et al. AdvMat 2017.
Author for Correspondence: noel.clark@colorado.edu   

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According to the dominance prescription of point-ions in the non-linear Poisson-Boltzmann theory, proposed by Valleau and Torrie more than 30 years ago, the equilibrium microscopic and thermodynamic properties of an asymmetric binary electrolyte converge asymptotically to those of a completely symmetric electrolyte, in the limit of an infinite colloidal surface charge density, if the properties of the counterions are the same in both instances. In order to determine if such a dominance can be observed dynamically, in this work we study the electrophoretic mobility of highly charged spherical colloids immersed in several 1:z electrolytes, when the counterions have the same properties. We observe that there is an inversion in the precedence order of the electrophoretic mobilities as a function of the surface charge density of the macroions. Moreover, our theoretical calculations based in the Wirsema, O'Brien and White theory suggest that, in the limit of an infinite colloidal charge density, the electrophoretic mobility does not converge to the same value for different valences of coions even if the monovalent counterions have the same properties.