Colloidal Rods Research Articles

2D phase behaviors of colloidal ellipsoids and rods

In recent years, the shape of particles has emerged as a key factor influencing their self-assembly and phase behaviors. Understanding the phase behaviors of systems containing shape anisotropic particles remains challenging. In this mini-review, we will summarize our recent experimental studies on the phase behaviors of colloidal systems in which all or part of particles have anisotropic shapes. We focus on two types of shape anisotropic particles, ellipsoids and rods. In particular, it was found that 1) in equilibrium, the anisotropic particles form a nematic phase in 2D, 2) when quenched, they can form a stable glassy state with a two-step glass transition, and 3) when they are dispersed as impurities in a 2D colloidal crystal of spheres, they can destroy the 2D crystal phase. We will discuss the current challenges in the field.

Machine-learning effective many-body potentials for anisotropic particles using orientation-dependent symmetry functions.

Spherically symmetric atom-centered descriptors of atomic environments have been widely used for constructing potential or free energy surfaces of atomistic and colloidal systems and to characterize local structures using machine learning techniques. However, when particle shapes are non-spherical, as in the case of rods and ellipsoids, standard spherically symmetric structure functions alone produce imprecise descriptions of local environments. In order to account for the effects of orientation, we introduce two- and three-body orientation-dependent particle-centered descriptors for systems composed of rod-like particles. To demonstrate the suitability of the proposed functions, we use an efficient feature selection scheme and simple linear regression to construct coarse-grained many-body interaction potentials for computationally efficient simulations of model systems consisting of colloidal particles with an anisotropic shape: mixtures of colloidal rods and non-adsorbing polymer coils, hard rods enclosed by an elastic microgel shell, and ligand-stabilized nanorods. We validate the machine-learning (ML) effective many-body potentials based on orientation-dependent symmetry functions by using them in direct coexistence simulations to map out the phase behavior of colloidal rods and non-adsorbing polymer coils. We find good agreement with the results obtained from simulations of the true binary mixture, demonstrating that the effective interactions are well described by the orientation-dependent ML potentials.

Group formation and collective motion of colloidal rods with an activity triggered by visual perception.

We investigate the formation of cohesive groups and the collective diffusion of colloidal spherocylinders with a motility driven by a simple visual perception model. For this, we perform Brownian dynamics simulations without hydrodynamic interactions. The visual perception is based on sight cones attached to the spherocylinders and perception functions quantifying the visual stimuli. If the perception function of a particle reaches a predefined threshold, an active component is added to its motion. We find that, in addition to the opening angle of the cone of sight, the aspect ratio of the particles plays an important role for the formation of cohesive groups. If the elongation of the particles is increased, the maximum angle for which the rods organize themselves into such groups decreases distinctly. After a system forms a cohesive group, it performs a diffusive motion, which can be quantified by an effective diffusion coefficient. For increasing aspect ratios, the spatial expansion of the cohesive groups and the effective diffusion coefficient of the collective motion increase, while the number of active group members decreases. We also find that a larger particle number, a smaller propulsion velocity of the group members, and a smaller threshold for the visual stimulus increase the maximum opening angle for which cohesive groups form. Based on our results, we expect anisotropic particles to be of great relevance for the adjustability of visual perception-dependent motility.

(Invited) One-Dimensionally Aligned Quantum Rods for Generation of Highly-Pure Circularly Polarized Light with High Light Intensity

Converting non-polarized visible light (natural light) into circular polarized (CP) light is currently an important issue because of the attractive properties of CP light such as acceleration of photosynthesis and enhancement of conversion efficiency of photovoltaics[1,2]. Until now, various methods for creating CP visible light, such as filtration by circular polarizer, selective reflection by chiral liquid crystal structures, and circular polarized luminescence (CPL) from chiral luminophores[3], have been reported. However, creation of highly-pure CP visible light with high light intensity is still considered as challenging issue. In this presentation, we propose an approach for creating highly-pure CP visible light with high light intensity by converting linearly polarized luminescence (LPL) using quarter-wave plate.In this work, we put spotlight on colloidal semiconductor quantum rods (QRs) because of its unique advantages described as follows: (1) Absorption coefficient and emission quantum yield are relatively high. (2) Sharp emission peaks are expected. (3) Variety of QRs with different emission colors can be excited by irradiation of single-wavelength light. We selected CdSe/CdS core/shell QRs as LPL generating 1D nanostructure. CdSe/CdS core/shell QRs were synthesized by hot-injection method. As shown in Figure 1a, the obtained suspension showed orange-colored strong emission under UV light irradiation. TEM image of the dried sample of the QRs toluene suspension indicates that the average length, average width of the obtained rod-like nanostructures were about 30 nm and 4.5 nm, respectively. After QRs/toluene suspension was mixed with poly(ethylene-co-vinyl acetate) (EVA)/toluene solution, the mixture was casted on glass slide and dried it under air. Then, the prepared composite polymer film was stretched in one direction and was attached on glass slide. When the luminescence from the QRs/EVA stretched film was observed under the linear polarizer placed in parallel direction to the stretching direction, the observed light was brighter than that of perpendicular direction (Figure 1c). By measuring the photoluminescence of parallel (I//) and perpendicular (I⊥) components under depolarized excitation at 450 nm, the QRs/EVA stretched film showed clear LPL property with constant ratio (CR) = 5.4, defined as CR = I// / I⊥ (Figure 1b). Furthermore, when the QRs/EVA stretched film was combined with a quarter-wave plate placed at 45° to the stretching direction and was observed through the right-handed (RH) or left-handed (LH) circular polarizer, brighter light was observed through LH circular polarizer (Figure 1d). By quantitative evaluation of RH- and LH-CP light components, total emission includes 82% of LH-CP light and 18% of RH-CP light (Figure 1e). These results indicate that the combination of the QRs/EVA stretched film and quarter-wave plate generate highly-pure CP light which can be distinguished by the naked eye (Figure 2d). Interestingly, the handedness of CP light can be switched by simply changing the angle between fast axis of the quarter-wave plate and stretching direction of LPL films (Figure 1f). In this presentation, we will also discuss about multiplexing of optical information by using CP light conversion.[1] W. Qin et. al., J. Phys. Chem., 2018, 122, 12566-12571. [2] B. Hu et. al., ACS Photonics, 2017, 4, 2821-2827. [3] P. Duan, M. Liu et. al., Adv. Mater. 2020, 32, 1900110. Figure 1

Alignment of Colloidal Rods in Crowded Environments.

Understanding the hydrodynamic alignment of colloidal rods in polymer solutions is pivotal for manufacturing structurally ordered materials. How polymer crowding influences the flow-induced alignment of suspended colloidal rods remains unclear when rods and polymers share similar length scales. We tackle this problem by analyzing the alignment of colloidal rods suspended in crowded polymer solutions and comparing that to the case where crowding is provided by additional colloidal rods in a pure solvent. We find that the polymer dynamics govern the onset of shear-induced alignment of colloidal rods suspended in polymer solutions, and the control parameter for the alignment of rods is the Weissenberg number, quantifying the elastic response of the polymer to an imposed flow. Moreover, we show that the increasing colloidal alignment with the shear rate follows a universal trend that is independent of the surrounding crowding environment. Our results indicate that colloidal rod alignment in polymer solutions can be predicted on the basis of the critical shear rate at which polymer coils are deformed by the flow, aiding the synthesis and design of anisotropic materials.

Underlying mechanism of shear-banding in soft glasses of charged colloidal rods with orientational domains

Soft glasses of colloidal rods (fd-virus particles) with orientational domains were recently shown to exhibit inhomogeneous flow profiles [Dhont et al., Phys. Rev. Fluids 2, 043301 (2017)]: fracture and accompanied plug flow at small shear rates, which transits to gradient shear-banding on increasing the shear rate, while a uniform flow profile develops at sufficiently high shear rates. These flow profiles coexist with Taylor-vorticity bands. The texture of such glasses under flow conditions consists of domains with varying orientations. The observed gradient shear-banding was solely attributed to the strong shear thinning behavior of the material inside the domains (henceforth abbreviated as domain-interior), without considering the texture stress that is due to interactions between the glassy domains. Here, we present new experiments on the shear-banding transition to assess the role played by the texture stress in comparison to the domain-interior stress. For a large concentration, well into the glassy state, it is found that both texture stress and domain-interior stress contribute significantly to the gradient shear-banding transition in the shear-rate region where it occurs. On the other hand, for a small concentration close to the glass-transition concentration, the domains are shown to coalesce within the shear-rate range where gradient shear-banding is observed. As a result, the texture stress diminishes and the domain-interior stress increases upon coalescence, leading to a stress plateau. Thus, a subtle interplay exists between the stresses arising from the structural order on two widely separated length scales from interactions between domains and from the rod-rod interactions within the domain-interior for both concentrations.

Coupling between splay deformations and density modulations in splay-bend phases of bent colloidal rods.

Using a grand-canonical Landau-de Gennes theory for colloidal suspensions of bent (banana-shaped) rods, we investigate how spatial deformations in the nematic director field affect the local density of twist-bend and splay-bend nematic phases. The grand-canonical character of the theory naturally relates the local density to the local nematic order parameter S. In the splay-bend phase, we find S and hence the local density to modulate periodically along one spatial direction. As a consequence the splay-bend phase has the key symmetries of a smectic rather than a nematic phase. By contrast we find that S and hence the local density do not vary in space in the twist-bend phase, which is therefore a proper nematic phase. The theoretically predicted one-dimensional density modulations in splay-bend phases are in agreement with recent simulations.

Effects of polymer nonideality on depletion-induced phase behaviour of colloidal disks and rods

Colloidal dispersions composed of either platelets or rods exhibit liquid crystalline phase behaviour that is strongly influenced by the addition of nonadsorbing polymers. In this work we examined how polymer segment–segment interactions affect this phase behaviour as compared to using either penetrable hard spheres (PHS) or ideal (‘ghost’) chains as depletants. We find that the simplified polymer description predicts the same phase diagram topologies as the more involved polymer descriptions. Therefore the PHS description is still adequate for qualitative predictions. For sufficiently large polymer sizes we find however that the precise polymer description significantly alters the locations of the phase coexistence regions. Especially the stability region of isotropic–isotropic coexistence is affected by the polymer interactions. To illustrate the quantitative effects some examples are presented.

Long-time relaxation dynamics in nematic and smectic liquid crystals of soft repulsive colloidal rods.

Understanding the relaxation dynamics of colloidal suspensions is crucial to identifying the elements that influence the mobility of their constituents, assessing their macroscopic response across the relevant time and length scales, and thus disclosing the fundamentals underpinning their exploitation in formulation engineering. In this work, we specifically assess the impact of long-ranged ordering on the relaxation dynamics of suspensions of soft repulsive rodlike particles, which are able to self-organize into nematic and smectic liquid-crystalline phases. Rods are modeled as soft repulsive spherocylinders with a length-to-diameter ratio L^{★}=5, interacting via the truncated and shifted Kihara potential. By performing dynamic Monte Carlo simulations, we analyze the effect of translational and orientational order on the diffusion of the rods along the relevant directions imposed by the morphology of the background phases. To provide a clear picture of the resulting dynamics, we assess its dependence on temperature, which can dramatically determine the response time of the system relaxation and the self-diffusion coefficients of the rods. The computation of the van Hove correlation functions allows us to identify the existence of rods that diffuse significantly faster than the average and whose concentration can be accurately adjusted by a suitable choice of temperature.

Understanding enhanced rotational dynamics of active probes in rod suspensions.

Active Brownian particles (APs) have recently been shown to exhibit enhanced rotational diffusion (ERD) in complex fluids. Here, we experimentally observe ERD and numerically corroborate its microscopic origin for a quasi-two-dimensional suspension of colloidal rods. At high density, the rods form small rafts, wherein they perform small-amplitude, high-frequency longitudinal displacements. Activity couples AP-rod contacts to reorientation, with the variance therein leading to ERD. This is captured by a local, rather than a global relaxation time, as used in previous phenomenological modeling. Our result should prove relevant to the microrheological characterization of complex fluids and furthering our understanding of the dynamics of microorganisms in such media.

Gelation phase diagrams of colloidal rod systems measured over a large composition space

Rheological modifiers tune product rheology with a small amount of material. To effectively use rheological modifiers, characterizing the rheology of the system at different compositions is crucial. Two colloidal rod system, hydrogenated castor oil and polyamide, are characterized in a formulation that includes a surfactant (linear alkylbenzene sulfonate) and a depletant (polyethylene oxide). We characterize both rod systems using multiple particle tracking microrheology (MPT) and bulk rheology and build phase diagrams over a large component composition space. In MPT, fluorescent particles are embedded in the sample and their Brownian motion is measured and related to rheological properties. From MPT, we determine that in both systems: (1) microstructure is not changed with increasing colloid concentration, (2) materials undergo a sol–gel transition as depletant concentration increases and (3) the microstructure changes but does not undergo a phase transition as surfactant concentration increases in the absence of depletant. When comparing MPT and bulk rheology results different trends are measured. Using bulk rheology we observe: (1) elasticity of both systems increase as colloid concentration increases and (2) the storage modulus does not change when PEO or LAS concentration is increased. The differences measured with MPT and bulk rheology are likely due to differences in sensitivity and measurement method. This work shows the utility of using both techniques together to fully characterize rheological properties over a large composition space. These gelation phase diagrams will provide a guide to determine the composition needed for desired rheological properties and eliminate trial-and-error experiments during product formulation.

Quasi-solid-state self-assembly of 1D-branched ZnSe/ZnS quantum rods into parallel monorail-like continuous films for solar devices

Translating the extraordinary optoelectric properties of colloidal quantum rods (QRs) into functional devices requires multiscale structural control to preserve the nanoscale attributes as well as to introduce micro- and macroscale interactions between the building blocks. Self-assembly of anisotropic QRs into ordered nanostructures can tailor the photoelectric properties of the QRs, such as in light absorption, and charge separation and transfer. However, it remains a challenge to assemble anisotropic nanomaterial into centimeter-sized, multilayered continuous films that retain nanoscale properties in the fabricated macroscopic devices. We have developed a quasi-solid-state self-assembly of randomly oriented nanostructures for overcoming this challenge, demonstrated by the re-assembly of randomly packed ZnSe/ZnS QRs into aligned and ordered parallel monorails (PMs). These ZnSe/ZnS PMs show significant enhancement in photo-excited charge transport, boosting photocatalytic oxygen evolution rates and the enhancement of photoelectrochemical activities, with a photocurrent density of 18 μA/cm 2 , 5 times higher than the parent random packing of ZnSe/ZnS QRs. The ZnSe/ZnS PMs enrich the p-n heterojunctions, which can modulate charge carrier separation and transport at the interfaces. The new method has applicability for re-assembling randomly packed films of anisotropic nanoparticles into ordered nanostructures. Importantly, the extraordinary photoelectro-energy conversion behavior of the Type-I core/shell quantum materials illuminates the pathways for novel designed materials by tailoring the hierarchical structures at all scales. • The quasi-solid-state self-assembly of randomly oriented 1-D nanostructures to close packed paralleled monorail alignment. • A general fabrication strategy for designing macroscopic solar devices at all scales. • A ‘electron-leaky’ Type-I core/shell quantum rods with significantly enhanced photoelectrochemical activities.

Multiphase Coexistences in Rod-Polymer Mixtures.

Using recently derived analytical equations of state for hard rod dispersions, we predict the phase behavior of athermal rod–polymer mixtures with free volume theory. The rods are modeled as hard spherocylinders, while the nonadsorbing polymer chains are described as penetrable hard spheres. It is demonstrated that all of the different types of phase states that are stable for pure colloidal rod dispersions can coexist with any combination of these phases if polymers are added, depending on the concentrations, rod aspect ratio, and polymer–rod size ratio. This includes novel two-, three-, and four-phase coexistences and isostructural coexistences between dilute and concentrated phases of the same kind, even for the more ordered (liquid) crystal phases. This work provides insight into the conditions at which particular multiphase coexistences are expected for well-defined model colloidal rod–polymer mixtures. We provide a quantitative map detailing the various types of isostructural coexistences, which confirms an early qualitative hypothesis by Bolhuis et al. (J. Chem. Phys.107, 19971551).

Rheological properties of phase transitions in polydisperse and monodisperse colloidal rod systems

AbstractRheological modifiers are added to formulations to tune rheology, enable function and drive phase changes requiring an understanding of material structure and properties. We characterize two colloidal rod systems during phase transitions using multiple particle tracking microrheology, which measures the Brownian motion of probes embedded in a sample. These systems include a colloid (monodisperse polyamide or polydisperse hydrogenated castor oil), surfactant (linear alkylbenzene sulfonate [LAS]), and nonabsorbing polymer (polyethylene oxide [PEO]) which drives gelation by depletion interactions. Phase transitions are characterized at all concentrations using time‐cure superposition. We determine that rheological evolution depends onLAS:colloid. The critical PEO concentration required to form a gel,cc/c*, is independent ofLAS:colloid, critical relaxation exponent,n, is dependent onLAS:colloid, and both are independent of colloid polydispersity.nindicates the material structure at the phase transition. AtLAS:colloid > 16, the scaffold is a tightly associated network and atLAS:colloid = 16 a loosely associated network.

Numerical Studies on Electrostatic Interaction Forces and the Free Energy between Parallel Colloidal Rods of Finite Size in Skewed Configurations.

Formulas for interaction forces F(s) and the free energy G(s) between two parallel charged prismatic rods of various scaled values of d, ψs, and L in skewed configurations are obtained, where s is the lengthwise positional difference between the front-end faces of the respective rods, and d is the minimal distance between the opposing faces of the rods, ψs is the electric surface potential, L is the length of the rods. To obtain the free-energy function G(s), (i) 3D spatial distributions of the electric potential ψ around two rods were determined by numerically solving the nonlinear Poisson-Boltzmann equation with a finite element method, (ii) with the ψ distributions so determined, the lengthwise interaction electrostatic Maxwell stress tangential to the midplane between the rods was calculated to obtain the (discrete) s dependence of the stress, and (iii) by introducing two different fitting functions, the discrete s dependence was transformed into a continuous force function, F(s), which was then lengthwise integrated to derive G(s). It was found that the curves of G(s) linearly decreased with increasing s between 1 and L + 1 due to a localization of the stress. Although natural, it is of interest that the values of G(0) calculated for rods of various values of d, ψs, and L were in good agreement with those of the interaction free energy obtained in our preceding work by the widthwise integration of repulsive electrostatic forces normal to the midplane between the parallel rods in nonskewed configurations.

Nematoelasticity of hybrid molecular-colloidal liquid crystals.

Colloidal rods immersed in a thermotropic liquid-crystalline solvent are at the basis of so-called hybrid liquid crystals, which are characterized by tunable nematic fluidity with symmetries ranging from conventional uniaxial nematic or antinematic to orthorhombic [Mundoor et al., Science 360, 768 (2018)SCIEAS0036-807510.1126/science.aap9359]. We provide a theoretical analysis of the elastic moduli of such systems by considering interactions between the individual rods with the embedding solvent through surface-anchoring forces, as well as steric and electrostatic interactions between the rods themselves. For uniaxial systems, the presence of colloidal rods generates a marked increase of the splay elasticity, which we found to be in quantitative agreement with experimental measurements. For orthorhombic hybrid liquid crystals, we provide estimates of all 12 elastic moduli and show that only a small subset of those elastic constants play a relevant role in describing the nematoelastic properties. The complexity and possibilities related to identifying the elastic moduli in experiments are briefly discussed.

Crystal nucleation in colloidal rod suspensions: The effect of depletant size.

In order to better control the assembly of nanorods, knowledge of the pathways by which they form ordered structures is desirable. In this paper, we characterize crystal nucleation in suspensions of spherocylindrical rods with aspect ratio L/D = 2.3 in the presence of both small and large polymer depletants. Using a combination of Langevin dynamics and Monte Carlo simulations, together with biased sampling techniques, we show that the preferred pathway always involves the formation of monolayer assemblies irrespective of the volume fraction of the initial isotropic phase and the diameter of the depletants. This includes the previously neglected case of nucleation from the colloidal liquid phase and shows that the presence of depletion attraction can alter nucleation pathways even when the initial phase is dense.

Are Langmuir Trough Studies Useful? Unexpected Emulsification Behavior Using Colloidal Rods.

While studies carried out in a Langmuir trough have rigorously demonstrated that, at high surface pressure, ellipsoidal particles do flip and spherocylinders (rods) can flip, much less is known about the practical situation on the surface of a droplet or bubble. We present emulsification studies using colloidal rods and find that the droplets are bridged by the rods independent of shear rate and particle concentration and are only weakly dependent on the pH of the continuous phase. In a trough, it is the low aspect ratio rods which flip and the high aspect ratio rods which form bilayers; on the surface of a droplet we found that the high aspect ratio rods always bridge whereas the shorter rods show random bridging behavior. Hence, the behavior of anisotropic particles "in action" is essentially opposite to expectations from trough studies.

Phase stability of colloidal mixtures of spheres and rods.

We determined the phase boundaries of aqueous mixtures containing colloidal rod-like fd-viruses and polystyrene spheres using diffusing-wave spectroscopy and compared the results with free volume theory predictions. Excluded volume interactions in mixtures of colloidal rods and spheres lead to mediated depletion interactions. The strength and range of this attractive interaction depend on the concentrations of the particles, the length L and diameter D of the rods, and the radius R of the spheres. At strong enough attraction, this depletion interaction leads to phase separation. We experimentally determined the rod and sphere concentrations where these phase transitions occur by systematically varying the size ratios L/R and D/R and the aspect ratio L/D. This was done by using spheres with different radii and modifying the effective diameter of the rods through either the ionic strength of the buffer or anchoring a polymeric brush to the surface of the rods. The observed phase transitions were from a binary fluid to a colloidal gas/liquid phase coexistence that occurred already at very low concentrations due to the depletion efficiency of highly anisotropic rods. The experimentally measured phase transitions were compared to phase boundaries obtained using free volume theory (FVT), a well established theory for calculating the phase behavior of colloidal particles mixed with depletants. We find good correspondence between the experimental phase transitions and the theoretical FVT model where the excluded volume of the rod-like depletants was explicitly accounted for in both the reservoir and the system.

65‐5: Improved Brightness and Efficiency of Green Quantum‐Rod‐Based Light‐Emitting Diodes

Semiconductor colloidal quantum rods (QRs) are evolving as potential candidates for future lighting and display applications due to numerous advantages. QRs offer various aids over quantum dots (QDs) in terms of polarized emission, large global Stokes shift, and double the outcoupling efficiency. In this paper, we synthesized CdSe/CdS/ZnS (core/shell/shell) green QRs with an emission wavelength at 520 nm and employed them in QR based light‐emitting diodes (QRLEDs). The QRLED device performance was further optimized by inserting a thin poly (methyl methacrylate) (PMMA) layer in‐between emissive layer and electron transport layer (ETL) (ZnO NP). The PMMA layer is acting as a barrier between the emissive layers to the ETL interface, and thus limit the excess electron injection into the QRs and helps to maintain the balanced charge injection. The fabricated green QRLEDs exhibit excellent optoelectronic performance, such as high brightness of 11890 cd/m2 and an impressive peak external quantum efficiency (EQE) of 5.6 %. Through further tuning the device with a PMMA, we elevated the peak‐EQE up to 6.8%. To the best of our knowledge, these are the best‐reported numbers for the QR based green light‐emitting diodes.