Interplay of self-organization of microtubule asters and crosslinking protein condensates.

The cytoskeleton is a major focus of physical studies to understand organization inside cells given its primary role in cell motility, cell division, and cell mechanics. Recently, protein condensation has been shown to be another major intracellular organizational strategy. Here, we report that the microtubule crosslinking proteins, MAP65-1 and PRC1, can form phase separated condensates at physiological salt and temperature without additional crowding agents in vitro. The size of the droplets depends on the concentration of protein. MAP65 condensates are liquid at first and can gelate over time. We show that these condensates can nucleate and grow microtubule bundles that form asters, regardless of the viscoelasticity of the condensate. The droplet size directly controls the number of projections in the microtubule asters, demonstrating that the MAP65 concentration can control the organization of microtubules. When gel-like droplets nucleate and grow asters from a shell of tubulin at the surface, the microtubules are able to re-fluidize the MAP65 condensate, returning the MAP65 molecules to solution. This work implies that there is an interplay between condensate formation from microtubule-associated proteins, microtubule organization, and condensate dissolution that could be important for the dynamics of intracellular organization.

Optical-Tweezers-integrating-Differential-Dynamic-Microscopy maps the spatiotemporal propagation of nonlinear strains in polymer blends and composites.

How local stresses propagate through polymeric fluids, and, more generally, how macromolecular dynamics give rise to viscoelasticity are open questions vital to wide-ranging scientific and industrial fields. Here, to unambiguously connect polymer dynamics to force response, and map the deformation fields that arise in macromolecular materials, we present Optical-Tweezers-integrating-Differential -Dynamic-Microscopy (OpTiDMM) that simultaneously imposes local strains, measures resistive forces, and analyzes the motion of the surrounding polymers. Our measurements with blends of ring and linear polymers (DNA) and their composites with stiff polymers (microtubules) uncover an unexpected resonant response, in which strain alignment, superdiffusivity, and elasticity are maximized when the strain rate is comparable to the entanglement rate. Microtubules suppress this resonance, while substantially increasing elastic storage, due to varying degrees to which the polymers buildup, stretch and flow along the strain path, and configurationally relax induced stress. More broadly, the rich multi-scale coupling of mechanics and dynamics afforded by OpTiDDM, empowers its interdisciplinary use to elucidate non-trivial phenomena that sculpt stress propagation dynamics-critical to commercial applications and cell mechanics alike.

Self-shaping liquid crystal droplets by balancing bulk elasticity and interfacial tension.

The shape diversity and controlled reconfigurability of closed surfaces and filamentous structures, universally found in cellular colonies and living tissues, are challenging to reproduce. Here, we demonstrate a method for the self-shaping of liquid crystal (LC) droplets into anisotropic and three-dimensional superstructures, such as LC fibers, LC helices, and differently shaped LC vesicles. The method is based on two surfactants: one dissolved in the LC dispersed phase and the other in the aqueous continuous phase. We use thermal stimuli to tune the bulk LC elasticity and interfacial energy, thereby transforming an emulsion of polydispersed, spherical nematic droplets into numerous, uniform-diameter fibers with multiple branches and vice versa. Furthermore, when the nematic LC is cooled to the smectic-A LC phase, we produce monodispersed microdroplets with a tunable diameter dictated by the cooling rate. Utilizing this temperature-controlled self-shaping of LCs, we demonstrate life-like smectic LC vesicle structures analogous to the biomembranes in living systems. Our experimental findings are supported by a theoretical model of equilibrium interface shapes. The shape transformation is induced by negative interfacial energy, which promotes a spontaneous increase of the interfacial area at a fixed LC volume. The method was successfully applied to many different LC materials and phases, demonstrating a universal mechanism for shape transformation in complex fluids.

Motor-Driven Restructuring of Cytoskeleton Composites Leads to Tunable Time-Varying Elasticity.

The composite cytoskeleton, comprising interacting networks of semiflexible actin and rigid microtubules, generates forces and restructures by using motor proteins such as myosins to enable key processes including cell motility and mitosis. Yet, how motor-driven activity alters the mechanics of cytoskeleton composites remains an open challenge. Here, we perform optical tweezers microrheology and confocal imaging of composites with varying actin-tubulin molar percentages (25-75, 50-50, and 75-25), driven by light-activated myosin II motors, to show that motor activity increases the elastic plateau modulus by over 2 orders of magnitude by active restructuring of both actin and microtubules that persists for hours after motor activation has ceased. Nonlinear microrheology measurements show that motor-driven restructuring increases the force response and stiffness and suppresses actin bending. The 50-50 composite exhibits the most dramatic mechanical response to motor activity due to the synergistic effects of added stiffness from the microtubules and sufficient motor substrate for pronounced activity.

DNA Conformation Dictates Strength and Flocculation in DNA-Microtubule Composites.

Polymer topology has been shown to play a key role in tuning the dynamics of complex fluids and gels. At the same time, polymer composites, ubiquitous in everyday life, have been shown to exhibit emergent desirable mechanical properties not attainable in single-species systems. Yet, how topology impacts the dynamics and structure of polymer composites remains poorly understood. Here, we create composites of rigid rods (microtubules) polymerized within entangled solutions of flexible linear and ring polymers (DNA) of equal length. We couple optical tweezers microrheology with confocal microscopy and scaled particle theory to show that composites with linear DNA exhibit a strongly nonmonotonic dependence of elasticity and stiffness on microtubule concentration due to depletion-driven polymerization and flocculation of microtubules. In contrast, composites containing ring DNA show a much more modest monotonic increase in elastic strength with microtubule concentration, which we demonstrate arises from the decreased conformational size and increased miscibility of rings.

Amphiphilic Comb Polymers as New Additives in Bicontinuous Microemulsions.

It has been shown that the thermodynamics of bicontinuous microemulsions can be tailored via the addition of various different amphiphilic polymers. In this manuscript, we now focus on comb-type polymers consisting of hydrophobic backbones and hydrophilic side chains. The distinct philicity of the backbone and side chains leads to a well-defined segregation into the oil and water domains respectively, as confirmed by contrast variation small-angle neutron scattering experiments. This polymer-microemulsion structure leads to well-described conformational entropies of the polymer fragments (backbone and side chains) that exert pressure on the membrane, which influences the thermodynamics of the overall microemulsion. In the context of the different polymer architectures that have been studied by our group with regards to their phase diagrams and small-angle neutron scattering, the microemulsion thermodynamics of comb polymers can be described in terms of a superposition of the backbone and side chain fragments. The denser or longer the side chain, the stronger the grafting and the more visible the brush effect of the side chains becomes. Possible applications of the comb polymers as switchable additives are discussed. Finally, a balanced philicity of polymers also motivates transmembrane migration in biological systems of the polymers themselves or of polymer-DNA complexes.

Toward Bioinspired Wet Adhesives: Lessons from Assessing Surface Structures of the Suction Disc of Intertidal Clingfish.

The clingfish attaches to rough surfaces with considerable strength using an intricate suction disc, which displays complex surface geometries from structures called papillae. However, the exact role of these structures in adhesion is poorly understood. To investigate the relationship between papillae geometry and adhesive performance, we developed an image processing tool that analyzed the surface and structural complexity of papillae, which we then used to model hydrodynamic adhesion. Our tool allowed for the automated analysis of thousands of papillae in specimens across a range of body sizes. The results led us to identify spatial trends in papillae across the complex geometry of the suction disc and to establish fundamental structure-function relationships used in hydrodynamic adhesion. We found that the surface area of papillae changed within a suction disc and with fish size, but that the aspect ratios and channel width between papillae did not. Using a mathematical model, we found that the surface structures can adhere considerably when subjected to disturbances of moderate to high velocities. We concluded that a predominant role of the papillae is to leverage hydrodynamic adhesion and wet friction to reinforce the seal of the suction disc. Overall, the trends in papillae characteristics provided insights into bioinspired designs of surface microstructures for future applications in which adhesion is necessary to attach to diverse surfaces (in terrestrial or aquatic environments), even when subjected to disturbance forces of randomized directionality.

Unexpected entanglement dynamics in semidilute blends of supercoiled and ring DNA.

Blends of polymers of different topologies, such as ring and supercoiled, naturally occur in biology and often exhibit emergent viscoelastic properties coveted in industry. However, due to their complexity, along with the difficulty of producing polymers of different topologies, the dynamics of topological polymer blends remains poorly understood. We address this void by using both passive and active microrheology to characterize the linear and nonlinear rheological properties of blends of relaxed circular and supercoiled DNA. We characterize the dynamics as we vary the concentration from below the overlap concentration c* to above (0.5c* to 2c*). Surprisingly, despite working at the dilute-semidilute crossover, entanglement dynamics, such as elastic plateaus and multiple relaxation modes, emerge. Finally, blends exhibit an unexpected sustained elastic response to nonlinear strains not previously observed even in well-entangled linear polymer solutions.

Filament Rigidity Vies with Mesh Size in Determining Anomalous Diffusion in Cytoskeleton.

The diffusion of microscopic particles through the cell, important to processes such as viral infection, gene delivery, and vesicle transport, is largely controlled by the complex cytoskeletal network, comprised of semiflexible actin filaments and rigid microtubules, that pervades the cytoplasm. By varying the relative concentrations of actin and microtubules, the cytoskeleton can display a host of different structural and dynamic properties that, in turn, impact the diffusion of particles through the composite network. Here, we couple single-particle tracking with differential dynamic microscopy to characterize the transport of microsphere tracers diffusing through composite in vitro networks with varying ratios of actin and microtubules. We analyze multiple complementary metrics for anomalous transport to show that particles exhibit anomalous subdiffusion in all networks, which our data suggest arises from caging by networks. Further, subdiffusive characteristics are markedly more pronounced in actin-rich networks, which exhibit similarly more prominent viscoelastic properties compared to microtubule-rich composites. While the smaller mesh size of actin-rich composites compared to microtubule-rich composites plays an important role in these results, the rigidity of the filaments comprising the network also influences the anomalous characteristics that we observe. Our results suggest that as microtubules in our composites are replaced with actin filaments, the decreasing filament rigidity competes with increasing network connectivity to drive anomalous transport.

Synergistic Interactions Between DNA and Actin Trigger Emergent Viscoelastic Behavior.

Composites of flexible and rigid polymers are ubiquitous in biology and industry alike, yet the physical principles determining their mechanical properties are far from understood. Here, we couple force spectroscopy with large-scale Brownian dynamics simulations to elucidate the unique viscoelastic properties of custom-engineered blends of entangled flexible DNA molecules and semiflexible actin filaments. We show that composites exhibit enhanced stress stiffening and prolonged mechanomemory compared to systems of actin or DNA alone, and that these nonlinear features display a surprising nonmonotonic dependence on the fraction of actin in the composite. Simulations reveal that these counterintuitive results arise from synergistic microscale interactions between the two biopolymers. Namely, DNA entropically drives actin filaments to form bundles that stiffen the network but reduce the entanglement density, while a uniform well-connected actin network is required to reinforce the DNA network against yielding and flow. The competition between bundling and connectivity triggers an unexpected stress response that leads equal mass DNA-actin composites to exhibit the most pronounced stress stiffening and the most long-lived entanglements.

Gelation Kinetics and Network Structure of Cellulose Nanocrystals in Aqueous Solution.

Cellulose nanocrystals (CNC) are rod-like biosourced nanoparticles that are widely used in a range of applications. Charged CNC was obtained by acid extraction from cotton and dispersed in aqueous solution using ultrasound and characterized by light scattering. Aggregation and gelation of CNC induced by addition of NaCl was investigated by light scattering as a function of the NaCl concentration (30-70 mM), the CNC concentration (0.5-5 g/L), and the temperature (10-60 °C). Formation of fractal aggregates was observed that grow with time until they percolate and form a weak system spanning network. The aggregation rate and gel time were found to decrease very steeply with increasing NaCl concentration and more weakly with increasing CNC concentration. A decrease of the gel time was also observed with increasing temperature for T > 20 °C. The structure of the CNC networks was studied using confocal laser scanning microscopy and light scattering. The local structure of the networks was fractal and reflected that of the constituting aggregates. The gels were homogeneous on length scales larger than the correlation length, which decreased with increasing CNC concentration. The CNC gels flowed when tilted for C < 12 g/L and sedimentation was observed macroscopically for C < 4 g/L due to the collapse of the CNC network under gravity. The speed and extent of sedimentation was investigated as a function of the ionic strength and the CNC concentration. Gelled CNC could be completely redispersed by applying ultrasound.

Stabilization of Water-in-Water Emulsions by Nanorods.

Water-in-water (W/W) emulsions formed by mixing incompatible water-soluble polymers cannot be stabilized with molecular surfactants. However, they can be stabilized by particles through the so-called Pickering effect. Recently, it was shown that its stabilization can be achieved also with nanoplates. Here, we show for the first time that even nanorods in the form of cellulose nanocrystals (CNCs) can efficiently stabilize W/W emulsions. Static light scattering and confocal microscopy techniques were used to determine the surface coverage by CNCs. In the presence of 50 mM NaCl very weak gels were formed by excess CNCs in the continuous phase. In this way creaming of the dispersed phase could be arrested. The nontoxicity, sustainability, and low cost of CNCs and the abundant availability of cellulose render these nanorods potentially highly suited for preparing W/W emulsions.

Myelin structures formed by thermotropic smectic liquid crystals.

We report on transient structures, formed by thermotropic smectic-A liquid crystals, resembling the myelin figures of lyotropic lamellar liquid crystals. The thermotropic myelin structures form during the solubilization of a smectic-A droplet in an aqueous phase containing a cationic surfactant at concentrations above the critical micelle concentration. Similar to the lyotropic myelin figures, the thermotropic myelins appear in an optical microscope as flexible tubelike structures growing at the smectic/aqueous interface. Polarizing microscopy and confocal fluorescence microscopy show that the smectic layers are parallel to the tube surface and form a cylindrically bent arrangement around a central line defect in the tube. We study the growth behavior of this new type of myelins and discuss similarities to and differences from the classical lyotropic myelin figures.

Lasing and waveguiding in smectic A liquid crystal optical fibers.

We demonstrate a new class of soft matter optical fibers, which are self-assembled in a form of smectic-A liquid crystal microtubes grown in an aqueous surfactant dispersion of a smectic-A liquid crystal. The diameter of the fibers is highly uniform and the fibers are highly birefringent. They are characterized by a line topological defect in the core of the fiber with an optical axis pointing from the defect core towards the surface. We demonstrate guiding of light along the fiber and Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber. The light guiding as well as the lasing threshold are significantly dependent on the polarization of the excitation beam. The observed threshold for WGM lasing is very low (≈ 75µJ/cm(2)) when the pump beam polarization is perpendicular to the direction of the laser dye alignment and is similar to the lasing threshold in nematic droplets. The smectic-A fibers are soft and flexible and can be manipulated with laser tweezers demonstrating a promising approach for realization of soft photonic circuits.

Solubilization of thermotropic liquid crystal compounds in aqueous surfactant solutions.

We study the micellar solubilization of three thermotropic liquid crystal compounds by immersing single drops in aqueous solutions of the ionic surfactant tetradecyltrimethylammonium bromide. For both nematic and isotropic drops, we observe a linear decrease of the drop size with time as well as convective flows and self-propelled motions. The solubilization is accompanied by the appearance of small aqueous droplets within the nematic or isotropic drop. At low temperatures, nematic drops expell small nematic droplets into the aqueous environment. Smectic drops show the spontaneous formation of filament-like structures which resemble the myelin figures observed in lyotropic lamellar systems. In all cases, the liquid crystal drops become completely solubilized, provided the weight fraction of the liquid crystal in the system is not larger than a few percent. The solubilization of the liquid crystal drops is compared with earlier studies of the solubilization of alkanes in ionic surfactant solutions.