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Bipolar membranes (BPMs) possess the potential to optimize pH environments for electrochemical synthesis applications when employed in reverse bias. Unfortunately, the performance of BPMs in revers...

The development and characterization of active and selective catalysts is critical for the simulation and optimization of electrochemical synthesis of chemicals and fuels using renewable energy. The rate of electrochemical generation of a specific product as a function of electrode potential can be described by a Tafel equation, which depends on two parameters: the Tafel slope (or the related transfer coefficient) and the exchange current density. However, common methods for calculating Tafel slopes are subjective and limited by data insufficiency resulting from challenges associated with product quantification, and, as shown here, the effects of mass transport, bulk reaction occurring in the mass-transfer boundary layer, and the occurrence of competitive surface reactions. Errors in the Tafel slope extracted from experimental data can also lead to errors in the exchange current density estimation. To address these issues, we present a technique that leverages statistical learning methods informed by physics-based modeling to calculate kinetic parameters (the transfer coefficient and exchange current density) with quantified uncertainty. The method is applied to 21 sets of data for the electrochemical reduction of CO2 to CO and H2 on Ag catalysts acquired under similar experimental conditions. We find that fitted values for the transfer coefficient and exchange current density do not converge to a unique set of values, and that there is an apparent correlation of these parameters; however, the most probable value of the exchange coefficient for CO and H2 formation correspond reasonably well with the DFT-predicted values of this parameter. While the system explored is relatively simple, the techniques developed can be used to evaluate the transfer coefficient and exchange current density for many other electrochemical processes.

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.

Ring polymers are a unique class of macromolecules that lack free ends and show qualitatively different dynamics compared to linear chains. Despite recent progress, the nonequilibrium flow behavior...

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.

Over the past few decades, microrheology has emerged as a widely used technique to measure the mechanical properties of soft viscoelastic materials. Optical tweezers offer a powerful platform for performing microrheology measurements and can measure rheological properties at the level of single molecules out to near macroscopic scales. Unlike passive microrheology methods, which use diffusing microspheres to extract rheological properties, optical tweezers can probe the nonlinear viscoelastic response, and measure the space- and time-dependent rheological properties of heterogeneous, nonequilibrium materials. In this Viewpoint, I describe the basic principles underlying optical tweezers microrheology, the instrumentation and material requirements, and key applications to widely studied soft biological materials. I also describe several sophisticated approaches that include coupling optical tweezers to fluorescence microscopy and microfluidics. The described techniques can robustly characterize noncontinuum mechanics, nonlinear mechanical responses, strain-field heterogeneities, stress propagation, force relaxation dynamics, and time-dependent mechanics of active materials.

We optically drive a microsphere at constant speed through entangled actin networks of 0.2 - 1.4 mg/ml at rates faster than the critical rate controlling the onset of a nonlinear response. By measuring the resistive force exerted on the microsphere during and following strain we reveal a critical concentration $c^{*}\simeq0.4$ mg/ml for nonlinear features to emerge. For $c>c^{*}$, entangled actin stiffens at short times with the degree of stiffening $S$ and corresponding timescale $t_{stiff}$ scaling with the entanglement tube density, i.e. $S\sim t_{stiff}\sim d_t^{-1}\sim c^{3/5}$. The network subsequently yields to a viscous regime with the yield distance $d_y$ scaling linearly with yield force $f_y$ and inversely with the entanglement length ($f_y\sim d_y\sim l_{e}^{-1} \sim c^{2/5}$). Stiffening and yielding dynamics are consistent with recent theoretical predictions for nonlinear cohesive breakdown of entanglements. We further show that above $c^{*}$ force relaxation proceeds via slow filament disengagement from dilated tubes coupled with $\sim$10x faster lateral hopping, with the corresponding concentration dependences in agreement with recent theoretical predictions for entangled rigid rods.

We track the deformation of discrete entangled actin segments while simultaneously measuring the resistive force the deformed filaments exert in response to an optically driven microsphere. We precisely map the network deformation field to show that local microscale stresses can induce filament deformations that propagate beyond mesoscopic length scales (60 μm, >3 persistence lengths lp). We show that the filament persistence length controls the critical length scale at which distinct entanglement deformations become driven by collective network mechanics. Mesoscale propagation beyond lp is coupled with nonlinear local stresses arising from steric entanglements mimicking cross-links.

Chain topology has a profound impact on the flow behavior of single macromolecules. For circular polymers, the absence of free ends results in a unique chain architecture compared to linear or branched chains, thereby generating distinct molecular dynamics. Here, we report the direct observation of circular DNA dynamics in transient and steady flows for molecular sizes spanning the range of 25.0–114.8 kilobase pairs (kbp). Our results show that the longest relaxation times of the rings follow a power-law scaling relation with molecular weight that differs from that of linear chains. Also, relative to their linear counterparts, circular DNA molecules show a shifted coil-to-stretch transition and less diverse “molecular individualism” behavior as evidenced by their conformational stretching pathways. These results show the impact of chain topology on dynamics and reveal commonalities in the steady state behavior of circular and linear DNA that extends beyond chain architecture.

Microrheology has emerged as a powerful approach for elucidating mechanical properties of soft materials and complex fluids, especially biomaterials. In this technique, embedded microspheres are used to determine viscoelastic properties via generalized Stokes–Einstein relations, which assume the material behaves as a homogeneous continuum on the length scale of the probe. However, this condition can be violated if macromolecular systems form characteristic length scales that are larger than the probe size. Here we report observations of the onset of this effect in DNA solutions. We use microspheres driven with optical tweezers to determine the frequency dependence of the linear elastic and viscous moduli and their dependence on probe radius and DNA length. For well-entangled DNA, we find that the threshold probe radius yielding continuum behavior is ∼3× the reptation tube diameter, consistent with recent theoretical predictions. Notably, this threshold is significantly larger than the mesh size of the pol...

We measure the forces confining the displacement of a single DNA molecule embedded within a concentrated solution of long relaxed circular DNA molecules (115 kbp at 1 mg/mL) using optical tweezers. We compare these measurements with our previous measurements of forces imposed by entangled linear DNA molecules of the same length and concentration. A tube-like confining field and three relaxation modes were observed, but the tube radius was 25% smaller (=0.6 Im) and the longest relaxation time 3 times shorter (=11 s) than with linear molecules, consistent with recent theoretical predictions by Iyer, Lele, and Juvekar. For displacements greater than the tube radius, the confining force imposed by circular polymers was substantially lower and shorter range than that measured with linear polymers.

Imaging of single DNA molecules has enabled detailed studies of dilute polymer dynamics and rigorous testing of assumptions and predictions of molecular theories. It is of interest to extend these methods to the study of entangled polymers and to correlate molecular dynamics with rheology measurements. Progress in this direction has been hampered, however, by a lack of available DNA samples in sufficient quantities and covering a wide range of lengths. Here we describe the preparation of a suitable set of molecules ranging in length from ∼3 to 300 kilobase pairs. These constructs are replicated as plasmids or as fosmids or bacterial artificial chromosomes fitted with an inducible high-copy number origin of replication. DNA sequences were chosen to allow molecules to be linearized by single-cutting restriction enzymes. We show that these molecules can be imaged and characterized by fluorescence microscopy and can be prepared in sufficient quantities for bulk rheology measurements.