Stephan A. Koehler

On self-propulsion of micro-machines at low Reynolds number: Purcell's three-link swimmer

Using slender-body hydrodynamics in the inertialess limit, we examine the motion of Purcell’s swimmer, a planar, fore–aft-symmetric three-link flagellum or propulsive mechanism that translates by alternately moving its front and rear segments. Purcell (1976) concluded via symmetry arguments that the net displacement of such a swimmer must follow a straight line, but the direction and other details of the motion have never been investigated. Numerical results indicate that the direction of net translation and the speed of Purcell’s swimmer depend on the angular amplitude of the swimming strokes as well as on the relative length of the links. Analytical results are presented for small rotations about the straightened configuration, and physical arguments are given to qualitatively explain the propulsive dynamics. The optimal swimmer configurations under the conditions of constant forcing and of minimum mechanical work are determined. We use a definition of efficiency based on the straightened configuration as a reference state to compare Purcell’s swimmer with the previously treated swimming motions of an undulating rod and a rotating helix. Finally, we demonstrate the importance of the anisotropy in the local hydrodynamic slender-body drag to swimming motions at low Reynolds number by showing that, in general, any inextensible swimmer in an otherwise quiescent fluid cannot alter its average position under conditions of locally isotropic drag.

Perspectives on foam drainage and the influence of interfacial rheology

Recent research related to foam drainage is surveyed with emphasis on the influence of interfacial rheology. Activ er es earch directions are highlighted and the possible impact of these studies on macroscopic rheology is indicated.

Surface-Functionalized Porous Lignin for Fast and Efficient Lead Removal from Aqueous Solution

The development of ecofriendly sorbents for fast and efficient removal of heavy metals from aqueous media still remains a significant challenge. Here, we report that this task can be addressed by creating a porous naturally occurring polymer, as illustrated by functionalizing lignin with large numbers of mesopores and functional groups. We show that surface-functionalized porous lignin (SFPL), obtained by a two-step process, has a large surface area of 22.3 m2/g, 12 times that of lignin, and a high density of dithiocarbamate groups (2.8 mmol/g). SFPL was found to exhibit an excellent adsorption performance toward lead ions dissolved in water. For example, 99% of the lead ions from 50 mL of a solution containing 20 mg/L lead ions was removed in just 30 min by 0.01 g of SFPL. The saturated adsorption capacity of SFPL for lead ions was found to be 188 mg/g, which is 13 times that of the original lignin and 7 times that of activated carbon. The adsorption process is endothermic and involves intraparticle diffusion and chemical adsorption between lead ions and the functional groups of SFPL. The cost effectiveness and environmental friendliness of SFPL make it a promising material for removing lead and other heavy metals from wastewater.

Rapid Assembly of Heterogeneous 3D Cell Microenvironments in a Microgel Array.

Heterogeneous 3D cell microenvironment arrays are rapidly assembled by combining surface-wettability-guided assembly and microdroplet-array-based operations. This approach enables precise control over individual shapes, sizes, chemical concentrations, cell density, and 3D spatial distribution of multiple components. This technique provides a cost-effective solution to meet the increasing demand of stem cell research, tissue engineering, and drug screening.

Microcapsules for Enhanced Cargo Retention and Diversity

Prevention of undesired leakage of encapsulated materials prior to triggered release presents a technological challenge for the practical application of microcapsule technologies in agriculture, drug delivery, and cosmetics. A microfluidic approach is reported to fabricate perfluoropolyether (PFPE)-based microcapsules with a high core-shell ratio that show enhanced retention of encapsulated actives. For the PFPE capsules, less than 2% leakage of encapsulated model compounds, including Allura Red and CaCl2 , over a four week trial period is observed. In addition, PFPE capsules allow cargo diversity by the fabrication of capsules with either a water-in-oil emulsion or an organic solvent as core. Capsules with a toluene-based core begin a sustained release of hydrophobic model encapsulants immediately upon immersion in an organic continuous phase. The major contribution on the release kinetics stems from the toluene in the core. Furthermore, degradable silica particles are incorporated to confer porosity and functionality to the otherwise chemically inert PFPE-based polymer shell. These results demonstrate the capability of PFPE capsules with large core-shell ratios to retain diverse sets of cargo for extended periods and make them valuable for controlled release applications that require a low residual footprint of the shell material.

Twirling elastica: kinks, viscous drag, and torsional stress.

Biological filaments such as DNA or bacterial flagella are typically curved in their natural states. To elucidate the interplay of viscous drag, twisting, and bending in the overdamped dynamics of such filaments, we compute the steady-state torsional stress and shape of a rotating rod with a kink. Drag deforms the rod, ultimately extending or folding it depending on the kink angle. For certain kink angles and kink locations, both states are possible at high rotation rates. The agreement between our macroscopic experiments and the theory is good, with no adjustable parameters.

Locomotor benefits of being a slender and slick sand swimmer

Squamates classified as ‘subarenaceous’ possess the ability to move long distances within dry sand; body elongation among sand and soil burrowers has been hypothesized to enhance subsurface performance. Using X-ray imaging, we performed the first kinematic investigation of the subsurface locomotion of the long, slender shovel-nosed snake (Chionactis occipitalis) and compared its biomechanics with those of the shorter, limbed sandfish lizard (Scincus scincus). The sandfish was previously shown to maximize swimming speed and minimize the mechanical cost of transport during burial. Our measurements revealed that the snake also swims through sand by propagating traveling waves down the body, head to tail. Unlike the sandfish, the snake nearly followed its own tracks, thus swimming in an approximate tube of self-fluidized granular media. We measured deviations from tube movement by introducing a parameter, the local slip angle, βs, which measures the angle between the direction of movement of each segment and body orientation. The average βs was smaller for the snake than for the sandfish; granular resistive force theory (RFT) revealed that the curvature utilized by each animal optimized its performance. The snake benefits from its slender body shape (and increased vertebral number), which allows propagation of a higher number of optimal curvature body undulations. The snakes low skin friction also increases performance. The agreement between experiment and RFT combined with the relatively simple properties of the granular ‘frictional fluid’ make subarenaceous swimming an attractive system to study functional morphology and bauplan evolution.

Rapid, targeted and culture-free viral infectivity assay in drop-based microfluidics

A key viral property is infectivity, and its accurate measurement is crucial for the understanding of viral evolution, disease and treatment. Currently viral infectivity is measured using plaque assays, which involve prolonged culturing of host cells, and whose measurement is unable to differentiate between specific strains and is prone to low number fluctuation. We developed a rapid, targeted and culture-free infectivity assay using high-throughput drop-based microfluidics. Single infectious viruses are incubated in a large number of picoliter drops with host cells for one viral replication cycle followed by in-drop gene-specific amplification to detect infection events. Using murine noroviruses (MNV) as a model system, we measure their infectivity and determine the efficacy of a neutralizing antibody for different variants of MNV. Our results are comparable to traditional plaque-based assays and plaque reduction neutralization tests. However, the fast, low-cost, highly accurate genomic-based assay promises to be a superior method for drug screening and isolation of resistant viral strains. Moreover our technique can be adapted to measuring the infectivity of other pathogens, such as bacteria and fungi.

Drying kinetics driven by the shape of the air/water interface in a capillary channel.

Abstract.We look at the drying process in a simple glass channel with dominant capillary effects as is the case in microfluidics. We find drying kinetics commonly observed for confined geometry, namely a constant period followed by a falling rate period. From visualization of the air/water interface with high resolution, we observe that the drying rate decreases without a drying front progression although this is the usually accepted mechanism for confined geometries. We show with FEM that in our specific geometry the falling rate period is due to changes in the shape of the air-water interface at the free surface where most evaporation occurs. Our simulations show that the sensitivity of the drying rate to the shape of the first air-water interface from the sample free surface implies that slight changes of the wetting or pinning conditions can significantly modify the drying rate.Graphical abstract

Artifact-Free Quantification and Sequencing of Rare Recombinant Viruses by Using Drop-Based Microfluidics.

Recombination is an important driver in the evolution of viruses and thus is key to understanding viral epidemics and improving strategies to prevent future outbreaks. Characterization of rare recombinant subpopulations remains technically challenging because of artifacts such as artificial recombinants, known as chimeras, and amplification bias. To overcome this, we have developed a high‐throughput microfluidic technique with a second verification step in order to amplify and sequence single recombinant viruses with high fidelity in picoliter drops. We obtained the first artifact‐free estimate of in vitro recombination rate between murine norovirus strains MNV‐1 and WU20 co‐infecting a cell (Prec=3.3×10−4±2×10−5) for a 1205 nt region. Our approach represents a time‐ and cost‐effective improvement over current methods, and can be adapted for genomic studies requiring artifact‐ and bias‐free selective amplification, such as microbial pathogens, or rare cancer cells.