Tangling of tethered swimmers: interactions between two nematodes

Micropipette force sensors for in vivo force measurements on single cells and multicellular microorganisms

Measuring forces from the piconewton to millinewton range is of great importance for the study of living systems from a biophysical perspective. The use of flexible micropipettes as highly sensitive force probes has become established in the biophysical community, advancing our understanding of cellular processes and microbial behavior. The micropipette force sensor (MFS) technique relies on measurement of the forces acting on a force-calibrated, hollow glass micropipette by optically detecting its deflections. The MFS technique covers a wide micro- and mesoscopic regime of detectable forces (tens of piconewtons to millinewtons) and sample sizes (micrometers to millimeters), does not require gluing of the sample to the cantilever, and allows simultaneous optical imaging of the sample throughout the experiment. Here, we provide a detailed protocol describing how to manufacture and calibrate the micropipettes, as well as how to successfully design, perform, and troubleshoot MFS experiments. We exemplify our approach using the model nematode Caenorhabditis elegans, but by following this protocol, a wide variety of living samples, ranging from single cells to multicellular aggregates and millimeter-sized organisms, can be studied in vivo, with a force resolution as low as 10 pN. A skilled (under)graduate student can master the technique in ~1-2 months. The whole protocol takes ~1-2 d to finish.

Hydrodynamic Choreographies of Microswimmers

We unveil orbital topologies of two nearby swimming microorganisms using an artificial microswimmer, called Quadroar. Depending on the initial conditions of the microswimmers, we find diverse families of attractors including dynamical equilibria, bound orbits, braids, and pursuit–evasion games. We also observe a hydrodynamic slingshot effect: a system of two hydrodynamically interacting swimmers moving along braids can advance in space faster than non-interacting swimmers that have the same actuation parameters and initial conditions as the interacting ones. Our findings suggest the existence of complex collective behaviors of microswimmers, from equilibrium to rapidly streaming states.

The effects of groove height and substrate stiffness on C. elegans locomotion

The physical environment surrounding an animal has a significant impact on its behavior. The nematode Caenorhabditis elegans (C. elegans) has proved to be an excellent choice for understanding the adaptability of organisms crawling on soft surfaces. In this work, we investigate the modulation of C. elegans' behavioral kinematics in response to changes in the stiffness of the substrate and study the effect of grooves incised by the worms on their locomotion speed and efficiency. We measure the height of the grooves created by the animals on surfaces with different rigidity using confocal microscopy. Our results indicate that the kinematic properties of C. elegans, including amplitude (A), wavelength (λ) and frequency (f) of head turns depend strongly on surface properties and the height of the grooves created by them. During crawling, we observe that the animal assumes two distinct shapes depending on the stiffness of substrates. As the stiffness increases, the worm's body shape changes gradually from a 'W' shape, which is characterized by low amplitude curvature to the more common 'S' shape, which is characterized by high amplitude curvature, at intermediate values and back to 'W' on stiffer substrates. Although the efficiency is found to vary monotonically with surface stiffness, the forward velocity shows a non-monotonic behavior with the maximum on a surface, where the animal makes the 'S' shape.

The nematode C. elegans as a complex viscoelastic fluid

The viscoelastic material properties of the model organism C. elegans were probed with a micropipette deflection technique and modelled with the standard linear solid model. Dynamic relaxation measurements were performed on the millimetric nematode to investigate its viscous characteristics in detail. We show that the internal properties of C. elegans can not be fully described by a simple Newtonian fluid. Instead, a power-law fluid model was implemented and shown to be in excellent agreement with experimental results. The nematode exhibits shear thinning properties and its complex fluid characteristics were quantified. The bending-rate dependence of the internal damping coefficient of C. elegans could affect its gait modulation in different external environments.