arXiv subjects

Matilda Backholm

Publications and source records attributed to Matilda Backholm.

7 recordsLinked to original sources

arXiv2026

Interaction of vortex rings generated by two unsynchronised drop impacts

A liquid drop falling into a deep pool can create a vortex ring at the right impact conditions. Such drop-formed vortex rings are of importance in nature and technology and the dynamics of rings created by single and synchronised double drop impacts have been extensively studied. In practice, two neighbouring drops rarely impact a liquid surface exactly at the same time, yet the interaction of two unsynchronised vortex rings have not been studied. Here, we have performed experiments with two water drops impacting a water pool at varying time differences $\Delta t$. By using particle image velocimetry, we have quantified the time-evolution of the resulting vortex rings. We find four distinct categories of vortex ring evolution depending on $\Delta t$. At $\Delta t<0.5$ ms, fully symmetric merging of the vortex rings occur. An unsynchronisation larger than this drastically influences the collision and merging, which either becomes asymmetric and incomplete ($0.5 < \Delta t<7$ ms) or does not happen at all ($\Delta t \geq 7$ ms). At $7 \leq \Delta t<80$ ms, the creation of the second vortex ring is impaired, whereas at $\Delta t\geq 80$ ms, the first impact no longer affects the formation of the second ring and eventually the two rings evolve without influencing each other. We show that these different regimes can be explained by the capillary waves created by the first droplet. Our results demonstrate the importance of the time difference between drop impacts in the creation and subsequent interaction of two adjacent drop-formed vortex rings, which is important for achieving uniform and controlled mixing in high-throughput applications.

physics.flu-dyn↗

arXiv2025

Ultra-slow capillary rise on hydrogel surfaces

Capillary rise occurs when a thin tube contacts a liquid, which rises against gravity due to the capillary force. This phenomenon is present in a wide range of everyday and industrial settings and provides the means to measure the physical properties of liquids. Here, we report on the unusual ultra-slow capillary rise on a solid-like material of agarose hydrogels. The observed meniscus motion cannot be described with classical capillary rise models, and we develop a new model based on the fluid transport through the porous hydrogel network. Our model is in good agreement with the temporal scaling observed in our experiments with agarose gels made with five different concentrations and with two different viscosities of the liquid flowing inside the gel. Our results provide a non-invasive technique to directly estimate the permeability of hydrogel interfaces with high spatial resolution, which is important in the implementation of hydrogels in advanced biomedical applications.

cond-mat.soft↗

arXiv2025

Soft matter mechanics of immune cell aggregates

T-cells are a crucial subset of white blood cells that play a central role in the immune system. When T-cells bind antigens, it leads to cell activation and the induction of an immune response. If T-cells are activated by antigens in vivo or artificially in vitro, they form multicellular aggregates. The mechanical properties of such clusters provide valuable information on different T-cell activation pathways. Furthermore, the aggregate mechanics capture how T-cells are affected by mechanical forces and interact within larger conglomerates, such as lymph nodes and tumours. However, an understanding of collective T-cell adhesion and mechanics following cell activation is currently lacking. Probing the mechanics of fragile and microscopically small living samples is experimentally challenging. Here, the micropipette force sensor technique was used to stretch T-cell aggregates and directly measure their Young's modulus and ultimate tensile strength. A mechanistic model was developed to correlate how the stiffness of the mesoscale multicellular aggregate emerges from the mechanical response of the individual microscopic cells within the cluster. We show how the aggregate elasticity is affected by different activators and relate this to different activation pathways in the cells. Our soft matter mechanics study of multicellular T-cell aggregates contributes to our understanding of the biology behind immune cell activation.

physics.bio-ph↗

arXiv2025

Forces and symmetry breaking of a living meso-swimmer

Swimming is ubiquitous in nature and crucial for the survival of a wide range of organisms. The physics of swimming at the viscosity-dominated microscale and inertia-dominated macroscale is well studied. However, in between lies a complicated mesoscale with swimmers affected by non-linear and time-dependent fluid mechanics. The intricate motility strategies, combined with complex and periodically changing body shapes add extra challenges for accurate meso-swimming modelling. Here, we have further developed the micropipette force sensor to directly probe the swimming forces of the meso-organism Artemia. Through deep neural network-based image analysis, we show how Artemia achieves an increased propulsive force by increasing its level of time-reversal symmetry breaking. We present a universal force-based scaling law for a wide range of micro- to meso-organisms with different body shapes, swimming strategies, and level of inertia at the mesoscale. These results capture fundamental aspects of biological meso-swimming dynamics and provide guidance for future biomimicking meso-robot designs.

cond-mat.soft↗

arXiv2018

Wetting of ferrofluids: phenomena and control

Ferrofluids are liquids exhibiting remarkably strong response to magnetic fields, which leads to fascinating properties useful in various applications. Understanding the wetting properties and spreading of ferrofluids is important for their use in microfluidics and magnetic actuation. However, this is challenging as magnetically induced deformation of the ferrofluid surface can affect contact angles, which are commonly used to characterize wetting properties in other systems. In addition, interaction of the magnetic nanoparticles and solid surface at nanoscale can have surprising effects on ferrofluid spreading. In this review we discuss these issues with focus on interpretation of ferrofluid contact angles. We review recent literature examining ferrofluid wetting phenomena and outline novel wetting related ferrofluid applications. To better understand wetting of ferrofluids, more careful experimental work is needed.

cond-mat.soft↗

arXiv2014

Capillary levelling of a cylindrical hole in a viscous film

The capillary levelling of cylindrical holes in viscous polystyrene films was studied using atomic force microscopy as well as quantitative analytical scaling arguments based on thin film theory and self-similarity. The relaxation of the holes was shown to consist of two different time regimes: an early regime where opposing sides of the hole do not interact, and a late regime where the hole is filling up. For the latter, the self-similar asymptotic profile was derived analytically and shown to be in excellent agreement with experimental data. Finally, a binary system of two holes in close proximity was investigated where the individual holes fill up at early times and coalesce at longer times.

cond-mat.soft↗

arXiv2013

Relaxation and Intermediate Asymptotics of a Rectangular Trench in a Viscous Film

The surface of a thin liquid film with nonconstant curvature flattens as a result of capillary forces. While this leveling is driven by local curvature gradients, the global boundary conditions greatly influence the dynamics. Here, we study the evolution of rectangular trenches in a polystyrene nanofilm. Initially, when the two sides of a trench are well separated, the asymmetric boundary condition given by the step height controls the dynamics. In this case, the evolution results from the leveling of two noninteracting steps. As the steps broaden further and start to interact, the global symmetric boundary condition alters the leveling dynamics. We report on full agreement between theory and experiments for: the capillary-driven flow and resulting time dependent height profiles; a crossover in the power-law dependence of the viscous energy dissipation as a function of time as the trench evolution transitions from two noninteracting to interacting steps; and the convergence of the profiles to a universal self-similar attractor that is given by the Green's function of the linear operator describing the dimensionless linearized thin film equation.

cond-mat.soft↗