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As the closest unicellular relatives of animals, choanoflagellates serve as useful model organisms for understanding the evolution of animal multicellularity. An important factor in animal evolution was the increasing ocean oxygen levels in the Precambrian, which are thought to have influenced the emergence of complex multicellular life. As a first step in addressing these conditions, we study here the response of the colony-forming choanoflagellate Salpingoeca rosetta to oxygen gradients. Using a microfluidic device that allows spatio-temporal variations in oxygen concentrations, we report the discovery that S. rosetta displays positive aerotaxis. Analysis of the spatial population distributions provides evidence for logarithmic sensing of oxygen, which enhances sensing in low oxygen neighborhoods. Analysis of search strategy models on the experimental colony trajectories finds that choanoflagellate aerotaxis is consistent with stochastic navigation, the statistics of which are captured using an effective continuous version based on classical run-and-tumble chemotaxis. Most animals are made up of millions of cells, yet all animals evolved from ancestors that spent their whole lives as single cells. Today the closest single-celled relatives of animals are a group of aquatic organisms called choanoflagellates. Certain species of choanoflagellates can also form swimming colonies. This kind of multicellularity might resemble that seen in the earliest of animals. As such, studies into modern-day choanoflagellates can give insights into how the first animals to evolve might have behaved. Many organisms can find their way towards favorable areas using different strategies. For instance, bacteria can bias their tumbling to gradually swim towards food, and algae can turn and move directly towards light. While choanoflagellates require oxygen, it was not known if they could also actively navigate towards it, or any other resource. Now, Kirkegaard et al. find that the choanoflagellate Salpingoeca rosetta can indeed navigate towards oxygen – an ability called aerotaxis. This was true for both individual cells and for colonies made up of many cells. This discovery suggests that the transition from living as a single cell to living as a simple multicellular organism could still have allowed the earliest animals to seek out and move towards resource-rich areas. Aerotaxis requires cells to both sense oxygen and react appropriately to changes in its concentration. Kirkegaard et al. watched choanoflagellate colonies swimming under controlled conditions and varied the oxygen concentration in the water over time. These experiments revealed that the colonies navigate based on the logarithm of the oxygen concentration, so that at low oxygen levels the cells were even more sensitive to small changes in oxygen concentration. This type of ‘logarithmic sensing’ is similar to how our ears sense sounds and our eyes sense light. Kirkegaard et al. went on to conclude that the colonies were not actively steering in the correct direction directly. Instead, the colonies appeared to choose directions at random and later decide whether such a turn was correct. It remains unclear whether the common ancestor of animals and choanoflagellates could also perform aerotaxis, and if so what mechanisms this involved. Further studies to compare aerotaxis and aerotaxis-related genes in simple animals and other single-celled relatives of animals would be needed to illuminate this. Future studies could also explore the maximum and minimum oxygen concentrations that choanoflagellates can detect, and how well they navigate at these upper and lower limits.

As reported in 1756 by Johann Gottlob Leidenfrost, volatile liquids on hot solids form “gleaming drops resembling quicksilver”, a consequence of their levitation on a vapour cushion1,2. This makes the drops spectacularly mobile, moving away as soon as they are deposited—an observation commonly attributed to gravity or surrounding airflows. This mobility has been exploited to manipulate drops, because tiny forces such as those generated on asymmetric substrates can move them in well-defined directions3–5, a situation that also provides heat evacuation6. Here we report that Leidenfrost droplets initially at rest on horizontal substrates self-rotate and self-propel in the direction they are rolling, in the absence of any source of asymmetry or external force. Their rapid internal flow is found to be accompanied by a tilting of their base, which creates a permanent ratchet-like mechanism, entraining the rolling liquid despite the fact that it is not in contact with its substrate.

Volatile liquids (water, alcohol, etc.) poured on hot solids levitate above a layer of vapor. Unexpectedly, these so-called Leidenfrost drops often suddenly start to oscillate with star shapes, a phenomenon first reported about 140 y ago. Similar shapes are known to be triggered when a liquid is subjected to an external periodic forcing, but the unforced Leidenfrost case remains unsolved. We show that the levitating drops are excited by an intrinsic periodic forcing arising from a vibration of the vapor cushion. We discuss the frequency of the vibrations and how they can excite surface standing waves possibly amplified under geometric conditions of resonance—an ensemble of observations that provide a plausible scenario for the origin, mode selection, and sporadic nature of the Leidenfrost stars.

In the version of this Letter originally published, the Supplementary Videos were incorrectly labelled; the descriptions of 1–4 should have gone with the videos of 6–9, and the descriptions of 5–9 should have gone with the videos of 1–5. This has now been corrected.

As the closest unicellular relatives of animals, choanoflagellates serve as useful model organisms for understanding the evolution of animal multicellularity. An important factor in animal evolution was the increasing ocean oxygen levels in the Precambrian, which are thought to have influenced the emergence of complex multicellular life. As a first step in addressing these conditions, we study here the response of the colony-forming choanoflagellate Salpingoeca rosetta to oxygen gradients. Using a microfluidic device that allows spatio-temporal variations in oxygen concentrations, we report the discovery that S. rosetta displays positive aerotaxis. Analysis of the spatial population distributions provides evidence for logarithmic sensing of oxygen, which enhances sensing in low oxygen neighborhoods. Analysis of search strategy models on the experimental colony trajectories finds that choanoflagellate aerotaxis is consistent with stochastic navigation, the statistics of which are captured using an effective continuous version based on classical run-and-tumble chemotaxis.

When moist air meets a cold surface, it creates a breath figure characterized by numerous small droplets. The central question is how the vapor flux is distributed between the growth of previously condensed drops and the nucleation of new ones. Here, we investigate the nucleation, growth, and coalescence of droplets on soft crosslinked polymer networks. The number of droplets initially remains constant, until drops start to coarsen according to a universal law; both phenomena are explained via the formation of a saturated boundary layer. Although nucleation occurs at a scale where the polymer network resembles a melt, we quantitatively unveil an algebraic sensitivity of the number of droplets on the substrate elasticity. Our findings suggest that nucleation follows a surprisingly low-energy pathway, influenced by the degree of crosslinking. Consequently, breath figures offer a macroscopic approach to probe the molecular characteristics of the polymer interface.

Breath figures are the complex patterns that form when water vapor condenses into liquid droplets on a surface. The primary question concerning breath figures is how the condensing vapor is allocated between the growth of existing droplets and the nucleation of new ones. Although numerous theoretical studies have concentrated on scenarios resulting in highly polydisperse droplet ensembles, a companion paper [Bouillant et al., submitted] demonstrates that nearly monodisperse patterns can be achieved on defect-free substrates in a diffusion-controlled regime. The objective of this work is to present a theoretical framework that elucidates the formation and evolution of nearly-monodisperse patterns in breath figures. We discover that, following a short nucleation phase, the number of droplets remains constant over an extensive range of timescales due to collective effects mediated by the diffusion of vapor. The spatial extent of these diffusive interactions is identified through asymptotic matching, based on which we provide an accurate description of breath figures through a mean-field model. The model accounts for the sub-diffusive growth of droplets as well as for the arrest of nucleating new droplets, and reveal the scaling laws for the droplet density observed in experiments. Finally, droplets expand and ultimately coalesce, which is shown to trigger a scale-free coarsening of the breath figures.

Breath figures are the complex patterns that form when water vapor condenses into liquid droplets on a surface. The primary question concerning breath figures is how the condensing vapor is allocated between the growth of existing droplets and the nucleation of new ones. Although numerous theoretical studies have concentrated on scenarios resulting in highly polydisperse droplet ensembles, a companion paper [Bouillant et al., submitted] demonstrates that nearly monodisperse patterns can be achieved on defect-free substrates in a diffusion-controlled regime. The objective of this work is to present a theoretical framework that elucidates the formation and evolution of nearly-monodisperse patterns in breath figures. We discover that, following a short nucleation phase, the number of droplets remains constant over an extensive range of timescales due to collective effects mediated by the diffusion of vapor. The spatial extent of these diffusive interactions is identified through asymptotic matching, based on which we provide an accurate description of breath figures through a mean-field model. The model accounts for the sub-diffusive growth of droplets as well as for the arrest of nucleating new droplets, and reveal the scaling laws for the droplet density observed in experiments. Finally, droplets expand and ultimately coalesce, which is shown to trigger a scale-free coarsening of the breath figures.

The breakup and coalescence of drops are elementary topological transitions in interfacial flows. The breakup of a drop changes dramatically when polymers are added to the fluid. With the strong elongation of the polymers during the process, long threads connecting the two droplets appear prior to their eventual pinch-off. Here, we demonstrate how elasticity affects drop coalescence, the complement of the much studied drop pinch-off. We reveal the emergence of an elastic singularity, characterised by a diverging interface curvature at the point of coalescence. Intriguingly, while the polymers dictate the spatial features of coalescence, they hardly affect the temporal evolution of the bridge. These results are explained using a novel viscoelastic similarity analysis and are relevant for drops created in biofluids, coating sprays and inkjet printing.

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flows structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10\\(^\\)C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the baseflows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius \\(R\\), yields the most unstable, thus, dominant, azimuthal modes (of wavenumber \\(m\\)). Our theory predicts well the radii \\(R\\) for the mode transitions and cascade with decreasing wavenumbers from \\(m=3\\), \\(m=2\\), down to \\(m=1\\) (the eventual rolling mode that entails propulsion) as the drop shrinks in size. The effect of the escaping vapor is not taken into account here, which may further destabilize the inner flow and couple to the liquid/vapor interface to give rise to motion Bouillant et al. (2018) [8] and Brandao et al. (2020) [9].