The topography of the environment alters the optimal search strategy for active particles

In environments with scarce resources, adopting the right search strategy can make the difference between succeeding and failing, even between life and death. At different scales, this applies to molecular encounters in the cell cytoplasm, to animals looking for food or mates in natural landscapes, to rescuers during search-and-rescue operations in disaster zones, as well as to genetic computer algorithms exploring parameter spaces. When looking for sparse targets in a homogeneous environment, a combination of ballistic and diffusive steps is considered optimal; in particular, more ballistic L\'evy flights with exponent {\alpha} <= 1 are generally believed to optimize the search process. However, most search spaces present complex topographies, with boundaries, barriers and obstacles. What is the best search strategy in these more realistic scenarios? Here we show that the topography of the environment significantly alters the optimal search strategy towards less ballistic and more Brownian strategies. We consider an active particle performing a blind search in a two-dimensional space with steps drawn from a L\'evy distribution with exponent varying from {\alpha} = 1 to {\alpha} = 2 (Brownian). We demonstrate that the optimal search strategy depends on the topography of the environment, with {\alpha} assuming intermediate values in the whole range under consideration. We interpret these findings in terms of a simple theoretical model, and discuss their robustness to the addition of Brownian diffusion to the searcher's motion. Our results are relevant for search problems at different length scales, from animal and human foraging to microswimmers' taxis, to biochemical rates of reaction

Brownian Motion in a Speckle Light Field: Tunable Anomalous Diffusion and Deterministic Optical Manipulation

The motion of particles in random potentials occurs in several natural phenomena ranging from the mobility of organelles within a biological cell to the diffusion of stars within a galaxy. A Brownian particle moving in the random optical potential associated to a speckle, i.e., a complex interference pattern generated by the scattering of coherent light by a random medium, provides an ideal mesoscopic model system to study such phenomena. Here, we derive a theory for the motion of a Brownian particle in a speckle and, in particular, we identify its universal characteristic timescale levering on the universal properties of speckles. This theoretical insight permits us to identify several interesting unexplored phenomena and applications. As an example of the former, we show the possibility of tuning anomalous diffusion continuously from subdiffusion to superdiffusion. As an example of the latter, we show the possibility of harnessing the speckle memory effect to perform some basic deterministic optical manipulation tasks such as guiding and sorting by employing random speckles, which might broaden the perspectives of optical manipulation for real-life applications by providing a simple and cost-effective technique

Metastable Clusters and Channels Formed by Active Particles with Aligning Interactions

We introduce a novel model for active particles with short-range aligning interactions and study their behaviour in crowded environments using numerical simulations. When only active particles are present, we observe a transition from a gaseous state to the emergence of metastable clusters as the level of orientational noise is reduced. When also passive particle are present, we observe the emergence of a network of metastable channels.Comment: 11 pages, 7 figure

Sorting of Chiral Microswimmers

Microscopic swimmers, e.g., chemotactic bacteria and cells, are capable of directed motion by exerting a force on their environment. For asymmetric microswimmers, e.g., bacteria, spermatozoa and many artificial active colloidal particles, a torque is also present leading in two dimensions to circular motion and in three dimensions to helicoidal motion with a well-defined chirality. Here, we demonstrate with numerical simulations in two dimensions how the chirality of circular motion couples to chiral features present in the microswimmer environment. Levogyre and dextrogyre microswimmers as small as $50\,\mathrm{nm}$ can be separated and selectively trapped in \emph{chiral flowers} of ellipses. Patterned microchannels can be used as \emph{funnels} to rectify the microswimmer motion, as \emph{sorters} to separate microswimmers based on their linear and angular velocities, and as \emph{sieves} to trap microswimmers with specific parameters. We also demonstrate that these results can be extended to helicoidal motion in three dimensions.Comment: 9 pages, 7 figure

Longterm Influence of Inertia on the Diffusion of a Brownian Particle

We demonstrate experimentally that a Brownian particle is subject to inertial effects at long time scales. By using a blinking optical tweezers, we extend the range of previous experiments by several orders of magnitude up to a few seconds. The measured mean square displacement of a freely diffusing Brownian particle in a liquid shows a deviation from the Einstein-Smoluchowsky theory that diverges with time. These results are consistent with a generalized theory that takes into account not only the particle inertia but also the inertia of the fluid surrounding the particle. This can lead to a bias in the estimation of the diffusion coefficient from finite-time measurements. We show that the decay of the relative error is polynomial and not exponential and, therefore, can have significant effects at time scales relevant for experiments.Comment: 5 pages, 4 figure

Pose and Shape Reconstruction of a Noncooperative Spacecraft Using Camera and Range Measurements

Recent interest in on-orbit proximity operations has pushed towards the development of autonomous GNC strategies. In this sense, optical navigation enables a wide variety of possibilities as it can provide information not only about the kinematic state but also about the shape of the observed object. Various mission architectures have been either tested in space or studied on Earth. The present study deals with on-orbit relative pose and shape estimation with the use of a monocular camera and a distance sensor. The goal is to develop a filter which estimates an observed satellite's relative position, velocity, attitude, and angular velocity, along with its shape, with the measurements obtained by a camera and a distance sensor mounted on board a chaser which is on a relative trajectory around the target. The filter's efficiency is proved with a simulation on a virtual target object. The results of the simulation, even though relevant to a simplified scenario, show that the estimation process is successful and can be considered a promising strategy for a correct and safe docking maneuver

Writing and reading practices in fifteenth-century Chosŏn Korea: focusing on King Sejong’s reign (1418-1450) and the invention of the Korean vernacular script (Hunmin Chŏngŭm 訓民正音)

Fifteenth-century Chosŏn Korea was characterised by a complex and interconnected system of writing practices that included literary Sinitic (hanmun 漢文), vernacular transcription systems, foreign languages, and, since the middle of the century, a new vernacular script. This new script was invented and promulgated during the reign of Sejong 世宗 (1418-1450) with the name of Hunmin Chŏngŭm 訓民正音 (the correct sounds for the instruction of the people). Its invention allowed for the first time to transcribe the Korean language with ease and precision, opening new possibilities for larger sectors of the society to express themselves through writing and for written texts to reach a larger part of the population. Existing scholarship, however, does not seem to have devoted much attention to how, at a practical level, this was made possible. What seems to be often implied is that illiterate people could finally write with the vernacular script and read texts written with it. The present study, however, suggests that this vision of the vernacular script as aimed to increase the level of literacy among the Korean population does not offer a complete understanding of the significance of its invention. Instead, it urges to place the invention of the vernacular script in the broader context of the writing and reading practices of the time, often characterized by oral and social uses of the written texts. Thus, it considers how early Chosŏn texts were also intended to be read aloud and, in some cases, memorized so that they could be mediated for an audience of listeners in specific contexts. The production of the early vernacular texts compiled during the reign of Sejong is thus analysed by considering the social dynamics at play during their reading and how their textual and linguistic features were linked to their use. By doing this, it is argued that the vernacular script did not simply make possible an increase of literacy but unlocked new possibilities for the vocalization and mediation of texts