96 research outputs found
Periodic and Quasiperiodic Motion of an Elongated Microswimmer in Poiseuille Flow
We study the dynamics of a prolate spheroidal microswimmer in Poiseuille flow
for different flow geometries. When moving between two parallel plates or in a
cylindrical microchannel, the swimmer performs either periodic swinging or
periodic tumbling motion. Although the trajectories of spherical and elongated
swimmers are qualitatively similar, the swinging and tumbling frequency
strongly depends on the aspect ratio of the swimmer. In channels with reduced
symmetry the swimmers perform quasiperiodic motion which we demonstrate
explicitely for swimming in a channel with elliptical cross section
Experimental observation of flow fields around active Janus spheres
The phoretic mechanisms at stake in the propulsion of asymmetric colloids have been the subject of debates during the past years. In particular, the importance of electrokinetic effects on the motility of Pt-PS Janus sphere was recently discussed. Here, we probe the hydrodynamic flow field around a catalytically active colloid using particle tracking velocimetry both in the freely swimming state and when kept stationary with an external force. Our measurements provide information about the fluid velocity in the vicinity of the surface of the colloid, and confirm a mechanism for propulsion that was proposed recently. In addition to offering a unified understanding of the nonequilibrium interfacial transport processes at stake, our results open the way to a thorough description of the hydrodynamic interactions between such active particles and understanding their collective dynamics
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Benefits of cooperation in captive Damaraland mole-rats
AbstractAlthough the social mole-rats are commonly classified as eusocial breeders on the grounds that groups include a single breeding female (the âqueenâ) and a number of nonbreeding individuals (âhelpersâ) of both sexes, alloparental care is not highly developed in these species and there is no direct evidence that the presence or number of nonbreeders is associated with reductions in the workload of the âqueen.â An alternative interpretation of mole-rat groups is that the social mole-rats are cooperative foragers rather than cooperative or eusocial breeders. Here, in captive colonies of Damaraland mole-rats (Fukomys damarensis), we provide the first evidence that increases in the number of nonbreeding subordinates in mole-rat groups are associated with reductions in the workload of âqueensâ and with increases in their fecundity.European Research Council (294494), European Commission Horizon 2020 (H2020) ERC (742808), Human Frontier Science Program (HFSP) (RGP0051/2017
Dual parasitism of Fork-tailed Drongos by African and Jacobin Cuckoos §
Different species of brood parasitic birds, which lay their eggs in the nests of host foster-parents, rarely target the same host species population. We report brood parasitism of Fork-tailed Drongos Dicrurus adsimilis in the southern Kalahari Desert by both African Cuckoo Cuculus gularis and Jacobin Cuckoo Clamator jacobinus serratus. Drongos are the only known host for the African Cuckoo, and were more frequently parasitised by this species (21.8% nests). Nevertheless, parasitism rates suggest that in the Kalahari, drongos are also an important host for Jacobin Cuckoo (4.6% nests). Jacobin Cuckoos likely compete with African Cuckoos for drongo hosts, as exemplified by the occurrence of both African and Jacobin Cuckoo eggs in the same drongo clutch. The drongoâs defensive adaptations to parasitism by African Cuckoos, including egg rejection, may also curtail parasitism by Jacobin Cuckoos. The extent of competition between these cuckoo species and whether they possess adaptations to prevent one anotherâs access to drongo hosts remains to be explored.Keywords: alloxenia, brood parasitism, coevolution, host selection, offspring competitio
Electron impact excitation of methane: determination of appearance energies for dissociation products
In this work, we present an experimental study of dissociative excitation of CHâ utilizing a crossed electron molecular beam experiment. Methane was excited by nearly monochromatic electrons generated by a trochoidal electron monochromator. The dissociative products were identified on the basis of the emission spectra in the ultravioletâvisible (UV/VIS) spectral range. The excitation functions were recorded as the function of the electron energy for different emission bands of the fragments (Balmer series for H: n = 3,4...9â2, and moreover, CH: AÂČÎâXÂČÎ , CH: BÂČÎŁâ»â XÂČÎ , CH: CÂČÎŁâșâ XÂČÎ , CHâș: BÂčÎâ AÂčÎ , and CI: 2p3s ÂčPoââ2pÂČ ÂčSâ). From the experimental data we have determined the threshold energies for excitation of particular fragments. Present experimental results indicate that the threshold energies for some dissociative excitation channels could be lower by ~1â2 eV in comparison to earlier studies and indicate that different dissociative processes may be operative at the threshold than assumed in the former studies
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