Acoustic microswimmers present great potential for microuidic applications and targeted drug delivery. Here we introduce armoured microbubbles (size range, 10 − 20 µm) made by threedimensional microfabrication which allows the bubbles to last for hours even under forced oscillations. The acoustic resonance of the armoured microbubbles is found to be dictated by capillary forces and not by gas volume, and its measurements agree with a theoretical calculation. We further measure experimentally and predict theoretically the net propulsive ow generated by the bubble vibration. This ow, due to steady streaming in the uid, can reach 100 mm/s, and is aected by the presence of nearby walls. Finally, microswimmers in motion are shown, either as spinning devices or free swimmers. I.

International audienceWe have studied the nonlinear absorption spectra and optical limitingproperties of 2,2′-(9,9-dihexyl) bifluorene. Measurements were done in chloroform solution, in the visible range (450–650 nm), and for nanosecond time-duration pulses. The two-photon absorption(TPA)spectrum, obtained by the up-conversion fluorescence method, shows a resonance at λ=534 nm with a cross-section σ TPA =60 10 −50 cm 4 ⋅s /photon-molecule. Semiempirical quantum chemistry calculation on fluorene and bifluorene suggests an enhancement of the bifluorene TPA due to coupling effects between monomers. In nonlinear transmission measurements,two-photon absorption is reinforced by the excited-state absorption that occurs during the pulse duration. At resonance, the three-photon absorption coefficient is α 3 =14 000 cm 3 /GW 2 for a bifluorene concentration of 600 g/L. This strong nonlinear absorption leads to an efficient optical power limiting in the green and blue parts of the spectrum. The maximum transmitted energy is lower than 10 μJ for an input energy of up to 200 μJ in a F/5 optical geometry