Seminar über aktuelle Fragen zur Dynamik komplexer Fluide

Seminar über aktuelle Fragen zur Dynamik komplexer Fluide: Light moves liquids: flow, interface deformation and instability

  • Datum: 26.01.2018
  • Uhrzeit: 10:15 - 11:15
  • Vortragender: Dr. Jean-Pierre Delville
  • University of Bordeaux, CNRS, Laboratoire Ondes et Matière d’Aquitaine, Talence, France
  • Ort: Max-Planck-Institut für Dynamik und Selbstorganisation (MPIDS)
  • Raum: SR 0.77
  • Gastgeber: MPIDS/DCF
  • Kontakt: julien.petit@ds.mpg.de
Optofluidics refers to the microfluidic area where optics and fluids are coupled together to create optical functions in fluids or conversely, to actuate fluids at a micrometric scale with optics [1]. Opto-hydrodynamics is a recent discipline which corresponds to the latter case and studies the dynamics of fluids under light forcing [2]. Here we aim at presenting a large overview on opto-hydrodynamics in order to illustrate its large potential in microfluidics.
Conceptually, electromagnetic waves possess energy and momentum that can be exchanged with or transferred to matter. Surface and density forces may result from this interaction whenever photon momentum undergoes a modification in direction and/or in amplitude and these forces can produce flows and interface deformations in liquids.
First, photons may be reflected and/or transmitted at the interface separating two transparent fluids of different index of refraction. Momentum conservation at crossing results in the so-called optical radiation pressure that sets transiently in movement and deforms the interface up to instability at large laser intensities [3]. Momentum transfer to non-absorbing turbid liquids can as well be produced by elastic scattering of the incident light on the scatterers and trigger bulk flows and jets in presence of an interface [4].
Second, photons may be partially or totally absorbed by the crossed liquid, and leave energy which is often transformed into heat. The resulting local decrease of the density produces in turn a pressure gradient which triggers convective motion driven by light [5].
Finally, absorption of the incident laser also heat liquid interfaces. As surface tension depends on temperature, a thermal gradient sets a surface-tension gradient and, therefore, produces tangential stresses along the interface separating two immiscible fluids. These Marangoni or thermocapillary stresses, induce interfacial flows and may cause interface deformations and instabilities [6].
Opto-hydrodynamics may thus provide an optical toolbox for the investigation of fluid dynamics at small scale.

Références
1. V. R. Horowitz, et al., “Optofluidics: field or technique?” Lab Chip, 8, 1856-63 (2008).
2. J. P. Delville, et al. "Laser microfluidics: fluid actuation by light", J. Opt. A: Pure Appl. Opt. 11, 034015 (2009).
3. H. Chraïbi, et al, "Optohydrodynamics of soft fluid interfaces: Optical and viscous nonlinear effects", Eur. Phys. J. E 32, 43 (2010).
4. H. Chraïbi, et al., "Excitation of Fountain and Entrainment Instabilities at the Interface between Two Viscous Fluids Using a Beam of Laser Light", Phys. Rev. Lett. 111, 044502 (2013).
5. D. Rivière, et al. "Convection flows driven by laser heating of a liquid layer." Physical Review E 93 023112 (2016).
6. M. Robert de Saint Vincent, et al. "Fragmentation mechanisms of confined co-flowing capillary threads revealed by active flow focusing." Physical Review Fluids 1 043901 (2016).
 
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