Scientific Programme

Back to overview

Lecture

Can a material perform as an engine?

Tuesday (17.03.2020)
14:00 - 14:20
Part of:


Autonomous motion of small objects, (i) rotation, (ii) oscillation, or (iii) translation, requires a local energy input. We focus on LCST-hydrogel micro-objects, that swim by body shape-deformation, mimicking the motility of bacteria. Local temperature changes effect the volume changes and cause bending motions. For this purpose, the gels were equipped with 1-4 gold nanorods per cubic micrometer that absorb light at 780 nm and convert it practically instantaneously to heat. Stroboscopic irradiation causes temperature changes that are faster than swelling or shrinking and the body shape deformation takes place under thermodynamic non-equilibrium conditions. Motility, i.e., a directed forward motion, is achieved by cyclic deformations where “forward” and the “backward” strokes differ spatially and in time. The micro-objects undergo displacement directed by the object itself and not aligned from outside. The (i) geometrical shape, (ii) the material properties and (iii) the energy intake via IR-irradiation control the motility. In a second step, we succeeded to develop objects that move fully autonomously. Here, the energy uptake was not controlled by pulsed irradiation, but the repetitive deformation was powered by continuous irradiation. This required the introduction of a feed-back mechanism by which the energy uptake ceased upon raising the temperature and a hysteretic bistability by which we could prevent that the system could adopt a shape corresponding to a steady state equilibrium. We thus enabled a recurring deformation process, where in a first step temperature changed faster than the equilibrium volume change, and in a second step, a snap deformation, volume changed faster than the corresponding temperature change. In the third step, again temperature changed faster than the volume. In a last snap-like deformation, the cycle was completed. By this, the material went through a cycle that is characteristic for a thermal engine.

Speaker:
Prof. Dr. Martin Möller
DWI - Leibniz Institute for Interactive Materials
Additional Authors:
  • Dr. Ahmed Mourran
    DWI - Leibniz Institute for Interactive Materials
  • Prof. Dr. Gero von Plessen
    RWTH Aachen University
  • Dr. Dimitri Chigrin
    DWI - Leibniz Institute for Interactive Materials