3D-printed chiral microparticles show surprising oscillation dynamics in fluid flows

What is it about?

In nature, many small objects are chiral: Their shape seen in a mirror is different from their actual shape. Important examples are helical-shaped DNA strands, bacteria, or cholesteric crystals, which are either of left-handed or right-handed chirality. Using the nanoscribe microprinting technique researchers now managed to fabricate 3D-printed artificial chiral microparticles of different shape and different chiralities. They mimic the shapes of 'passive bacteria', consisting of a spherical 'head' and a helical 'tail'. When particles are put in shear flow, they are transported with the flow and they rotate. The pioneering work from Jeffery in 1922 showed that that the orientation of non-chiral particles such as spheres or rods in shear flow oscillates periodically in time. Francesca Tesser from ESPCI Paris now performed for the first time experiments using 3D-printed chiral microparticles and observed a very different behavior: In contrast to non-chiral particles the oscillation amplitude now increases or decreases over time. Interestingly, depending on the initial orientation of the particle, and of the particle handedness (left- or right-handed), the particle either gets stabilized pointing to the left or to the right with respect to the direction of the flow. Furthermore, this bistability is asymmetric, as there is a higher probability to go on either side, depending on the initial orientation of the particle. The researchers also identified a new dynamic “switching” of the particle orientation from the left to the right side for left-handed particles, and from the right to the left side for right-handed particles, which is absent for non-chiral particles. Andreas Zöttl from the University of Vienna, who developed the theoretical model and performed computer simulations, explains the occurring handedness-dependent bistability by two factors: First, theoretical considerations explain that left- and right-handed particles reorient away from the stable and periodic Jeffery oscillations in exactly the opposite way. Thereby the chirality of the particles is quantified by their 'chiral strength', a theoretical factor whose exact values had been determined by computer simulations performed by Daiki Matsunaga from Osaka University. Second, the fact that the head of the particles are slightly denser than the helical tail, makes the particles rotate pointing head-down under the influence of gravity. The combination of both factors will then lead to the observed asymmetric bistability.

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Photo by Warren Umoh on Unsplash

Photo by Warren Umoh on Unsplash

Why is it important?

The clean experimental measurements and the precise theoretical model enabled the researchers to match experimental and simulation data extremely well. The underlying work, which had just been published in PNAS, is the first observation of the complex oscillation dynamics of chiral particles in shear flows. Furthermore, the results are of relevance for many microscopic transport processes in nature and technology, such as bacteria transport or particle sorting.