Since the mid-twentieth century, the concept of transporting matter through a ratchet effect has intrigued researchers for many years. Essentially, this mechanism utilizes asymmetries in space or time to convert external fluctuations into a directed force and occurs in numerous biological and physical processes, including the translocation of proteins, molecular motors, and enzymes [1]. On a practical level, the ratchet effect provides an efficient method to extract useful work from thermodynamic systems and has been explored in diverse systems including dusty plasma, nanoparticles, artificial spin ice, domain walls, vortices in high Tc superconductors, and active matter [2].
Microscopic colloidal particles can be used as an experimentally accessible model system to investigate ratchet transport effects, with a diverse range of mechanisms. However, most of them proposed so far concern the use of a Newtonian fluid as carrier [3].
Shear thinning fluids represent a class of non-Newtonian media that display a decrease in apparent viscosity as the shear rate increases. In this work, we experimentally prove a deterministic ratchet effect in such media, which enables directed transport of micron-sized magnetic particles [4]. Particles are moved through a square wave magnetic force acting in opposite directions alternatively, always satisfying the condition F1·T1=F2·T2. This modulation is designed in such a way that it does not produce any average speed when the particles are dispersed in a Newtonian fluid (e.g., water). However, in a dilute biopolymer solution, we observe the emergence of a net colloidal current when the forcing wave is composed of different amplitudes and time duration within a single period. The shear thinning nature of the dispersing medium non-linearly raises the mean speed for strong forces, breaking the spatial symmetry of the particle displacement and generating a net colloidal transport.
We complement our results with numerical simulations that can explain the underlying physical mechanism, showing good agreement with the experimental results. Our technique to ratchet magnetic particles could be potentially extended in active microrheology to probe other non-Newtonian, complex fluids, and to infer the non-linear properties of viscoelastic materials.
Since the mid-twentieth century, the concept of transporting matter through a ratchet effect has intrigued researchers for many years. Essentially, this mechanism utilizes asymmetries in space or time to convert external fluctuations into a directed force and occurs in numerous biological and physical processes, including the translocation of proteins, molecular motors, and enzymes [1]. On a practical level, the ratchet effect provides an efficient method to extract useful work from thermodynamic systems and has been explored in diverse systems including dusty plasma, nanoparticles, artificial spin ice, domain walls, vortices in high Tc superconductors, and active matter [2].
Microscopic colloidal particles can be used as an experimentally accessible model system to investigate ratchet transport effects, with a diverse range of mechanisms. However, most of them proposed so far concern the use of a Newtonian fluid as carrier [3].
Shear thinning fluids represent a class of non-Newtonian media that display a decrease in apparent viscosity as the shear rate increases. In this work, we experimentally prove a deterministic ratchet effect in such media, which enables directed transport of micron-sized magnetic particles [4]. Particles are moved through a square wave magnetic force acting in opposite directions alternatively, always satisfying the condition F1·T1=F2·T2. This modulation is designed in such a way that it does not produce any average speed when the particles are dispersed in a Newtonian fluid (e.g., water). However, in a dilute biopolymer solution, we observe the emergence of a net colloidal current when the forcing wave is composed of different amplitudes and time duration within a single period. The shear thinning nature of the dispersing medium non-linearly raises the mean speed for strong forces, breaking the spatial symmetry of the particle displacement and generating a net colloidal transport.
We complement our results with numerical simulations that can explain the underlying physical mechanism, showing good agreement with the experimental results. Our technique to ratchet magnetic particles could be potentially extended in active microrheology to probe other non-Newtonian, complex fluids, and to infer the non-linear properties of viscoelastic materials.

References:
[1] P. Reimann. Brownian motors: noisy transport far from equilibrium. Phys. Reports 361, 57 (2002)
[2] C. J. O. Reichhardt and C. Reichhardt. Ratchet Effects in Active Matter Systems. Ann. Rev. Cond. Matt. Phys. 8, 51 (2017)
[3] B. B. Yellen, O. Hovorka, and G. Friedman, Arranging Matter by Magnetic Nanoparticle Assemblers, Proc. Nat. Acad. Sci. U.S.A. 102, 8860 (2005)
[4] G. Camacho, A. Rodríguez-Barroso, O. Martínez-Cano, J. R. Morillas, P. Tierno and J. de Vicente, Experimental Realization of a Colloidal Ratchet Effect in a non-Newtonian, Phys. Rev. Applied 19, L021001 (2023)
 

Unsteady magnetic fields,Non-stationary fields,Emerging systems,Field-driven colloids