@article{han_shields_bharti_arratia_velev_2020, title={Active Reversible Swimming of Magnetically Assembled "Microscallops" in Non-Newtonian Fluids}, volume={36}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.9b03698}, abstractNote={Miniaturized devices capable of active swimming at low Reynolds numbers are of fundamental importance and possess potential biomedical utility. The design of colloidal microswimmers requires not only miniaturizing reconfigurable structures, but also understanding their interactions with media at low Reynolds numbers. We investigate the dynamics of "microscallops" made of asymmetric magnetic cubes, which are assembled and actuated using magnetic fields. One approach to achieve directional propulsion is to break the symmetry of the viscous forces by coupling the reciprocal motions of such microswimmers with the nonlinear rheology inherent to non-Newtonian fluids. When placed in shear-thinning fluids, the local viscosity gradient resulting from non-uniform shear stresses exerted by time-asymmetric strokes of the microscallops generates propulsive thrust through an effect we term "self-viscophoresis". Surprisingly, we found that the direction of propulsion changes with the size and structure of these assemblies. We analyze the origins of their directional propulsion and explain the variable propulsion direction in terms of multiple counterbalancing domains of shear dissipation around the microscale structures. The principles governing the locomotion of these microswimmers may be extended to other reconfigurable microbots assembled from colloidal scale units.}, number={25}, journal={LANGMUIR}, author={Han, Koohee and Shields, C. Wyatt and Bharti, Bhuvnesh and Arratia, Paulo E. and Velev, Orlin D.}, year={2020}, month={Jun}, pages={7148–7154} }
 @article{ohiri_han_shields_velev_jokerst_2018, title={Propulsion and assembly of remotely powered p-type silicon microparticles}, volume={6}, ISSN={["2166-532X"]}, DOI={10.1063/1.5053862}, abstractNote={In this letter, we discuss how to prepare millions of uniform p-type silicon (Si) microparticles using top-down fabrication processes and how to remotely control their dynamics when they are suspended in water and powered by external alternating current (AC) electric fields. These microparticles present positively charged carrier types (majority carriers from boron atom doping in the intrinsic Si) and negatively charged carrier types (minority carriers from the free electrons in the Si lattice), which electrostatically affects their negatively charged surfaces and enables a variety of programmable behaviors, such as directional assembly and propulsion. At high AC electric field frequencies ( f > 10 kHz), the microparticles assemble by attractive dielectrophoretic polarization forces. At low electric field frequencies ( f ≤ 10 kHz), the microparticles propel by induced-charge electrophoretic flows. The ability to manipulate the electrostatic potential distribution within and around the microparticles (i.e., by controlling electronic carrier types through doping) is useful for designing a number of new dynamic systems and devices with precise control over their behaviors.}, number={12}, journal={APL MATERIALS}, author={Ohiri, Ugonna and Han, Koohee and Shields, C. Wyatt and Velev, Orlin D. and Jokerst, Nan M.}, year={2018}, month={Dec} }
 @misc{shields_velev_2017, title={The Evolution of Active Particles: Toward Externally Powered Self-Propelling and Self-Reconfiguring Particle Systems}, volume={3}, ISSN={["2451-9294"]}, DOI={10.1016/j.chempr.2017.09.006}, abstractNote={Active particles harvest energy from their environment to autonomously power their movement, organization, and reconfiguration. Much like molecular motors on the nanoscale to living organisms on the micro- and macroscale, active particles move by asymmetrically converting and directing energy to create local gradients of force. We discuss how active particles have evolved as a result of advances in particle fabrication by analyzing the relationship between their design and mechanism of self-propulsion. We constrain our focus to particles that transduce energy from externally applied fields because their movements can be directly controlled, they do not require chemical fuel, and they are potentially biocompatible. We provide a prospectus of the challenges that must be overcome if we are to secure their further evolution for use in chemical and biomedical applications such as remotely powered drug-delivery vehicles, efficient micromixers, the reconfiguration of assemblies for microsurgical devices, and swarms of particles for the next generation of self-healing materials.}, number={4}, journal={CHEM}, author={Shields, C. Wyatt and Velev, Orlin D.}, year={2017}, month={Oct}, pages={539–559} }