Transporting droplets through surface anisotropy

This review article examines digital microfluidic systems that manipulate droplets through surface anisotropy. These systems are categorized as surface tension driven or contact line driven. Surface tension driven systems include electrowetting on dielectric, Marangoni flow on microheater arrays, and chemical gradient surfaces, whereas contact line driven systems include anisotropic ratchet conveyors, nanostructured Parylene ratchets, and tilted pillar arrays. This article describes the operating principles and outlines the fabrication procedures for each system. We also present new equations that unify several previous models of contact line driven systems. The strengths and weaknesses of each system are compared, with a focus on their ability to perform the generation, switching, fusion, and fission of droplets. Finally, we discuss current and potential future applications of these systems.