Multiphysics of microfluidics and nanofluidics

Microfluidics and nanofluidics involves the study and development of new devices and techniques with multiple phenomena occurring simultaneously. These phenomena include transport of mass, charge, momentum, and energy with critical length scales of 1–100 μm for microfluidics and 1–100 nm for nanofluidics governing the transport.¹ The term “transport” in the context of microfluidics and nanofluidics also encompasses chemical and biological reactive processes. Continued advances in microscale and nanoscale fabrication, emerging materials, imaging, and computational capacity make possible the evaluation and use of molecular-scale mechanisms enabling new technologies in energy generation and storage,² water treatment,² and health-related³ applications. It is this interplay between multiple physico-chemical phenomena that provide the basis for microfluidics and nanofluidics being an inherently multiphysics discipline.⁴ In this topical collection, contributions from various researchers around the world emphasize the multiphysics aspects and provide new research discoveries.

Several fundamental scientific principles for the physical phenomena that govern microfluidics and nanofluidics have been in existence for over two centuries. Yet, the formal definition for the discipline as it exists today can be attributed to the advent of micro-total analytical systems (μ-TAS; now also referred to as Lab on a Chip) nearly 30 years ago.^5,6 Advances in nanomaterials, micro- and nanofabrication, and a growing understanding of transport at these length scales have therefore spawned many microdevices and nanodevices as essential tools in energy generation and storage, environmental remediation, and medical diagnostics and treatment—with impacts on biosensors, imaging systems, fuel cells, drug delivery, diagnostic tools, and molecular separations among a growing list of applications. Therefore, several reviews have described the progress in the state of art, physical insights, and innovative applications to the multiphysics phenomena observed with microscale and nanoscale transport.^7–12 

Lin et al. used dissipative particle dynamics to model DNA deposition with the aim of better understanding the immobilization of DNA molecules to a solid-surface.¹³ The ability to tether biomolecules (e.g., DNA and proteins) to surfaces continues to be an important topic of scientific and technological progress. Generating flow patterns, surface templating both through physical and chemical means, and use of geometrical features remains an active area of research for microscale and nanoscale fabrication strategies. While Ince et al. reported on the curvature of microchannels for inertial focusing of micron-scale particles for different flow rates,¹⁴ Tang et al. generated acoustic streaming vortices by arranging acoustic sources asymmetrically around the a microfluidic chamber.¹⁵ In a similar vein, Bohm and Runge reported on a simulation method to calculate interface shapes influenced by electric-fields.¹⁶ The methodology reported could have applications in droplet-based or electrowetting effects in microfluidics. Separations are an essential part of recent advances in microfluidics and nanofluidics with researchers reporting on sub-continuum transport¹⁷ and ultrafast transport.¹⁸ Furthering critical understanding, Rayabharam and Aluru describe the transport of protons and hydroxide ions across a 2D material for quantum-dominated transport towards water desalination.¹⁹ Similarly, Goyal and Datta report on separating nanoparticles by use of engineered grooves to alter surface topography.²⁰ 

Clearly, given the breadth of topics possible under the banner of multiphysics phenomena in microfluidics and nanofluidics, this topical collection provided a focus on the use of multiphysics, including flow, acoustic, and electrical fields, as well as their coupling in confined microscale and nanoscale systems, to design, fabricate, process, and manipulate materials and their respective properties for microfluidics and nanofluidics.

The guest editors thank all the authors who contributed to this Topical Collection of the Journal of Applied Physics. The guest editors also acknowledge the support from the AIP Publishing staff and the Associate Editor team of Dr. Bilek and Dr. Brousseau in managing the publication process of this topical collection.