Nanodrag

Nanodrag is a term used to describe dissipative forces experienced by nanoscale objects moving through fluids or within confined spaces. It encompasses hydrodynamic drag on nanoparticles, as well as additional contributions from electrokinetic and structural forces that become significant at nanometer scales.

Mechanisms: In nanoscale regimes, drag deviates from classical Stokes drag due to boundary slip, fluid layering near surfaces, and the influence of confinement. The presence of electric double layers around charged particles and in pores can produce electroviscous drag. Thermal fluctuations (Brownian motion) interact with shear to shape effective drag, and molecular-scale ordering of water near surfaces can alter effective viscosity.

Measurements and modeling: Drag forces are inferred from translational or rotational motion of particles using optical tweezers, atomic force microscopy, nanopore experiments, or microfluidic flows. Theoretical models modify the Stokes equation with slip length parameters, confinement corrections, and electrokinetic terms; at extreme nanoscale, molecular dynamics simulations supplement continuum approaches.

Applications: Understanding nanodrag informs nanofluidic device design, targeted drug delivery, nanopore sensing, energy harvesting, and sensor technologies where precise control of transport at the nanoscale is needed.

Challenges: Open questions include how to define and measure slip lengths consistently, how to bridge continuum and molecular descriptions, and how ionic strength, surface chemistry, and temperature alter drag. Experimental measurements are sensitive to noise and surface preparation.