Real-time tracking of ionic nano-domains under shear flow

Abstract

The behaviour of ions at solid–liquid interfaces underpins countless phenomena, from the conduction of nervous impulses to charge transfer in solar cells. In most cases, ions do not operate as isolated entities, but in conjunction with neighbouring ions and the surrounding solution. In aqueous solutions, recent studies suggest the existence of group dynamics through water-mediated clusters but results allowing direct tracking of ionic domains with atomic precision are scarce. Here, we use high-speed atomic force microscopy to track the evolution of Rb+, K+, Na+ and Ca2+ nano-domains containing 20 to 120 ions adsorbed at the surface of mica in aqueous solution. The interface is exposed to a shear flow able to influence the lateral motion of single ions and clusters. The results show that, when in groups, metal ions tend to move with a relatively slow dynamics, as can be expected from a correlated group motion, with an average residence timescale of ~ 1–2 s for individual ions at a given atomic site. The average group velocity of the clusters depends on the ions’ charge density and can be explained by the ion’s hydration state. The lateral shear flow of the fluid is insufficient to desorb ions, but indirectly influences the diffusion dynamics by acting on ions in close vicinity to the surface. The results provide insights into the dynamics of ion clusters when adsorbed onto an immersed solid under shear flow.

Nanoscale probing of local dielectric changes at the interface between solids and aqueous saline solutions

“…High-speed AFM measurements can reach a temporal resolution of micro-to milliseconds [34][35][36] with most measurements still taking tens of seconds. 7,13,19,37 In contrast, computer simulations 30,[38][39][40] and estimates from the bulk diffusion coefficient 41,42 suggest residence times and interfacial dynamics on the scale of nanoseconds for ions and moving molecules. This signicant temporal gap with existing AFM capabilities cannot be solved by brute force since it would require mechanical parts moving at hundreds of MHz in a controlled manner.…”

“…The dynamics of ions and charged molecules at solid-liquid interfaces is central to numerous phenomena in science and technology. Examples span from charge transfer along biological membranes [1][2][3] to electrochemical processes 4,5 and energy storage 6 , the growth of minerals [7][8][9] , nanofluidics 10,11 and lubrication 12,13 . Experimentally, deriving quantitative information about the motion of charges at and along interfaces can often be challenging.…”

“…However, most AFM measurements are temporally limited by the mechanical response of the probe. High-speed AFM measurements can reach a temporal resolution of micro-to milliseconds [34][35][36] with most measurements still taking tens of seconds 7,13,19,37 . In contrast, computer simulations 30,[38][39][40] and estimates from bulk diffusion coefficient 41,42 suggest residence times and interfacial dynamics on the scale of nanoseconds for ions and moving molecules.…”

“…Examples span from charge transfer along biological membranes [1][2][3] to electrochemical processes 4,5 and energy storage, 6 the growth of minerals, 7-9 nanouidics 10,11 and lubrication. 12,13 Experimentally, deriving quantitative information about the motion of charges at and along interfaces can oen be challenging. Firstly, solid-liquid interfaces are intrinsically nanoscale objects.…”

“…Examples span from charge transfer along biological membranes 1–3 to electrochemical processes 4,5 and energy storage, 6 the growth of minerals, 7–9 nanofluidics 10,11 and lubrication. 12,13…”