Exclusion-Zone Dynamics Explored with Microfluidics and Optical Tweezers

Abstract

Abstract:The exclusion zone (EZ) is a boundary region devoid of macromolecules and microscopic particles formed spontaneously in the vicinity of hydrophilic surfaces. The exact mechanisms behind this remarkable phenomenon are still not fully understood and are debated. We measured the short-and long-time-scale kinetics of EZ formation around a Nafion gel embedded in specially designed microfluidic devices. The time-dependent kinetics of EZ formation follow a power law with an exponent of 0.6 that is strikingly close to the value of 0.5 expected for a diffusion-driven process. By using optical tweezers we show that exclusion forces, which are estimated to fall in the sub-pN regime, persist within the fully-developed EZ, suggesting that EZ formation is not a quasi-static but rather an irreversible process. Accordingly, the EZ-forming capacity of the Nafion gel could be exhausted with time, on a scale of hours in the presence of 1 mM Na 2 HPO 4 . EZ formation may thus be a non-equilibrium thermodynamic cross-effect coupled to a diffusion-driven transport process. Such phenomena might be particularly important in the living cell by providing mechanical cues within the complex cytoplasmic environment. OPEN ACCESSEntropy 2014, 16 4323

Non-negligible Water-permeance through Nanoporous Ion Exchange Medium

“…surface charge density, porosity, Donnan equilibrium quantities, etc ) concerning the nanoporous material as well as the nanoscale ion exchange process. Hence, the natural ion depletion phenomenon induced by the ion exchange has been drawn significant attentions recently 22 , 25 – 31 , 33 , 34 . Despite the fact that the proton inside the medium is being dissociated by the absorbed water from the outer solution as shown in Fig.…”

Exclusion Zone Phenomena in Water—A Critical Review of Experimental Findings and Theories

“…Using laser tweezers, a forcefield has been measured inside the exclusion zone. Two independent experiments have found that the magnitude of the repulsive force decays as a function of distance from the surface in a manner consistent with the diffusiophoresis theory [ 1 , 3 ]. The presence of a force decaying from the surface is inconsistent with the theory that a new phase forms in the exclusion zone.…”

“…Further experiments and a computational study using COMSOL Multiphysics simulation by Esplandiu et al lend further support to the findings of Florea et al [ 11 ]. Huszár et al note that the growth of the exclusion zone with times follows a power law with an exponent of 0.6, very close to the exponent of 0.5 expected for a diffusion-driven process [ 3 ]. Using laser tweezers, a forcefield has been measured inside the exclusion zone.…”

“…For the case of highly hydrophillic surfaces these findings have now been reproduced by several independent research groups [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ] and constitute a genuine physical phenomena which is in need of a theoretical explanation. A few experimenters have reported EZs near metal surfaces [ 12 , 13 , 14 ], and one has reported it for cellulose [ 15 ].…”

“…Huszár et al have investigated two other possible explanations for EZ-formation [ 3 ]. Dissolution of Nafion, during which polymer strands diffusing out of the gel push the beads away from the surface.…”