Abstract
The existence of the exclusion zone (EZ), a layer of water in which plastic microspheres are repelled from hydrophilic surfaces, has now been independently demonstrated by several groups. A better understanding of the mechanisms which generate EZs would help with understanding the possible importance of EZs in biology and in engineering applications such as filtration and microfluidics. Here we review the experimental evidence for EZ phenomena in water and the major theories that have been proposed. We review experimental results from birefringence, neutron radiography, nuclear magnetic resonance, and other studies. Pollack theorizes that water in the EZ exists has a different structure than bulk water, and that this accounts for the EZ. We present several alternative explanations for EZs and argue that Schurr’s theory based on diffusiophoresis presents a compelling alternative explanation for the core EZ phenomenon. Among other things, Schurr’s theory makes predictions about the growth of the EZ with time which have been confirmed by Florea et al. and others. We also touch on several possible confounding factors that make experimentation on EZs difficult, such as charged surface groups, dissolved solutes, and adsorbed nanobubbles.
Highlights
Gerald Pollack’s group has provided many convincing experimental demonstrations of an exclusion zone (EZ) in water whereby particles such as plastic microspheres are repelled from a surface
Giudice et al propose that quantum electrodynamics calculations are necessary to understand the phase transition which occurs in EZ water [42,44,45], a claim that is forcefully disputed in a detailed work by Bier and Pravaca [46]
We presented new results from neutron beam radiography which do not support the idea of a higher density phase and discussed how flaws were discovered in Pollack’s birefringence measurements, which have been used as evidence for a structure change
Summary
Gerald Pollack’s group has provided many convincing experimental demonstrations of an exclusion zone (EZ) in water whereby particles such as plastic microspheres are repelled from a surface. For the case of highly hydrophillic surfaces these findings have 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. In this work we present a review of exclusion zone phenomena, including many recent experimental studies, and describe several mechanisms by which the EZ phenomena can arise. EZ phenomena may have important engineering applications in water filtration, reducing biofouling [16], and microfluidics [6]. EZ phenomena have obvious importance to understanding biological systems and resolving outstanding questions about “biological water” [17]
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