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

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.

Keywords: diffusiophoresis; exclusion zone; repulsive van der Waals; water.

Figure 1

Image produced by subtracting the natural logarithm of the neutron attenuation in the distilled water filled cell with and without two strips of Nafion. The yellow outline shows the region of interest for creating the 3D surface plot shown on the right.

Figure 2

(left) Homogeneous case. (right) Heterogeneous case leading to diffusiophoresis.