  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 and others have theorized 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.
