Achieving
control over the distribution of biocides across the
thickness of polymer nanocomposite films is one of the largest challenges
to develop efficient antibacterial surfaces. In such applications,
it is key to maximize the biocide presence at the film top surface
to ensure contact with bacteria. Here, we make use of evaporation
driven colloidal self-assembly to control the vertical distribution
of biocides in polymer composite films cast from colloidal blends
of polymer and zinc oxide (ZnO) nanoparticles. We present a thorough
study which shows that the evaporation rate and ZnO volume fraction
have a strong impact on the final film architecture and on its wetting
and antibacterial properties. For high enough ZnO volume fraction,
the ZnO nanoparticles assemble in superstructures on top of the film,
which are higher the slower the evaporation rate used, and maximum
ZnO surface coverage achieved through slow film drying. At high ZnO
volume fraction (ϕ = 0.29), the zone of inhibition diameter
against Escherichia coli increases as evaporation
rate decreases, with the nanocomposite films having the strongest
antibacterial activity when formed at slow evaporation rate. We propose
a model for the formation of these colloidal superstructures based
on the segregation of large (polymer) and small (ZnO) particles during
drying, followed by the assembly of small particles around packed
large particles due to differences in the surface charge of the two
populations. Our work provides valuable guidelines for the design
and assembly of not only antibacterial colloidal films but also a
wider range of functional colloidal polymer films including abrasion
resistant, self-cleaning, and others.
Nitrate accumulated deep (>100
cm) in the regolith (soil and saprolite)
threatens groundwater quality, but most studies focus only on nitrate
nearer the surface (<100 cm). Surface soil management versus regolith
interactions affect deep nitrate leaching, but their combined impact
remains unclear. This study measured how deep nitrate accumulation
was affected by crop practices including orchard/cropland planting
years, regolith structure, and soil properties in highly weathered
subtropical red soils. Deep nitrate storage varied from 43.6 to 1116.3
kg ha–1. Regolith thickness was positively correlated
with nitrate storage (R
2 = 0.43, p < 0.05). Reticulated red clay (110–838 cm) had
81% of the accumulated nitrate and overlapped with 79% of the nitrate
accumulation layer. All of the nitrate accumulation parameters (except
for peak depth (PD)) generally increased with the planting years.
The difference in peak nitrate concentration (9.0–20.0 mg kg–1) with planting year gradient (3–58 years)
varied by 2.2 times, and the difference in nitrate storage (43.6–425.7
kg ha–1) varied by 9.8 times. Texture and pH explain
41.6% of the variation in nitrate concentration. As soil management
practices interact with deeper regolith to control the spatial pattern
of nitrate accumulation, vulnerable regions could be identified to
avoid high accumulation.
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