104 research outputs found
Surface bubble nucleation phase space
Recent research has revealed several different techniques for nanoscopic gas
nucleation on submerged surfaces, with findings seemingly in contradiction with
each other. In response to this, we have systematically investigated the
occurrence of surface nanobubbles on a hydrophobised silicon substrate for
various different liquid temperatures and gas concentrations, which we
controlled independently. We found that nanobubbles occupy a distinct region of
this phase space, occurring for gas concentrations of approximately 100-110%.
Below the nanobubble phase we did not detect any gaseous formations on the
substrate, whereas micropancakes (micron wide, nanometer high gaseous domains)
were found at higher temperatures and gas concentrations. We moreover find that
supersaturation of dissolved gases is not a requirement for nucleation of
bubbles.Comment: 4 pages, 4 figure
To see or not to see: Imaging surfactant coated nano--particles using HIM and SEM
Nano--particles are of great interest in fundamental and applied research.
However, their accurate visualization is often difficult and the interpretation
of the obtained images can be complicated. We present a comparative scanning
electron microscopy and helium ion microscopy study of
cetyltrimethylammonium--bromide (CTAB) coated gold nano--rods. Using both
methods we show how the gold core as well as the surrounding thin CTAB shell
can selectively be visualized. This allows for a quantitative determination of
the dimensions of the gold core or the CTAB shell. The obtained CTAB shell
thickness of 1.0 nm--1.5 nm is in excellent agreement with earlier results
using more demanding and reciprocal space techniques.Comment: revised versio
Wetting of two-component drops: Marangoni contraction versus autophobing
The wetting properties of multi-component liquids are crucial to numerous
industrial applications. The mechanisms that determine the contact angles for
such liquids remain poorly understood, with many intricacies arising due to
complex physical phenomena, for example due to the presence of surfactants.
Here, we consider two-component drops that consist of mixtures of vicinal
alkane diols and water. These diols behave surfactant-like in water. However,
the contact angles of such mixtures on solid substrates are surprisingly large.
We experimentally reveal that the contact angle is determined by two separate
mechanisms of completely different nature, namely Marangoni contraction
(hydrodynamic) and autophobing (molecular). It turns out that the length of the
alkyl tail of the alkane diol determines which mechanism is dominant,
highlighting the intricate coupling between molecular physics and the
macroscopic wetting of complex fluids
Electrolytically Generated Nanobubbles on HOPG Surfaces
Electrolysis of water is employed to produce surface nanobubbles on highly
orientated pyrolytic graphite (HOPG) surfaces. Hydrogen (oxygen) nanobubbles
are formed when the HOPG surface acts as negative (positive) electrode.
Coverage and volume of the nanobubbles enhance with increasing voltage. The
yield of hydrogen nanobubbles is much larger than the yield of oxygen
nanobubbles. The growth of the individual nanobubbles during the electrolysis
process is recorded in time with the help of AFM measurements and correlated
with the total current. Both the size of the individual nanobubbles and the
total current saturate after typical 1 minute; then the nanobubbles are in a
dynamic equilibrium, meaning that they do not further grow, in spite of ongoing
gas production and nonzero current. The surface area of nanobubbles shows a
good correlation with the nanobubble volume growth rate, suggesting that either
the electrolytic gas emerges directly at the nanobubbles' surface, or it
emerges at the electrode's surface and then diffuses through the nanobubbles'
surface. Moreover, the experiments reveal that the time constants of the
current and the aspect ratio of nanobubbles are the same under all conditions.
Replacement of pure water by water containing a small amount of sodium chloride
(0.01 M) allows for larger currents, but qualitatively gives the same results.Comment: Langmuir, in pres
Characterization of ftsZ Mutations that Render Bacillus subtilis Resistant to MinC
Background: Cell division in Bacillus subtilis occurs precisely at midcell. Positional control of cell division is exerted by two mechanisms: nucleoid occlusion, through Noc, which prevents division through nucleoids, and the Min system, where the combined action of the MinC, D and J proteins prevents formation of the FtsZ ring at cell poles or recently completed division sites.
Methodology/Principal Findings: We used a genetic screen to identify mutations in ftsZ that confer resistance to the lethal overexpression of the MinC/MinD division inhibitor. The FtsZ mutants were purified and found to polymerize to a similar or lesser extent as wild type FtsZ, and all mutants displayed reduced GTP hydrolysis activity indicative of a reduced polymerization turnover. We found that even though the mutations conferred in vivo resistance to MinC/D, the purified FtsZ mutants did not display strong resistance to MinC in vitro.
Conclusions/Significance: Our results show that in B. subtilis, overproduction of MinC can be countered by mutations that alter FtsZ polymerization dynamics. Even though it would be very likely that the FtsZ mutants found depend on other Z-ring stabilizing proteins such as ZapA, FtsA or SepF, we found this not to be the case. This indicates that the cell division process in B. subtilis is extremely robust.
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