5,712 research outputs found
Coarse-graining MARTINI model for molecular-dynamics simulations of the wetting properties of graphitic surfaces with non-ionic, long-chain and T-shaped surfactants
We report on a molecular dynamics investigation of the wetting properties of
graphitic surfaces by various solutions at concentrations 1-8 wt% of
commercially available non-ionic surfactants with long hydrophilic chains,
linear or T-shaped. These are surfactants of length up to 160 [\AA]. It turns
out that molecular dynamics simulations of such systems ask for a number of
solvent particles that can be reached without seriously compromising
computational efficiency only by employing a coarse-grained model. The MARTINI
force field with polarizable water offers a framework particularly suited for
the parameterization of our systems. In general, its advantages over other
coarse-grained models are the possibility to explore faster long time scales
and the wider range of applicability. Although the accuracy is sometimes put
under question, the results for the wetting properties by pure water are in
good agreement with those for the corresponding atomistic systems and
theoretical predictions. On the other hand, the bulk properties of various
aqueous surfactant solutions indicate that the micellar formation process is
too strong. For this reason, a typical experimental configuration is better
approached by preparing the droplets with the surfactants arranged in the
initial state in the vicinity of contact line. Cross-comparisons are possible
and illuminating, but equilibrium contanct angles as obtained from simulations
overestimate the experimental results. Nevertheless, our findings can provide
guidelines for the preliminary assessment and screening of surfactants. [See
pdf file for full abstract]Comment: Revised version. Publication: http://dx.doi.org/10.1063/1.4747827.
Material: https://sites.google.com/site/material4sim
Growth and decay of localized disturbances on a surfactant-coated spreading film
If the surface of a quiescent thin liquid film is suddenly coated by a patch of surface active material like a surfactant monolayer, the film is set in motion and begins spreading. An insoluble surfactant will rapidly attempt to coat the entire surface of the film thereby minimizing the liquid's surface tension. The shear stress that develops during the spreading process produces a maximum in surface velocity in the region where the moving film meets the quiescent layer. This region is characterized by a shock front with large interfacial curvature and a corresponding local buildup of surfactant which creates a spike in the concentration gradient. In this paper, we investigate the sensitivity of this region to infinitesimal disturbances. Accordingly, we introduce a measure of disturbance amplification and transient growth analogous to a kinetic energy that couples variations in film thickness to the surfactant concentration. These variables undergo significant amplification during the brief period in which they are convected past the downstream tip of the monolayer, where the variation in concentration gradient and surface curvature are largest. Once they migrate past this sensitive area, the perturbations weaken considerably and the system approaches a stable configuration. It appears that the localized disturbances of the type we consider here, cannot sustain asymptotic instability. Nonetheless, our study of the dynamics leading to the large transient growth clearly illustrates how the coupling of Marangoni and capillary forces work in unison to stabilize the spreading process against localized perturbations
Impaction of spray droplets on leaves: influence of formulation and leaf character on shatter, bounce and adhesion
This paper combines experimental data with simple mathematical models to
investigate the influence of spray formulation type and leaf character
(wettability) on shatter, bounce and adhesion of droplets impacting with
cotton, rice and wheat leaves. Impaction criteria that allow for different
angles of the leaf surface and the droplet impact trajectory are presented;
their predictions are based on whether combinations of droplet size and
velocity lie above or below bounce and shatter boundaries. In the experimental
component, real leaves are used, with all their inherent natural variability.
Further, commercial agricultural spray nozzles are employed, resulting in a
range of droplet characteristics. Given this natural variability, there is
broad agreement between the data and predictions. As predicted, the shatter of
droplets was found to increase as droplet size and velocity increased, and the
surface became harder to wet. Bouncing of droplets occurred most frequently on
hard to wet surfaces with high surface tension mixtures. On the other hand, a
number of small droplets with low impact velocity were observed to bounce when
predicted to lie well within the adhering regime. We believe this discrepancy
between the predictions and experimental data could be due to air layer effects
that were not taken into account in the current bounce equations. Other
discrepancies between experiment and theory are thought to be due to the
current assumption of a dry impact surface, whereas, in practice, the leaf
surfaces became increasingly covered with fluid throughout the spray test runs.Comment: 19 pages, 6 figures, accepted for publication by Experiments in
Fluid
Fluorescent visualization of a spreading surfactant
The spreading of surfactants on thin films is an industrially and medically
important phenomenon, but the dynamics are highly nonlinear and visualization
of the surfactant dynamics has been a long-standing experimental challenge. We
perform the first quantitative, spatiotemporally-resolved measurements of the
spreading of an insoluble surfactant on a thin fluid layer. During the
spreading process, we directly observe both the radial height profile of the
spreading droplet and the spatial distribution of the fluorescently-tagged
surfactant. We find that the leading edge of spreading circular layer of
surfactant forms a Marangoni ridge in the underlying fluid, with a trough
trailing the ridge as expected. However, several novel features are observed
using the fluorescence technique, including a peak in the surfactant
concentration which trails the leading edge, and a flat, monolayer-scale
spreading film which differs from concentration profiles predicted by current
models. Both the Marangoni ridge and surfactant leading edge can be described
to spread as . We find spreading exponents, and for the ridge peak and
surfactant leading edge, respectively, which are in good agreement with
theoretical predictions of . In addition, we observe that the
surfactant leading edge initially leads the peak of the Marangoni ridge, with
the peak later catching up to the leading edge
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