What is it about?
A method to circumscribe a sphere around a collection of liquid molecules is developed and tested. The methodology can be implemented and used for projects on studding wetting and especially spreading, as it enables analysis of individual snapshots and thus wetting dynamics.
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Why is it important?
Studying spreading in particular is difficult because of its dynamic nature. By considering individual simulation snapshots it is possible to extract insights on wetting process (contact angle as a function of time, for example). Additionally, contact angle uncertainty in some technological applications like hydrogen or carbon dioxide geo-storage is an open problem. A method to analyze localized in time groups of snapshots is useful for these studies as well.
Perspectives
Presented method is unique, as it can be used to study spreading. In spreading the contact angle evolution as a function of time is vital. Molecular dynamics approaches to estimate the contact angle are based on long production runs after the equilibrium is reached. It is thus impossible to study the dynamics of wetting, i.e. spreading. The proposed method offers a possibility of estimating the contact angle using very few simulation snapshots, ultimately only one. The approach thus opens a way to have contact angle estimates at number of time intervals along a molecular dynamics trajectory. Moreover, it can also be used as an alternative or in conjunction with already available methods to study wetting, after the contact angle reaches its steady state value.
Aleksandr Abramov
Heriot-Watt University
Read the Original
This page is a summary of: Analysis of individual molecular dynamics snapshots simulating wetting of surfaces using spheroidal geometric constructions, The Journal of Chemical Physics, August 2019, American Institute of Physics,
DOI: 10.1063/1.5113852.
You can read the full text:
Resources
Application of the CLAYFF and the DREIDING Force Fields for Modeling of Alkylated Quartz Surfaces
To extend applicability and to overcome limitations of combining rules for nonbond potential parameters, in this study, CLAYFF and DREIDING force fields are coupled at the level of atomic site charges to model quartz surfaces with chemisorpt hydrocarbons. Density functional theory and Bader charge analysis are applied to calculate charges of atoms of the OC bond connecting a quartz crystal and an alkyl group. The study demonstrates that the hydrogen atom of the quartz surface hydroxyl group can be removed and its charge can be redistributed among the oxygen and carbon atoms of the OC bond in a manner consistent with the results calculated at the density functional level of theory. Augmented with modified charges of the OC bond, force fields can then be applied to a practical problem of evaluation of the contact angle of a water droplet on alkylated quartz surfaces in a carbon dioxide environment, which is relevant for carbon geo-sequestration and in a broader context of oil and gas recovery. Alkylated quartz surfaces have been shown to be extremely hydrophobic even when the surface density of hydroxyl groups is close to the highest naturally observed density of 6.2 OH groups per square nanometer.
Wettability of Fully Hydroxylated and Alkylated (001) α-Quartz Surface in Carbon Dioxide Atmosphere
Wettability of alkylated quartz surfaces is of primary importance in several technological applications, including the development of oil and gas reservoirs and carbon geo-sequestration. It is intuitively understood and experimentally confirmed that hydroxylated quartz surfaces are hydrophilic. By gradually saturating a hydroxylated (001) α-quartz surface with pentyl groups, we show using molecular dynamics simulations that the surface can also exhibit extreme hydrophobicity. Within a range of surface pentyl group density from 0.29 to 3.18/nm2, the contact angle of a water droplet under 10 MPa pressure of carbon dioxide at 300 K changes from 10–20 to 180°. This study has shown that a complete description of wettability of alkylated quartz surfaces requires three contact angles—one at the tip level of pentyl groups and two at the level of the quartz surface. The latter two are the contact angle of the spherical droplet and the hidden contact angle of a water “skirt” formed between the tip level of pentyl groups and the quartz surface. Analysis of the hidden contact angle unveils a binary wettability, where the surface relatively abruptly transforms from hydrophilic (the contact angle is less than 90°) to hydrophobic (the contact angle is 180°) with an increase in surface pentyl group concentration.
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