Seclusion of molecular layers in a confined simple liquid

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Seclusion of molecular layers in a confined simple liquid

Ali Gooneie, Tobias E. Balmer, and Manfred Heuberger

Phys. Rev. Research 2, 022026(R) – Published 4 May 2020

Abstract

We provide theoretical evidence by using molecular dynamics that a nanoconfined film of octamethylcyclotetrasiloxane divides into manifolds of secluded thermodynamic substates. We find equivalence between the splitting into substates and the formation of molecular layers. The seclusion of layers is validated in drainage experiments using an extended surface forces apparatus (eSFA). Furthermore, per-molecule evaluations of the configurational entropy provide evidence for an increased molecular packing upon confinement. The increasing trends in both layer seclusion and molecular packing with confinement explain the exponentially increasing surface forces measured in eSFA.

Received 11 November 2019
Revised 6 February 2020
Accepted 9 April 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.022026

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Confinement Phase transitions

Multilayer thin films Thin films

Fluid Dynamics

Authors & Affiliations

Ali Gooneie ^1,*, Tobias E. Balmer^2,†, and Manfred Heuberger ^1,2,‡

¹Laboratory of Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland
²Laboratory for Surface Science and Technology, ETH Zürich, Zürich, Switzerland

^*Corresponding author: ali.gooneie@empa.ch
^†Present address: Weibel AG, Bern, Switzerland.
^‡Corresponding author: manfred.heuberger@empa.ch

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Issue

Vol. 2, Iss. 2 — May - July 2020

Subject Areas

Chemical Physics

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Images

Figure 1
(a) Oscillatory surface forces in confined OMCTS measured by eSFA. Red points are measured during approach and blue points are measured during retraction succeeding different actuator inversion points. Points A and B correspond to the thicknesses of the parallel sections used in MD simulations. (b) The step size $Δ_{FTT}$ (filled symbols) and the net compression of the fluid film $Δ_{fc}$ (open symbols) during multiple FTTs. $Δ_{fc}$ is calculated from the total contact compression $Δ_{cc}$ and is corrected for the optical mica compression. See part (a) and Fig. S3 in the SM [30] for the definitions of these terms. The different symbols in (b) correspond to force measurements from different experiments showing a good reproducibility.
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Figure 2
(a) Schematics of double-wedge geometry and its dimensions used in MD simulations of mica-OMCTS. Note that the simulated depth of the wedges was constant everywhere and set to 9 nm. (b) Layered configuration of OMCTS molecules at equilibrium from MD. Each molecule is colored according to its per-molecule variables of state, once by its entropy and once by its enthalpy. (c) Lateral averages of entropy and enthalpy across the film (bin size 0.5 nm). The mean entropy exhibits discernable peaks in the wedged sections close to the layering transitions where the central layers merge in part (b) (arrows). The intensity of the entropy peaks is influenced by the slope of the wedge and the wall-liquid interactions (see Fig. S5 in the SM [30]). The average entropy is higher in the thicker portions of the parallel film indicating a less ordered structure.
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Figure 3
(a) Thermodynamic state diagram analysis of molecules in the two parallel sections of the wedge geometry; (top) illustration of layer-specific color scheme and indexing convention introduced to identify individual substates (layers) and their manifolds; (bottom) condensed form of the state diagram marking the mean substate loci by filled squares $σ_{[n, N]}$ (representing each individual layer), as well as the cumulative loci of the whole films shown by white circles $Σ_{[N]}$ (representing each parallel section containing multiple layers). The state diagram displaying every molecule in its layer-specific color is shown in Fig. S7a in the SM [30]. (b) The strong modulation of the density profile across the film $ρ_{z}$ in the $Σ_{[4]}$ and $Σ_{[5]}$ film states indicate secluded molecular layers.
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Figure 4
(a) Sequence of OSC images recorded during the liquid drainage experiments. The FTT was nucleated at the perimeter, then a $Σ_{[N]}$ island (bright blue) was trapped and continuously drained into a sea of $Σ_{[N - 1]}$ (dark blue). (b) Stacked illustration of film thickness profiles during layered island drainage experiments from [ $N$ ] to [ $N$ −1] layers, measured at different times $t_{0}$ to $t_{5}$ corresponding to the OSC images shown in part (a). The final profile is the measured film thickness and the preceding profiles are graphically offset by 0.2 nm for better visibility. Based on OMCTS molecular size and simulations $N$ equals eight layers. (c) The changes in the square root of island area $\sqrt{A_{N} (t)}$ over time suggest that interlayer diffusion, which limits the drainage rate, is predominantly occurring at the island perimeter. Note that the drainage rate is slower than that in parts (a), (b) for more accurate measurement of the island area.
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