1. Water spread over a patterned SiO2
substrate
INVESTIGATING SUBSTRATE WETTABILITY WITH MD SIMULATIONS
INTRODUCTION
In recent years nanotechnology has presented solutions for several
engineering problems. It is getting visibility with successful applications where the
interactions among materials occur at a dimension scale of less than 100nm. These
include the processors and memory chips in cell phones and computers, target
oriented drug release, high-density battery cells for energy storage, and many more.
The fabrication with materials at such a reduced scale, require specialized equipment,
which is expensive, have high operating cost and require long set up times. Hence, the
use of computer models can help optimize the design of the application and reduce the
cost and time expended with experiments.
OBJECTIVE
Investigate the effect of substrate material, topography, substrate proximity
with water and temperature allocation using Molecular Dynamics (MD) simulations.
METHODOLOGY
• Nanodroplet-substrate interaction modeled using NAMD and VMD;
• Models of 10nm droplets of water using a TIP3P structure (17,267 molecules);
• Flat gold and SiO2 substrates measuring 161Å x 161Å x 38Å and 283Å x 283Å x
48Å, respectively;
• SiO2 substrate with surface pattern of pillars with lateral dimensions of 25Å x 25Å,
with a height of 30Å, and were spaced 28Å from each other.
• Measure wettability using contact angles and stability based on RMSD.
• Vapor and liquid water is separated using density profiles.
RESULTS: REPLICATION CELL SIZE
ABSTRACT
Nanotechnology is being used to solve many engineering problems in the fields
of electronics, medicine and energy. However, nanomanufacturing and nano-
characterization of materials usually require time, specialized machinery and
laboratory facilities that are unavailable or expensive to operate. The use of
classical computer simulation of materials, such as finite element analysis has
limited representation at the nanoscale, because they fail to consider the atomic
configurations and forces. This research presents the use of Molecular
Dynamics (MD) modeling to characterize materials at the nanoscale. Atomistic
models of Gold and SiO2 were simulated to predict their wettability with nano
droplets of water. Results are presented on how the simulation environment
influences representation accuracy of the models. Analysis tools, such as
geometry measurements and average atomic position variability are used to
infer about the wettability of these materials with different surface pattern
topologies. The tools, analysis and results presented can be extended to
investigate other engineering problems that may have solutions at the
nanoscale.
Jhonatam Cordeiro1
Salil Desai2
Department of Industrial and Systems Engineering,
North Carolina A&T State University. Greensboro, NC
27411, US
1 jcrodrig@aggies.ncat.edu
2 sdesai@ncat.edu
RESULTS: TEMPERATURE ASSIGNMENT
CONCLUSIONS
• Simulations with 10nm water droplet on SiO2 showed the substrate is super
hydrophilic (contact angle of 0°).
• A SiO2 substrate with reduced surface exposure did not change the
wettability of the substrate. The water molecules fully spread in between the
pillars.
• The use of a replication cells in the z direction resulted in water nanodroplet
to be attracted to the neighboring top replication, illustrating the strength of
interaction between water and SiO2.
• When a temperature control of 373K was applied solely to the substrate the
droplet kept moving randomly around the substrate surface.
• When the temperature control was uniformly applied to the entire control
volume (droplet and substrate) the droplet maintained a fixed position and
presented increased evaporation.
• The simulation at higher temperature had shorter stabilization time and lower
final contact angles.
ACKNOWLEDGE NSF GRANT
The authors extend their gratitude to the US National Science
Foundation (NSF CMMI: Awards 0846562 and 1435649) for support towards
this research.
RESULTS: TEMPERATURE LEVEL
RESULTS: SUBSTRATE TOPOGRAPHY
0 ps 560 ps80 ps
0 0.5 1 1.5 2 2.5 3 3.5
110
120
130
140
150
160
170
time(ns)
Angle(degrees)
373K
293K
Contact angles of 10nm droplets
over a gold substrate
0 1 2 3 4 5
0
10
20
30
40
50
time (ns)
RMSD(A)
293K
373K
Water droplet over a gold substrate
with system equilibrated to 373K
10nm water droplet behavior over SiO2
substrate inside a replication cell of 285Å
x285Å x200Å in size
10nm water droplet behavior over SiO2
substrate inside a replication cell of
285Å x285Å x300Å in size
10nm water droplet spread over SiO2
substrate without replication cells.
vapor Liquid Water
Gold
Substrate
Density profile of 10nm water droplet over a gold substrate
Water droplet at 293K over a gold substrate heated to 373K
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0
10
20
30
40
50
60
time (ns)
RMSD(A)
Heated System
Heated Substrate
RMSD of droplets on a Heated
System and on a Heated Substrate
RMSD of 10nm water droplets over
gold substrate
Nano & Bio manufacturing Lab