Discussion:
The coupling of molecular simulations and engineering
principles as described here has the following advantages:
- Atomic
detail accuracy on individual component performance: Since the properties of the components
have been determined through molecular simulations, the energy
contributions of the different components are as accurate as the force
field parameters used for their estimations. Due to the large number of atoms in each component, and
given that the molecular simulation is classical, there should be no
significant loss of accuracy in the calculations when the different energy
contributions of the system components are calculated separately and then
added. In practice, these
potential losses in accuracy are taken into account through the
appropriate engineering safety factors.
In order to have a robust working system it is not necessary to
know the exact position of every atom, as long as it is known with
certainty that the system will be able to reach the required states under
definite controlled conditions: in this case, the valve position (on/off)
for specific pH levels of the surrounding environment.
- Scale-up
flexibility: Since
the energy curves of the components of the system are smooth varying
functions of the relevant parameters (curvature and dimensions), they can
be easily incorporated into multiple correlation functions, which allow
the design of devices of various length scales (within the ranges for
which the data were obtained) without having to repeat the calculations
for each individual device.
- Lower
computational cost, especially during prototype development: The approach presented here
requires molecular simulations only during the initial characterization of
the components of the system. Once
that information is available, modeling of the system becomes a very
simple classical physics exercise, which can be completed with negligible
computational cost. As research
progresses in fundamental areas which govern the behavior of the valve,
such as controlled charge distributions in charged monolayers or response
times to changes in pH (monolayer pH curves), these can be easily
incorporated into the classical engineering model without significantly
increasing the computational cost of prototype development. This provides the design teams with a
very accurate and powerful, yet efficient tool for fast prototyping in the
laboratory.
Areas of future research:
As mentioned above, there are several fundamental questions
which need to be answered in order to obtain accurate ab initio predictions of
the behavior of molecular devices that utilize functionalized cantilevers to
generate motion. We also point out that
there are other ways of generating motion in this device for which the current
analysis is still valid.
Areas that need immediate attention to fully characterize
these devices include:
- Monolayer
solvation properties: This
includes the prediction of charge distributions for the acid monolayers as
a function of the cantilever geometry (curvature and dimensions), and
environmental factors (pH, and concentrations and types of species present
in the solution in which the monolayer is immersed). The solvation properties of a monolayer
are expected to be different than those of the individual acid molecules
in solution because (1) the motion of the adsorbed molecules is
restricted, (2) the molecules are packed relatively close together, which
doesn't allow the solvent to freely penetrate between the molecules, and
(3) the change in energy, with respect to surface charge, depends on the
geometry and magnitude of the organic monolayer surface [10].
- Dynamic
testing: This includes
the study of the response times of the system to changes in the parameters
that generate motion. It can be
anticipated that several local and global factors will influence the
dynamic performance of the device.
Some of the local factors include the speed of change of the local
environment of each molecule (concentration distribution of the different
species present), the speed with which charges disperse through the
monolayer molecules, and the speed with which the cantilever material
yields to deformation. Some of the
global factors are the transport phenomena (either natural or forced) that
govern the speed with which the externally-induced changes made to the
environment reach the device, and the speed with which the local
distribution of the key parameters, such as ion concentrations, change as
a function of the global changes made to the system.
- Molecular
transport phenomena: This
includes the study of momentum transfer and diffusion in constrained
regions such as the inside of a SWCNT [19]. This will also require reconciling the
behavior of classical continuum systems with molecular systems, perhaps
through the development of new models and parameters to characterize
molecular behaviors, which are analogous to the traditional models and
parameters used in continuum approximations, such as Newton's law of
viscosity or Fick's laws of diffusion.
- Other
means of generating motion: Due to the anticipated lag time between the pH changes made
to the environment surrounding the device and the actual deflection of the
cantilever, the current design is not necessarily of the most responsive
type. This characteristic makes it
reliable only for on/off systems but not necessarily for continuous flow
control systems. There have been
significant developments in nanoelectronics (mainly nanowires and
switches), which can provide improved ways of generating motion that would
allow faster response times and a greater degree of positional control of
the device. Electronic operation of
the system could be accomplished by replacing the acid monolayer on the
cantilever with a metallic layer, which can be charged and discharged
through an electronic circuit. Two
additional advantages of nanoelectronic systems are that (1) they do not
require the solution environment, and (2) they are better able to
communicate with and be acted upon by external systems through electrical
signals. Electrical signals in
turn, offer a much more efficient and ubiquitous way of communication,
which can combine input and output signals in the same communication
channel.
Conclusions:
We have presented the design and static characterization of
a chemically feasible nanomechanical fluid control valve, 32.5 to 55 nm in
length, that utilizes a functionalized silicon cantilever to generate
motion. The device can be controlled
through variations in the pH of the surrounding environment. We have also illustrated the steps and
advantages of an engineering design procedure that combines molecular mechanics
with classical engineering in the design of nanomechanical systems, thus
overcoming the major limitations of both.
Finally, we have identified the key fundamental areas of future research
in the development of nanosystems where motion is generated through the deflection
of cantilever beams.