Discussion:

 

The coupling of molecular simulations and engineering principles as described here has the following advantages:

 

  1. 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.
  2. 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.
  3. 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:

 

  1. 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]. 
  2. 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.
  3. 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.
  4. 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.

 

 



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