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The Self-Propulsion of Droplets Along Gradients of Wettability in Fluidic Channels
Achievement/Results
Microscale systems implement biochemical and chemical process operations – separations, synthesis, sensing and diagnostics – on miniaturized chips using networks of microfluidic channels. The networks are used to orchestrate and sequence the fluid movements required to complete the process steps. On the horizon are systems using nanfluidic networks, which have the potential to undertake the processing operations on a single molecule level. Capillary forces largely control the motion of fluids in channels with cross-sectional dimensions of the order of nanometers to microns, and these forces can be used to design novel methods for fluid movement.
One scheme, which has attracted much attention, is to use wettability differences to self-propel aqueous droplets through the channel. In this concept, the inside surface of the channel is functionalized with chemical groups to tailor the surface polarity and water wettability (contact angle). An axial gradient of these groups is imprinted on the channel walls, and a droplet contacting the gradient moves towards the higher wettability, driven by the low contact angle at the leading edge of the drop relative to the larger angle at the trailing end. Experiments have shown, however, that as the drop moves, the contact angle at the front is larger than the equilibrium angle, and the contact angle at the back is smaller than at equilibrium. This contact angle hysteresis causes the drop motion to be arrested, and has presented a major difficulty in constructing wettability gradient schemes to self-propel droplets in fluidic conduits.
NSF IGERT Trainee John Halverson, working with CCNY faculty members Alex Couzis (Chemical Engineering), Joel Koplik (Physics), and Charles Maldarelli (Chemical Engineering), has developed a molecular dynamics program to model the interaction of water nanodroplets on surfaces with a prescribed polarity. The surfaces are functionalized with self assembled monolayers which consist of amphiphiles with hydrophobic chains. One end of the hydrophobic chain is attached to the surface and the opposite end displays a terminal group with a particular polarity which serves to tailors the surface wettability. The group has considered monolayers with varying mixtures of amphiphiles terminated in either hydroxyl and methyl groups, to provide a range of surface polarities.
For surfaces with a spatially uniform composition of methyl and hydroxyl groups, the molecular dynamics calculations accurately compute the contact angle of the droplet as a function of the surface composition, as verified by direct comparison to experimental measurements. They have also undertaken simulations of the self propulsion of water droplets on surfaces with a gradient in composition and therefore polarity. These calculations demonstrate the importance of hysteresis in gradient propulsion, and provide an understanding of how the gradient needs to be prescribed to maintain the self-propulsion of the droplets and minimize the hysteretic effect.
Address Goals
This molecular dynamics study of the self-propulsion of water nanodroplets on surfaces with gradients of wettability provides a rational molecular basis for designing gradient surfaces with reduced hysteresis. The reduction in hysteresis is the key to maintaining effective capillary driven motion of fluid droplets. As such, these results should be very useful in the development of prototype micro and nanofluidic devices that utilize energy gradient surfaces to position fluids and direct their movement.
Progress in the design of these devices, particularly ones that undertake biochemical and chemical separations, synthesis and diagnostics, are extremely important to maintaining the Nation’s leadership edge in biotechnologies. Lab-on-chip platforms have the advantage of dramatically reduced reagent consumption, and high sensitivity, and nanofluidic chips, with their potential for single molecule manipulation and detection, are essential tool for implementing many proposed nano-biotechnologies. Self propulsion in micro and nanofluidic networks also has the advantage that the device is elf contained, and consumers minimal amounts of energy.
The molecular dynamics programs which were developed as part of this research accurately model the interactions of water with functional groups attached to a surface, and account for the hydrogen bonding characteristics of water. In this sense, the programs are unique, since most molecular dynamics simulations model only the van der Waals or Lennard-Jones interactions dominant in non-aqueous systems. Programs incorporating water’s hydrogen bonding electrostatic interactions are essential in modeling realistically aqueous systems, particularly at the molecular level, and are essential in advancing the Nation’s nanotechnologies. In addition, the IGERT student trained on this project brings to the workforce this unique set of tools.
