NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-5, 2008
Grant # : 0XXXXXX
Interfacial Forces in Active Nanodevices
NSF NIRT Grant 0709187
PIs: S. Chen, J. Frechette, P. M. McGuiggan and M. O. Robbins
Johns Hopkins University
Project Objectives: As device dimensions shrink towards molecular scales, interfacial forces
become increasingly important. At the same time, traditional theories of interfacial forces
become inadequate, and fundamentally new phenomena appear. The goals of our project are to
determine the limits of traditional theories, identify new interfacial phenomena, develop general
models for interfacial forces at the nanometer scale, and explore processes that may enable new
active nanodevices. To achieve these goals we are developing and applying new experimental
and theoretical methods that allow measurement of interfacial forces on nanowires and in
nanometer gaps between solid surfaces and control of these forces using electric fields and light.
Progress on specific projects is outlined below [1].
New Simulation Algorithms: Simulations of interfacial forces are difficult because of the wide
range of length and time scales and the presence of long-range interactions. The team has
developed new algorithms that address these challenges. The first addresses time scale
separation in hybrid atomistic/continuum models that treat interfacial regions atomistically,
while using a more efficient continuum model in the larger surrounding regions. While this
greatly decreases the number of atoms that must be followed, their motion must still be
integrated on femtosecond time scales. This becomes inefficient as the size of the continuum
region, and the associated time scale, grow. The new algorithm [2] follows atomic motion over
short intervals and extrapolates the results to the characteristic time scale of continuum regions.
This allows a substantial speed up with an accuracy that grows as the time scale separation
increases. The second algorithm uses a multigrid approach to obtain the long-range contribution
of Coulomb interactions [3]. This allowed the studies of electrowetting described below.
Capillary Effects at the Nanoscale: The atomic force microscope (AFM) is being used to
measure capillary forces on nanowires as they are pulled through an air/liquid interface. The
effects of surface chemistry, roughness, nanowire radius, and velocity are being studied. Si,
InAs, and Ag2Ga nanowires with 50nm to 250 nm radius have already been mounted on AFM
cantilevers (below). A measured force curve of a 200 nm Ag2Ga nanowire pushed into water is
shown to the right. Surface tensions and contact
angles can be obtained from such curves and
compared to macroscopic values [4]. Future work
will examine changes induced by electric fields and
light, and the
potential for
switching the
interface
between different
states.
NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-5, 2008
Grant # : 0XXXXXX
Simulations are examining static and dynamic capillary forces at nanometer scales to help
interpret and guide the above experiments. Simulation results for capillary adhesion between a
spherical tip and a flat plate (below) are plotted
on the right. For separations greater than about
10 molecular diameters, continuum theory is
accurate. Molecular layering and disjoining
pressure effects lead to pronounced deviations at
smaller scales. Nanowire studies are in progress.
Electrowetting at the Nanoscale: Experiments are examining electrowetting in nanometer scale
gaps between surfaces in the Surface Force Apparatus [5]. Simulations have provided the first
test of continuum theories in nanoscale drops. The figure below shows the change in contact
angle θ with voltage U for model systems with a
very thin dielectric layer, and the prediction of the
Lippmann equation (solid line) [6]. The drop radius
is only about 15 molecular diameters and yet the
change in contact angle follows the continuum
prediction at low voltages. At higher voltages the
contact angle saturates. This is also observed in
macroscopic experiments and there has been debate
over its origin [7]. Our results suggest that
saturation is associated with charged molecules
leaving the drop. The barrier for this is larger for
longer chains (squares) than shorter chains
(triangles), leading to a higher saturation voltage.
References:
[1] For further information about this project email mr@jhu.edu.
[2] J. Liu, S. Y. Chen, X. Nie, and M. O. Robbins, “A Continuum-Atomistic Multi-Timescale Algorithm for
Micro/Nano Flows,” Commun. Comput. Phys. 4, 1279-1291 (2008).
[3] J. Liu, “Multi-Scale Simulation of Micro/Nano Flows and Granular Flows,” PhD Thesis, Johns Hopkins
University (2008) and to be published.
[4] J. F. Padday, A. R. Pitt and R. M. Pashley, “Menisci at A Free Liquid Surface - Surface-Tension from Maximum
Pull on A Rod," Faraday Trans. I 71, 1919-1931 (1975). P. M. McGuiggan and J. S. Wallace, “Maximum force
technique for the measurement of the surface tension of a small droplet by AFM," J. Adhesion 82, 997-1011 (2006).
[5] J. N. Israelachvili and G. E. Adams, “Direct Measurement of Long-Range Forces Between 2 Mica Surfaces in
Aqueous KNO3 Solutions,” Nature 262, 773-776 (1976). J. Frechette and T. K. Vanderlick, “Electrocapillary at
contact: Potential-dependent adhesion between a gold electrode and a mica surface," Langmuir 21, 985-991 (2005).
[6] G. Lippmann, “Relations entre les phénomènes électriques et capillaries,” Ann. Chim. Phys. 5 494-549 (1875).
[7] A. Quinn, R. Sedev, and J. Ralston, “Contact Angle Saturation in Electrowetting,” J. Phys. Chem. B 109, 62686275 (2005). A. G. Papathanasiou, A. T. Papaioannou, and A. G. Boudouvis, “Illuminating the connection between
contact angle saturation and dielectric breakdown in electrowetting through leakage current measurements,” J. Appl.
Phys. 103, 034901 (2008).