Interfacial Forces in Active Nanodevices
(NIRT 0709187)
S. Chen, S. Cheng, J. Frechette, R. Gupta, J. Liu, J. Ma, P. M. McGuiggan, M. O. Robbins: Johns Hopkins University
Project Overview
As device dimensions shrink into the nanometer range, interfacial forces
become increasingly important. At the same time, traditional continuum
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 the control of these forces
using electric fields and light.
Algorithmic Development: Multi-Timescales
Algorithmic Development: Multi-Grid Coulomb Method
A continuum-atomistic multi-timescale algorithm was developed [1].
Interfacial regions are treated atomistically, while a continuum
description is used for bulk regions. The two are coupled through
an overlap region.
There remains a large gap between the time scales of atomic and
bulk regions. The new algorithm integrates atomistic equations for
short intervals and then extrapolates over the bulk time interval.
Studies of electrowetting require efficient algorithms for the long-range
interaction between charges. An efficient multi-grid method has been
developed. It has enabled the studies of electrowetting described below, as
well as simulations of electro-osmosis.
Figures below show the charge distribution and effect on flow rate Q in rough
channels.
Effect of roughness on flow rate
Charge density distribution
Tests of Couette flow driven by an oscillating wall show that
substantial speedups can be achieved and that errors decrease as
the separation between time scales grows.
1. J. Liu, S. Chen. X. Nie & M. O. Robbins, Commun. Comput. Phys. 4, 1279 (2008).
Wetting Measurements of Nanowires by AFM
Simulations of Nanocapillaries:
Electrowetting at Nanoscales:
• Generic behavior studied first with Lennard-Jones interactions
Liquid – short chain molecules with FENE bonds
LJ units: energy e ~ 0.01eV, length s ~ 0.3nm, force e /s ~ 5pN.
• Study effect of atomic structure of surfaces
Rigid spheres (8 ~ 120nm), bent or cut, crystalline or amorphous
Rigid or elastic substrate, (111) surface of fcc
• Control q through solid-liquid interactions
• Compare adhesive force and internal capillary pressure to
continuum theory.
• Relate differences to molecular scale properties and structure.
50 nm radius InAs
nanowire2
250 nm radius Si
200 nm radius Ag2Ga nanowire4
nanowire3
Measuring the entire force curve as a nanowire is pushed/pulled
though an air/fluid interface gives independent information about
interfacial tension, contact angle, dynamic contact angles and
hysteresis[5]. Future work will examine changes induced by
electric fields and light, and the potential for switching the
interface between different states.
Capillary force on rods
Can ignore gravity for small rods
R ≡ r (rg/g)0.5 < 1
Away from end: F/r = 2p g cosq
Contact angle hysteresis →
Measure different angles as
advance qadv and recede qrec
Both vary with rate of motion
Interface pinned at end.
Peak force Fmax = 2pgr
Measured force on
200nm Ag2Ga
nanowire pushed into
and retracted from
water interface
Forces consistent with
bulk surface tension
and contact angles:
θadv = 58°, θrec = 47°
r
z
θ
Φ = 90 - θ


4
z  r sin   ln
 g E 
 (1  cos  R

Water
detaches
z
r
EWOD with 12.68 m PDMS on Gold
Results for capillary force F on sphere:
40
• At large h, exact theoretical results and the commonly
used circle approximation are almost identical.
Both are consistent with MD results.
• At h < 12σ ~ 4nm, MD results deviate from continuum.
There are large oscillatory forces related to layering of
fluid molecules that vary with R.
• The contributions to F can be resolved spatially into
components from the surface tension at the edge of the
drop, the Laplace pressure at intermediate r, and
structural forces in a central layered region.
• Discrepancies from continuum theory remain even after
removing the oscillatory component.
• Disjoining pressure effects lead to non-hydrostatic
pressures in the outer region of the drop.
• The pressure in the plane of the drop is consistent with
bulk expressions for Laplace pressure and the bulk g.
The adhesive force is determined by the z-component of
the pressure, which is systematically less negative.
Young-Lippmann Equation Fit
60
0.4
ΔV
Fringes of Equal Chromatic Order (FECO)
Water
contact
Transmitted spectrograph
light
with camera
objective
70
0.2
80
saturation
0
90
100
-0.2
0
50
100
150
200
250
Force with layering term removed
The surface force apparatus (SFA) allows study of liquids between
surfaces with nanometer separation. The thickness can be
measured optically with subnanometer resolution.
Simulations of Electrowetting at Nanoscales
The new multigrid Coulomb method described above allowed tests of the
Young-Lippmann equation in droplets as small as 15s (~5nm) in radius.
Short chain molecules like those in capillary adhesion simulations were used.
The density contours below show the decrease in contact angle as the number
of charges and the associated voltage increase. The charge remains highly
localized at the surface of the insulator.
The Young-Lippmann equation describes the changes in q in these nanoscale
drops. As in macroscopic experiments, there is a saturation at large voltages.
Increasing the chain length
increases the saturation voltage
by preventing molecules from
evaporating under the high
electrostatic force. This is a new
mechanism for saturation
Young-Lippmann
Droplet density contours
Current work shows that films can be condensed and evaporated
by an applied field, creating another mechanism for controlling fluid
geometry at nanometer scales.
Piezo
retract
disks
spring
White light
Image analysis
Double cantilever
spring (stiff)
Helical spring (soft)
5. McGuiggan PM, Wallace JS (2006) J. Adhesion, 82: 997-1011.
Lyons CJ, Elbing E, Wilson IR (1984) J. Coll. Int’ Sci., 102: 292-294.
R
v
r1
θ
l
xL
110
300
Voltage (V)
SFA experiments can measure the capillary forces described
above and changes in force from electrowetting. Applying voltages
to patterned electrodes on the mica surfaces can also change the
droplet configuration via electrowetting effects.
approach
50
Experimental Data
0.6
Nanoscale Electrowetting in the Surface Force Apparatus
Surface Forces Apparatus
Applying a voltage V between a fluid and an
electrode covered by an insulator of dielectric ed
and thickness d leads to a new contact angle qEW
At macroscopic scales this is described by the
Young-Lippmann equation:
cos qEW = cos q0 + e0ed V2/2dg ,
where q0 is the equilibrium angle and g the liquid surface tension.
The curves below show mm scale measurements by our team
Contact Angle, q
2. Prepared by Brian Swartzentruber, Doug Pete, and Tom Picraux as part of a Sandia CINT User
Proposal #U2008A160: Fabrication of Nanowires attached to AFM cantilevers
3. Prepared by Frank Zhu at Johns Hopkins University using a FIB to mill the nanowire from a Si
cantilever
4. Prepared from solution by NaugaNeedles, LLC
Active nanodevices require means of changing capillary forces. We are
exploring control of q by electric fields and optical illumination.
R
cos q
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, nanowire
roughness and radius, and velocity are being studied.
Examples of nanowires under study are shown below.
Charge density contours