Interfacial Forces in Active Nanodevices

1
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.
Wetting Measurements of Nanowires by AFM 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.

• Simulations of Nanocapillaries
• Generic behavior studied first with Lennard-Jones
  interactionsLiquid short chain molecules with
  FENE bondsLJ units energy ? 0.01eV, length ?
  0.3nm, force ? /? 5pN.
• Study effect of atomic structure of
  surfacesRigid spheres (8 120nm), bent or cut,
  crystalline or amorphousRigid 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.

Electrowetting at Nanoscales Active nanodevices
require means of changing capillary forces. We
areexploring control of q by electric fields and
optical illumination. Applying a voltage V
between a fluid and an electrode covered by an
insulator of dielectric edand thickness d leads
to a new contact angle qEW At macroscopic scales
this is described by theYoung-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
R

• Results for capillary force F on sphere
• At large h, exact theoretical results and the
  commonly used circle approximation are almost
  identical.Both are consistent with MD results.
• At h lt 12s 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.

200 nm radius Ag2Ga nanowire4
50 nm radius InAs nanowire2
250 nm radius Si nanowire3
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
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 hysteresis5. Future work will examine
changes induced by electric fields and light, and
the potential for switching the interface between
different states.
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 lengthincreases the
saturation voltageby preventing molecules
fromevaporating under the highelectrostatic
force. This is a new mechanism for saturation
Capillary force on rods Can ignore gravity for
small rods R r (rg/g)0.5 lt 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
r
z
?
F 90 - ?
Nanoscale Electrowetting in the Surface Force
Apparatus
Surface Forces Apparatus
The surface force apparatus (SFA) allows study of
liquids between surfaces with nanometer
separation. The thickness can be measured
optically with subnanometer resolution. 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. Current work shows that films can be
condensed and evaporated by an applied field,
creating another mechanism for controlling fluid
geometry at nanometer scales.
Measured force on200nm Ag2Ga nanowire pushed
into and retracted from water interface Forces
consistent with bulk surface tension and contact
angles ?adv 58, ?rec 47
Water detaches
Droplet density contours
approach
Water contact
retract
Charge density contours
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.