47000-G7
Microrheological Investigation of Particle-Laden Polymer/Polymer Interfaces
The goal of our PRF-G project is to measure the interfacial rheology of polymer-polymer interfaces that contain various surface-active species – examples include nanoparticles, colloids, block copolymers, dendrimers, and so on. During the past year, we have successfully demonstrated a novel technique for measuring interfacial visco-elasticity, and are presently using it to measure the interfacial rheology of a variety of model systems. (Note: I was advised by UCSB to segregate "patentable" parts of this work from those funded by the PRF; as such, the student who developed the microfabrication strategies and the technique itself has been supported on other funds, and the PRF has been used to support the fundamental scientific research – materials, supplies and equipment used in making the rheological measurements themselves, as described below).
Although its development was not specifically funded by our PRF-G funds, our technique is new and its details are important to understand the results presented below and our future plans. As such, we begin with its description. Using photolithography, we make micron-scale ferromagnetic disk probes with controllable surface chemistry and geometry, which we employ as rheological probes (fig. 1). Typically, we make circular disks that are 20 microns in diameter and 1 micron in thickness, upon which we deposit a thin (~100 nm) layer of ferromagnetic metal to impart the desired magnetic properties, as well as a thin layer of gold which allows us to use (very versatile) gold-thiol chemistry to functionalize the "top" of the disks with a desired thiol-terminated polymer. This latter step creates a disk that is itself amphiphilic and which thus irreversibly adsorbs to the interface in the desired orientation. We then use four electromagnets to exert a (known) torque upon the disk, while simultaneously measuring its orientation using fluorescent or bright-field microscopy and image processing. The relationship between the small-amplitude, oscillatory torque and the (measured) oscillatory orientation can then be analyzed to give the interfacial visco-elastic moduli, so long as the rotational drag force upon the disk is dominated by the interface. The small scale (~10 micron) of the probe naturally renders its motion exquisitely sensitive to visco-elastic forces from the interface, rather than the bulk. Our microdisks should be two to three orders of magnitude more sensitive than existing techniques. Furthermore, they enable such measurements to be performed with smaller sample volumes, as we will describe below.
Fig. 1: (left) SEM image showing the
microfabricated, 20-micron diameter and (middle) 1-micron tall ferromagnetic
disks. (Right) bright-field image
of ferromagnetic micro-disk sitting at a water-decane interface . With the support of our
PRF-G grant, we have been using these ferromagnetic microdisks to measure the
visco-elastic properties of several model interfaces, building towards
investigating polymer-polymer interfaces with synthetic surfactants. Fig. 2 shows one such measurement of a
Langmuir monolayer of Palmitic (Hexadecanoic) acid, at an air-water interface,
measured using a (macro-scale) floating needle viscometer. Data from 20 and 50 micron microdisk
probes are shown against the floating needle data. A Langmuir trough allowed control over the surfactant
surface concentration (surface pressure).
Significant features are:
(1) at intermediate
pressures (15-22) all methods give consistent measurements; (2) at low surface
pressure (<15), the microdisks measure the viscosity accurately, whereas the
needle drag is dominated by the water, with no effect from the interface (3) at
high pressure (>22, above Palmitic Acid's liquid-solid transition) the
needle and disks measure the same viscosity, but the disks also capture a
strong elastic response (triangles) not captured by the needle. We have also measured the nonlinear
rheology of colloidal monolayers at water/decane interfaces, which show
shear-melting at high surface concentration, although these results are
preliminary and meant to highlight
the ability to track the deformation of the interface in additional to
measuring drag. More quantitative
measurements are underway. We are currently working
with a mini-trough developed by J. Israelachvili and J. Zasadzinski to begin
measurements with polymer-polymer systems, in continued collaboration with an
NSF-funded IRG in the Materials Research Laboratory at UCSB. Interfacially-active particles and
polymers will be synthesized by C. Hawker's group, and the ability to use small volumes will
be essential.
F
Fig. 2:
(left) Surface viscosity
vs. surface pressure (~concentration) of a Palmitic Acid monolayer at a
water/air interface, as measured by (macro) floating needle viscometer
(circles), 20 micron disks (+), 50 micron disks (green squares for viscosity
and triangles for elasticity).
Note the increased sensitivity of the disks at low surface pressure,
agreement between all techniques at intermediate and high values, and
elasticity measurements with the disks.
(right) Measured velocity of a colloidal monolayer interface as a function
of distance from the probe. The
velocity decays with the 1/r decay expected for interfacially-dominated flows
(plotted lines), with two discontinuous drops in velocity due to shear-banding.
