Research Interests (updated
July, 2007)
- The primary purpose of this page is to inform prospective group members
and collaborators of the type of work we are currently doing. While this list
does not include all of our current work, it is a good overall description
of the major research thrusts within our group. To make the connection between
these projects and the graduate students who are working on them, go to our
members link.
Much of our work is motivated and enhanced by the growing importance of nanomaterials
and biomaterials. While we are often interested in direct
characterization of materials on the nanometer scale, much of our experimental
work probes
'microscopic'
length scales that exist between the 'nanoscopic', molecular level and the
bulk, macroscopic level. Our aim is to understand how molecular processes
affect observable behavior at larger length scales. Our work falls
into the general areas listed below. Overlap exists between some of these
categories. For example, micromechanical techniques designed
to study adhesive materials and polymer gels are also well suited for
studying a range of biomaterials. Nevertheless, the following breakdown
is a useful way of defining
the
core
expertise of our laboratory.
Adhesion and Interfacial Properties of Soft Solids
How do chemical bonds between two surfaces manifest themselves in macroscopic
adhesive forces? Often, these forces are much larger than what would be expected,
based just on the strength of the bonds themselves. We have developed experimental
methods for quantifying the underlying physics of adhesive interactions in
'soft'
materials. Ref. 50 is a recent review
that describes the underlying concepts in more detail. Materials of interest
to us include common pressure sensitive adhesives used in 'sticky'
tape, in
addition to materials used in demanding applications such as electronic packaging
and
wound healing. The following list of examples is representative
of our work in this area:
- In situ characterization of pressure sensitive adhesive films
(ref. 32, ref.
40, ref. 48 and ref.
75).
- Viscoelastic properties of films made from mixtures of 'hard' and 'soft'
particles (ref. 53).
- Deformation and debonding mechanisms of soft adhesive layers, including the
role of geometrical confinement (ref. 47, ref.
61 and ref. 68).
- Adhesive transfer of a thin elastomeric film from an elastomeric substrate
to a rigid surface (ref. 70, ref.
80).
In many cases we find that existing experimental methods are not suitable
for obtaining the desired information, or that commonly employed methods
contain additional information that is not conventionally extracted from
the experimental
data. We are involved in developing a series of techniques that
are highly sensitive to interfacial structure and adhesion. Examples include
the following:
- Development of a membrane contact technique for quantifying adhesion (ref.
92).
- Use of drop shape analysis to monitor monolayer formation at the oil/water
interface.
- Use of the quartz crystal microbalance (QCM) as a contact sensor. We have
published extensively in this area in recent years. Important examples of
the most
recent work are found in ref. 78, where we
discuss geometrical effects that are important in a contact experiment where
the crystal is not uniformly
loaded, and ref. 86, where we describe the
quantitative analysis for a general multilayer system, and apply it to a
grafted polymer brush that is in contact
with a polymeric membrane. An appropriately chosen membrane increases the
base sensitivity of the quart crystal by a factor of 100, greatly enhancing
the utility of the QCM as a simple tool for assessing the
structure of interfacial layers
in aqueous systems.
Self-Assembling Polymer Gels
We have been increasingly interested in polymer gels over the past several
years. We are partially motivated by our interest in the mechanical response
of these materials. Examples include the following:
- The
high-strain
properties of alginate hydrogels (ref.
65) These naturally occurring materials are often used in biomedical
applications because of their excellent mechanical toughness, and their
ease of crosslinking by the addition of calcium ions.
- Thermoreversible acrylic triblock gels that form tough elastic gels at room
temperature, but form low viscosity solutions when warmed to about 70 C. We
have used these materials in a lot of our fundamental studies of adhesion
and mechanical response because they have a very high mechanical strength,
relative to the low-strain elastic modulus. Ref.
60 and ref. 89 summarize
our understanding of the origins of the mechanical response of these unique
materials. Click here for a brief description of the
ordering process that gives rise to the thermoreversible elastic response
in these materials.
The ability of these materials to rapidly transform from low-viscosity solutions
to high-strength elastic gels has given rise to a materials processing
application called thermoreversible gelcasting, described in
more detail at this
link.
Ceramics processing applications have been developed in collaboration with the
Faber group, and metals processing applications are being developed in collaboration
with the Dunand group. Applications so far include the following:
- Development of a near-net-shape thermoreversible gelcasting technique for
the formation of ceramic objects into complex shapes (ref.
56).
- Use of thermoreversible gelcasting to make ceramic laminates with a graded
pore structure (ref. 59).
- Development of a titanium hydride route for casting bulk or porous titanium
into complex shapes (in progress).
We are also
developing a series of self-assembling hydrogels that rely on similar principles.
These materials, which consist of acrylic triblock copolymers that produced
gels in water by a solvent exchange process, are remarkably easy to form (see
ref. 90). These materials are the focus of much of our ongoing work in the
biomaterials area, as described in more detail below.
Biomaterials
Our interest in adhesion and in the mechanical response of
very soft materials has led to an emerging research program in the general
area
of biomaterials. Much of our work in this area is done collaboratively, and
includes the following examples:
- Adhesion of hydrogels modified with the adhesive peptide, DOPA (dihydroxy
phenylalanine). See ref. 64, ref.
82, and ref. 83 for examples of this
collaboration with Phil Messersmith and his group here at Northwestern.
- Mechanical properties of tissue engineering scaffolds. Ref.
63 and ref.
73, are examples of our collaboration with the Shea
group.
- Hydrogels with high toughness and low friction (in progress). The aim
of this work is to understand the origins of the remarkable mechanical
properties of cartilage, and to develop guidelines for the synthesis of
cartilage replacement.
- Mechanical and chemical influences on the cell and tissue growth (in
progress). The aim here is to use the triblock hydrogels as model systems
with controlled moduli in the kilopascal range, and with chemical signals
of interest incorporated into the midblock.
Polymer/nanoparticle composites
Polymer-based nanocomposites are a natural extension of our work on polymer
interfaces, because nanocomposites have a very high surface to volume ratio.
Examples of our work in this area include the following:
- Dynamics of metal particle nanoparticle diffusion at long times (ref.
38)
and at times that are comparable to the relaxation time of the polymer (ref.
69). The short-time measurements utilize an x-ray standing wave technique
that provides sub-nanometer resolution on the particle position. This work
is done
in collaboration with
Dr. Jin Wang and others at Argonne National Laboratory. (Click
here for more info)
- Use of our self-assembling gels as a dispersing medium for carbon nanotubes
(in progress). These systems are well-suited for fundamental investigations
of elasticity in fibrous gels. This work is done in collaboration with the
Brinson group.
Collaborators
Our work benefits enormously from collaborations with a variety of labs
at Northwestern and throughout the world. Here's a list of some of our
active collaborations:
- Tristan Baumberger and Christiane Carol, CNRS-Université Paris 6 (Paris,
France): Fracture of gels.
- Cate
Brinson, Northwestern University: Polymer nanocomposites.
- Wes
Burghardt, Northwestern University: Rheology of polymer gels.
- Costantino Creton, ESPCI (Paris, France): Adhesion of soft materials.
- David Dunand, Northwestern University: Thermoreversible gelcasting of titanium.
- Kathy Faber, Northwestern University: Thermoreversible gelcasting of ceramics.
- Kim
Fisher, Northwestern University (Speech and Communication Disorders):
Mechanics and adhesion of vocal fold tissue.
- Dan Harrington, Northwestern University (Medical School): Biomedical applications
of triblock hydrogels.
- Diethelm
Johannsmann, Technische Universität Clausthal (Clausthal, Germany):
Advanced uses of the quartz crystal microbalance.
- Phil Messersmith, Northwestern University: Adhesion of DOPA-containing hydrogels.
- Lonnie
Shea, Northwestern University: Mechanical properties of tissue scaffolds.
- Jin Wang, Argonne National Laboratory. Synchrotron x-ray studies of polymer/metal
nanocomposite systems and of polymer interfaces.