New England Complex Fluids Workgroup

Abstracts for Invited Talks and Sound Bites:

Invited Talks

1. Invited Speaker(s); Clare M. Waterman-Storer

The Scripps Research Institute, La Jolla, CA

"Cytoskeletal Systems Integration in Cell Migration"
The directed locomotion of vertebrate tissue cells involves spatiotemporal coordination of protrusion, adhesion, and contraction. This necessitates complex and dynamic interactions between cytoskeletal systems and the extracellular environment. My lab uses quantitative microscopy of protein dynamics in living cells and in vitro biochemistry to understand how seemingly distinct cytomechanical systems are integrated with one another to promote the polarized morphogenic activity that drives directed cell movement. My lab aims to answer questions such as how the microtubule and actin cytoskeletons interact to polarize a motile cell or how the acto-myosin contractile system interfaces with the extracellular matrix via focal adhesions to generate traction force that drives cell movement. To aid our studies of molecular dynamics, we pioneered a method called quantitative Fluorescent Speckle Microscopy (qFSM), which allows quantitative analysis of the dynamics of and interactions between proteins within macromolecular assemblies such as the cytoskeleton and focal adhesions in living cells.
Keywords: cell migration, actin cytoskeleton

2. Invited Speaker(s); Peter G. Vekilov

University of Houston

"Phase Transitions in Protein Solutions "
Dense liquid, gel-like, and solid, ordered in three, two, or one dimension, or completely disordered phases form in protein solutions and underlie physiological and patho-physiological, laboratory, and technological processes. Two aspects of the phase transitions will be discussed. The first one is the role of water, structured at the hydrophobic and hydrophilic patches on the surface of the protein molecules. Examples will be provided illustrating that this structuring often determines the entropy and enthalpy balance of the phase transition, leads to unusual intermolecular interaction potentials with one or more outlying maxima, which severely affect the phase diagrams, and that the dynamics of destruction of the water shell is the major determinant of the kinetics of association of molecules into solid phases. Because of the water structuring, the fastest pathway of nucleation of ordered solid phases is not the one with the lowest free-energy barriers. The second aspect is the interaction between the phases. Examples from the nucleation of two types of ordered solid phases: three-dimensional crystals and the polymers of sickle cell hemoglobin, which have one-dimensional translational symmetry, show that nucleation proceeds via a disordered liquid-like intermediate. In crystal nucleation, the structuring of the intermediate is the rate determining step in the nucleation process, while in the nucleation of the HbS polymers, the formation of the intermediate determines the overall nucleation rate.
Keywords: phase transitions, protein

3. Invited Speaker(s); Peter Schall

Harvard University

"Growth and Deformation of Colloidal Crystals and Glasses"
Plastic deformation in atomic crystals is governed by dislocations – line defects in the crystalline lattice. Understanding how these dislocations propagate, multiply, and interact is central for understanding the plastic response of a crystalline material to an applied stress. Unlike for crystals, not much is known about the deformation mechanism of glasses, and the microscopic processes leading to macroscopic deformation are still highly speculative. We use colloidal crystals and glasses as models to study the behavior of their atomic counterparts under applied stress. We use confocal microscopy to determine the position of the individual particles and to study defect propagation on the particle scale. The colloidal crystals exhibit dislocations that show remarkable similarities to dislocations in atomic crystals. The slow time scale of the colloidal suspension allows us to directly observe the nucleation of dislocations as the crystal is deformed. We have built a laser diffraction microscope, which is inspired by a transmission electron microscope used to study dislocations in hard materials, to image the strain field of the dislocation defects. This technique enables us to study dislocation motion and dislocation interaction on a much larger length scale. In the amorphous suspension, we are able to follow the motion of the individual particles and identify local shear events that give rise to the macroscopic deformation. Recent results indicate a correlation between the location of shear events and regions of low local particle density or high free volume. Plastic deformation in atomic crystals is governed by dislocations – line defects in the crystalline lattice. Understanding how these dislocations propagate, multiply, and interact is central for understanding the plastic response of a crystalline material to an applied stress. Unlike for crystals, not much is known about the deformation mechanism of glasses, and the microscopic processes leading to macroscopic deformation are still highly speculative. We use colloidal crystals and glasses as models to study the behavior of their atomic counterparts under applied stress. We use confocal microscopy to determine the position of the individual particles and to study defect propagation on the particle scale. The colloidal crystals exhibit dislocations that show remarkable similarities to dislocations in atomic crystals. The slow time scale of the colloidal suspension allows us to directly observe the nucleation of dislocations as the crystal is deformed. We have built a laser diffraction microscope, which is inspired by a transmission electron microscope used to study dislocations in hard materials, to image the strain field of the dislocation defects. This technique enables us to study dislocation motion and dislocation interaction on a much larger length scale. In the amorphous suspension, we are able to follow the motion of the individual particles and identify local shear events that give rise to the macroscopic deformation. Recent results indicate a correlation between the location of shear events and regions of low local particle density or high free volume.
Keywords: colloidal crystals growth deformation glasses shear

4. Invited Speaker(s); Anubhav Tripathi

Brown University

"Microfluidic Separation of Proteins in Semi-dilute Polymer Solutions"
We present a systematic study of the electrophoretic migration of protein fragments (10- 200 kDa) in dilute-polymer solutions using microfluidic chips. The electrophoretic mobility and dispersion of protein samples were measured in a series of monodisperse polydimethylacrylamide (PDMA) polymers of different molecular weights (243, 443 & 764 kDa, polydisperseivity index = 2) with varying concentrations. Experiments were also performed in PDMA solutions with a polydispersivity index of about 5 and in mixtures of polymers with different molecular weights. The polymers solutions were characterized using rheometry and optical techniques. Prior to its loading onto the microchip, the polymer solution was mixed with known concentrations of SDS surfactant and staining dye. The SDS-denatured protein samples were electrokinetically injected, separated and detected in the microchip using electric fields ranging from 100 to 300 V/cm. Our results show that the electrophoretic mobility of protein fragments decreases exponentially with the concentration c of the polymer solution. The mobility was found to decrease logarithmically with the molecular weight of the protein fragment. In addition, the mobility was found to be independent of the electric field in the separation channel. The results appear to suggest that the denatured protein molecules migrate as rigid rod-like molecules. The migration mechanism for protein molecules will be discussed using polymer physics and hydrodynamic drag arguments. The protein migration was found to depend strongly on the SDS concentration in the polymer solution. Moreover, the optimal concentration of SDS depends on the polydisperisity of the PDMA polymer. Specifically, repeated injection of protein samples through a single load of polymer solution was achieved using the optimized value of the SDS concentration. A simple mass balance analysis is used to explain the behavior. The results from experiments using a matrix composed of polyethylene oxide (PEO) and PDMA-PEO blends of varying composition will also be presented.
Keywords: protein separation, microfluidics

5. Invited Speaker(s); Krystyn J.Van Vliet

Massachusetts Institute of Technology

"Guiding and Determining Cell State through Chemomechanical Contact "
Living cells can be idealized as chemomechanically coupled material systems, in that cells respond actively to changes in mechanical state through biomolecular synthesis, and likewise respond actively to changes in biochemical state through mechanical actuation. This feature of cells not only enables all biological processes, but also presents opportunities to guide cell development and to engineer robust transducers based on this coupling. Here, we discuss experimental means to manipulate and characterize the cell as an "active material" via nanomechanics. First, we will demonstrate the effects of cyclic mechanical strain on vascular endothelial cell phenotype. Second, we will discuss robust polyelectrolyte substrata by which we can independently control the mechanical compliance and biomolecular surface profile, such that the mechanical and chemical environments to which can be decoupled. Using this system, we show that phenotypic differentiation of vascular endothelial cells can be manipulated directly via the synthesis-dependent compliance of polymeric substrata. Finally, we present an approach to track the effects of external stimuli on the living cell, through chemomechanical mapping of molecules at or near the cell surface
Keywords: cell state, mechanical properties

Sound Bites

1. Aleksander Roshi; S. Barjami, G.S. Iannacchione, D. Paterson, I. McNulty

Worcester Polytechnic Institute, Argonne National Laboratory

"Structure and Dynamics of a Nano-colloidal Silica Gel Dispersion"
We have used multispeckle X-Ray Intensity Fluctuation Spectroscopy to study the structure and the dynamics of a nano-colloidal silica gel dispersed in an organic solvent (octylcyanobiphenyl) as a function of the silica density. The silica density of the dispersed aerosil gel samples ranged from 0.030 to 0.200 gcm^-3 and the silica scattering were probed over the q range from 0.03 to 0.15 nm^-1 (corresponding to length scales from 42 to 209 nm) at a constant temperature of 301.7 K. The gel structure has a fractal dimension in this density range of d_f ~ 2.15 and all samples showed no sign of restructuring (aging) during the experiment. The time autocorrelation functions of the gels show clear density dependent, nonergodic, complex dynamics. The gel relaxation times are very long and (become) bimodal with nonergodic character for densities from 0.100 to 0.160 gcm^-3. In this same density range, the fluctuation contrast (strength) is a minimum while a diffusion analysis finds a pseudo-diffusion coefficient near zero.
Keywords: gel dynamics, quenched random disorder, liquid crystals, X-ray Intensity Fluctuation Spectroscopy

2. Amy Rowat; P.L. Hansen, K.J. Jensen*, J.H. Ipsen

MEMPHYS - Centre for Biomembrane Physics, University of Southern Denmark, Odense, Denmark; *Department of Chemistry, Royal Veterinary & Agricultural University, Frederiksberg, Denmark

"Experimental evidence of the electrostatic contribution to membrane bending rigidity "
We have investigated the thermal fluctuations of giant unilamellar vesicles in the presence of adsorbing non-ionic and ionic surfactants (peptides) with identical apolar chains. Using vesicle fluctuation analysis, the effects of both charged and uncharged surfactants on membrane bending rigidity have been determined in the case of no added salt and the electrostatic contribution thereby isolated. We interpret these experimental findings in terms of mean-field theory for the adsorption of charged surfactants to a lipid bilayer and couple these results to theory to describe the electrostatic contribution to membrane bending rigidity.
Keywords: membranes, elasticity, electrostatics

3. Chanjoong Kim; David Weitz

Harvard University

"Creaming of Emulsion Aggregations and Gels"
Even though creaming of emulsion has been considered as a simple phenomenon due to the hydrodynamics interaction and the density mismatch between dispersed phase and continuous phase, actual creaming behavior is not so simple once emulsion droplets interact with each other. Interaction between droplets was controlled by depletion force, which depends on the concentration of smaller micelles. Creaming behaviors can be categorized to three different groups; 1) The boundary moves up fast at a constant speed with low surfactant concentrations, 2) it moves slowly in the beginning but suddenly collapses up at intermediated concentrations, and 3) it moves up slowly without collapsing at high concentrations. These behaviors are interpreted in terms of poroelatic model.
Keywords: emulsions, poroelasticity

4. Charles Kerbage; C. Kerbage, K. Ahn, T.P. Hunt, R.M. Westervelt, and D.A. Weitz

Department of Physics and Division of Engineering and Applied Sciences, Harvard University,

"Optical Detection and Magnetic Manipulation of Drops in Microfluidic Devices"
We demonstrate an integrated magneto-optic microfluidic device for drop detection and sorting. Optical detection of water drops formed in a continuous oil phase flow is performed using optical fibers which are integrated into the channels of the Polydimethylsiloxane based microfluidic device. The size and the velocity of the drops can be determined by measuring the transmission intensity as a function of time. We also show that such a device can be used to detect fluorescent materials introduced in the drop itself. Moreover, introducing nano-scale magnetic particles into the water drops allows for drop sorting by means of a magnetic field gradient
Keywords: microlfuidics, optical detection, magnetic sorting

5. Dan Blair; Jacinta Conrad, Eric Dufresne, David Weitz

DEAS/Physics Harvard

"Crowded Colloids: Inherrent structures in sedimented colloidal glasses"
Using scanning laser confocal microscopy we investigate colloidal glasses and supercooled fluids in the presence of gravity. By measuring the effective free volume per particle we observe signifigant spatial correlations. That is, particles with lower free volume tend to be spatially clustered along the axis of principle stress.
Keywords:

6. David Vader; C. Brangwynne, K. Kasza, D.Weitz

Harvard University, DEAS

"Quantifying Cell Motion in a 3D Brain Tumor Model"
We use a quasi-spherical aggregate (spheroid) of ~500 glioblastoma brain tumor cells in a collagen gel to study the invasive behavior and motility of cells. The cells are transfected so that the nuclei can be observed through fluorescence microscopy, and subsequently tracked using particle tracking algorithms.
Keywords: cell, tracking, brain, tumor

7. Deniz Kaya; M Muthukumar

Umass-Amherst

"pattern formation in drying polyelectrolyte/salt drops"
we are studying the patterns formed after drying of aqueous drop of NaPSS/NaCl on flat substrate. The patterns exhibit larger variety than the "drying coffee drop" experiment.
Keywords:

8. Germano Iannacchione; Saimir Barjami

WPI

"High-Resolution AC-Calorimetry by RF-Field Heating for Complex Fluids"
A new, high-resolution, modulation calorimetric technique using Radio Frequency or Dielectric heating of the sample, has been developed for the study of complex fluids. This powerful method is sensitive to both the heat capacity and permittivity of organic liquids. The employment of this calorimeter on dispersions of aerosils in the liquid crystal octylcyanobiphenyl (8CB) is described. There is no enhancement of the dielectric permittivity through SmA to N transition in the disordered liquid crystal at the studied density consistent with recent NMR work, suggesting decoupling of the nematic and smectic order parameters.
Keywords: Calorimetry

9. Guanglai Li; Qi Wen, Jay X. Tang

Brown University

"A Domino toppling model for the switch of bacterial flagellar motor"
Most of our understandings of the switch of bacterial flagellar motor come from the studies of E. coli. Here, we introduce Caulobacter crescentus, a gram-negative bacterium whose flagellar motor is similar to that of E. coli. We have studied the switch of flagellar motor of Caulobacter crescentus and found a peak in the distributions of the time interval of its motor staying in CW and CCW rotation. This interval distribution is obviously different from the exponential distribution predicted by the two-state model for E.coli. We have proposed a Domino toppling model to understand the switch mechanism. In this model, the noise induces the FliM to switch between CCW and CW states. The switch of any FliM will cause the switch of all the other FliMs in the C-ring, thus the motor switches. This model can interpret the interval distribution of Caulobacter flagellar motor and dependence of CW bias on the CheY-P concentration measured for E. coli.
Keywords: Caulobacter, flagellar motor, switch model

10. Ho-Young Kim; L. Mahadevan

DEAS, Harvard University

"Capillary rise between elastic sheets"
When a paintbrush is dipped in a bucket of paint, its hair bundle as the liquid (paint) impregnates them due to capillarity. We consider such capillary interaction of deformable objects using a pair of hydrophilic sheets impregnated by water. The static configuration of the sheets after the steady state is reached is analyzed by a theoretical model. The prediction results are in a good agreement with those of experiments. We show that there exist three different regimes of sheet configuration, depending on how easily the sheets bend due to capillary effects.
Keywords:

11. Hyeran Kang; Kelly Molloy

Brown University

"In vitro study of actin-based motility"
Bacterial pathogens such as Listeria and Shigella are propelled through the cytoplasm of host cells by polymerizing actin into ‘comet tail’ structures. We studied this process in cell extract using polystyrene beads coated with either full-length N-WASP,or VCA, a partial domain of N-WASP. VCA was not sufficient to induce bead motility, though actin cloud formation was observed. However, N-WASP induced various forms of actin tails with different sizes of beads. Our goal is to understand the mechanism of actin-based motility by elucidating the conditions under which helical tails form.
Keywords: Actin-based motility, VCA, N-WASP

12. Jay X Tang

"Why is length distribution of self-assembled protein filaments non-exponential?"
Simple statistical theory of filamentous assembly predicts an exponential length distribution. In vitro polymerization of cytoskeletal filaments such as F-actin and microtubules, however, can lead to nonexponential distribution in length. The distribution of microtubules is known to be bell-shaped with peak occurence at the range of several microns. Similar length distribution is found for F-actin when polymerized in the absence of nucleating or capping proteins. In spite of extensive characterization in the literature for actin assembly, neither experimentally nor theoretically has a consencus been reached at the present. I will take a quick sound bite to express my current concerns towards understanding the true length distribution of F-actin in the experimentally defined test tube conditions.
Keywords:

13. Jorge Viamontes; Jay X.Tang

Brown University

"The orientational order parameter of nematic liquid crystalline phase of F-actin "
The cytoskeletal protein actin self-assembles to form long and stiff filaments, F-actin, which serves essential functions in cells, such as control of cell shape, division, and motility. Suspensions of F-actin form either entangled isotropic networks or a nematic liquid crystalline phase. Depending on the average filament length, the isotropic-nematic (I-N) liquid crystalline transition occurs at a concentration of 2 mg/ml or above. We have measured the orientational order parameter of F-actin traversing the I-N phase transition using a combination of techniques, including fluorescence microscopy, local birefringence measurement, and x-ray scattering. With actin concentrations above the region of I-N transition, the order parameter approaches a saturated value of 0.75. This value implies significant extent of misalignment or entanglement among long actin filaments even in the nematic phase. At concentrations slightly below the I-N transition, non zero values of the order parameter were detected within a time window on the order of an hour following the sample preparation, which tends to cause unintended initial alignment. This result shows extremely slow rotational kinetics of F-actin in the entangled networks.
Keywords: Actin, order parameter, isotropic, nematic

14. Keunho Ahn; Darren R. Link, Andrew D. Griffiths, David A. Weitz

Harvard university

"Enzyme-Inhibitor Assay Using Microdroplets"
We performed an enzyme-inhibitor assay using microdroplets. A buffer solution containing enzyme and inhibitor was introduced into a microchannel with hexadecane as a continuous fluid and broken up into droplets by perfluorodecalin (PFD) droplets. PFD droplets act as spacers between aqueous reagent droplets and keep them from coalesing. Substrates were directly infused after homogeneous mixing of the reagents. Each reagent concentration was controlled by changing pumping ratio while keeping total flow rate the same. Reaction rates were measured with long time exposure through fluoresce microscopy. Each fluorescent droplet was recorded by an image intensified fast camera to check probability of inter-droplet penetration of reagents.
Keywords: microfluidics

15. Leo Tsai; M. Klein, T. Pavlin, M. Rosen, D. Candela, and R. Walsworth

Harvard-Smithsonian Center for Astrophysics

"The role of interstitial gases in density-segregation of vertically-vibrated granular beds"
We report experimental studies of the effect of interstitial gas on density-segregation in vertically-vibrated mixture of 100 micron-diameter bronze and glass spheres. Sufficiently strong vibration in the presence of interstitial gas induces vertical segregation into sharply separated bronze and glass layers. We find that the segregated steady-state (i.e., bronze or glass layer on top) is a sensitive function of gas pressure and viscosity, as well as vibration frequency and amplitude. We present a qualitative physical picture for this segregation behavior based on the key role played by vibration-induced gas pressure gradients, and the resultant competition between density-dependent particle mobility and bulk separation of particle species in the presence of gas flow.
Keywords:

16. Ling Chao; Alice Gast

MIT

"Planar supported bilayers as model membranes used to investigate lipid microdomains and lipid-protein interactions"
Shingomyelinases are enzymes that catalyze the hydrolysis of sphingomyelin into ceramide and phosphorylcholine which changes the properties of bio-membranes and the properties of membranes also influence the activity of the enzyme. Sphingomylin is a major component of lipid rafts and the interaction with Shingomyelinases takes an important part in cell signaling. Planar supported bilayers are good model systems since they capture the membrane fluidity and facilitate the measurement and interpretation of structural and dynamic data. The poly(dimethylsiloxane) (PDMS) stamping proteins are used as barriers to form corrals and direct lipid bilayer self assembly on a single substrate. Incorporated with microfludic device, the vesicles flow through the channels laminarly and fuse to the glass surface. The lipid composition gradients in the corrals are constructed by diffusion of vesicles in different streams of flow and hence can be controlled by flow parameters. Thus, a single substrate can give huge amount of information about the microdomains and the influence of composition and physical state of lipids on the enzymes.
Keywords:

17. Moumita Das; B. Chakrabarti, S. Ramaswamy, C. Dasgupta, A. K. Sood

"Routes to spatiotemporal chaos in the rheology of nematogenic fluids"
With a view to understanding the ``rheochaos'' observed in recent experiments in a variety of orientable fluids, we study numerically the equations of motion of the spatiotemporal evolution of the traceless symmetric order parameter of a sheared nematogenic fluid. In particular we establish, by decisive numerical tests, that the the irregular oscillatory behavior seen in a region of parameter space where the nematic is not stably flow-aligning is in fact spatiotemporal chaos. We outline the dynamical phase diagram of the model and study the route to the chaotic state. We find that spatiotemporal chaos in this system sets in via a regime of {\em spatiotemporal intermittency}, with a power-law distribution of the widths of laminar regions, as in H. Chat\'{e} and P. Manneville, Phys. Rev. Lett. {\bf 58}, 112 (1987). Further, the evolution of the histogram of band sizes shows a growing length-scale as one moves from the chaotic towards the flow aligned phase.
Keywords:

18. Petia Vlahovska

"Drift of a surfactant-covered drop in a wall-bounded shear flow"
Surfactants modify interfacial properties and significantly affect drop behavior in flow. I study the hydrodynamic interactions of surfactant-covered drops. Results indicate that the nonuniform surfactant distribution causes a spherical drop to migrate away from the wall.
Keywords:

19. Ruopeng Wang; Ross Mair, Mattew Rosen, Donald Candela, Ronald Walsworth

Harvard Smithsonian Center for Astrophysics; University of Massachusetts

"Measurement of gas exchange rate among different phases in gas-fluidized bed using hyperpolarized 129Xe NMR"
Gas fluidization is a process in which solid particles experience fluidlike suspension in upflowing gas-phase fluids. Efficient mixing is easily achievable among fluidized solid particles, in which concentration and temperature gradients can be quickly relaxed. Existence of bubbles promotes the mixing but also provides a path via which gas escapes the bed without much contact with the particles. We report our experimental work on non-invasive measurement of gas exchange rate among bubbles, emulsion and adsorption phases in a 3D gas-fluidized bed, by using hyperpolarized 129Xe NMR. Alumimum oxide powers were used as the particles, which provided a contrast mechanism differentiating the phases in xenon spectrum. The results agree well with prediction based on previous modeling works.
Keywords: Fluidization, emulsion, bubble, exchange rate

20. Shang Tee; M.P. Brenner, D.A. Weitz

Harvard University

"Dynamics of Fluidized Bed"
Fluidized bed dynamics at low Reynolds number is investigated. Particle concentrations, velocity fluctuations and correlation lengths are found to vary strongly with bed height. Surprisingly, despite all these complications, we find that....
Keywords: velocity fluctuations, correlation lengths

21. Stella Park; C.J. Russo, H.A. Stone, D. Branton

Harvard University

"Electroosmosis through a Bottleneck"
Although using an applied electrical field to drive flows becomes desirable as channels become smaller, most discussions of electroosmosis treat the case of thin Debye layers. Here electroosmotic flow (EOF) through a constricted cylinder is presented for arbitrary Debye lengths $\kappa^{-1}$ using a perturbation approach. The varying diameter of the cylinder produces radially and axially varying effective electric fields, as well as an induced pressure gradient. We predict the existence of eddies for certain constricted geometries and propose the possibility of electrokinetic trapping in these regions. Eddies can be found both in the center of the channel and along the perimeter, and the presence of the eddies is a consequence of the induced pressure gradient that accompanies electrically driven flow into a narrow constriction.
Keywords: electrokinetic flow, electroosmosis, eddy

22. Suliana Manley; Margaret Horton, Alice Gast

MIT

"Mechanics of protein-coated vesicles"
Protein coats are ubiquitous nature, from viral capsids to the S-layer protein coats on the surfaces of prokaryotic cells. Protein coats may also be useful in providing specificity and rigidity to vesicles, which can be used for encapsulation. We will use a model system of giant unilamellar vesicles coated with streptavidin to study the effect of protein coats on membrane mechanical properties.
Keywords:

23. Wesley Wong; Volkmar Heinrich, Ken Halvorsen, Evan Evans

Harvard University; Boston University

"Exploring the dynamics of weak single-molecule interactions through the 3D tracking and trapping of a microsphere"
I present a novel technique that we have developed for exploring the forward and reverse kinetics of weak bonds near equilibrium at the single-molecule level. In this method, molecular events, such as the association/dissociation of single intermolecular bonds, or the folding/unfolding of protein domains, are encoded in the Brownian fluctuations of a functionalized microsphere held weakly near a reactive substrate by an optical trap. By combining an interference technique with high-speed video microscopy, we are currently able to track the position of the bead with nanometer resolution (~1 nm vertically, 2-3 nm laterally) at ~200 Hz. A feedback-stabilized optical trap is used to explore the force-dependent bond kinetics, and to establish a well-defined reaction volume. I will give some preliminary results, including (very!) recent work demonstrating the detection of unfolding events in spectrin.
Keywords: single-molecule interactions, photonic force microscopy

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