XPCS and Science Opportunities at NSLS-II

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XPCS and Science Opportunities at NSLS-II
Bob Leheny Johns Hopkins University
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X-ray Photon Correlation Spectroscopy
(Image from B. Stevenson, ANL)
Dynamic light scattering with x-rays
Coherent Beam
Autocorrelation of intensity
I(Q,t)
t
Gives dynamic structure factor
g2(Q,t)
t
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Examples of XPCS topics to date
Hard matter
Order-disorder transitions in alloys
Charge density wave motion
Antiferromagnetic domain motion
Soft matter
Smectic liquid crystals
Polymers
Colloids
gels
surface interfacial fluctuations
glass transitions
reptation
phase separation and mesophase ordering
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Prospects for NSLS-II
Signal-to-Noise in g2(Q,t)
accumulation time ( minimum delay time t)
(Falus et al., JSR 2006)
source brilliance
cross section per volume
energy bandpass
Potential improvement at NSLS-II over APS (8-ID)
x 30
Intrinsic brilliance
x 10
Optimization of coherent flux
- vertical focusing
- wider
Consequences
Weaker scatterers become accessible.
Minimum delay time shortens substantially
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What occurs in 100 ns?
Overlap with Neutron Spin Echo in reach!
S(Q,t) from 10-11 s lt t lt 104 s
E.g., a 6 nm sphere in water diffuses its diameter
Nanoscale dynamics in aqueous solution become
accessible to XPCS
Suggests studies of nanoparticle
motion/self-assembly in low-viscosity
solutions in bulk and on surfaces
biologically relevant systems
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Fluctuations in lipid membranes
(Image from E. Marcotte, UT Austin)
t 10-6 s
at Q 0.03 - 0.1 nm-1
NSE of higher Q dispersion indicates
Potentially interesting range of length scales
could be accessible at NSLS-II
protein conformation
membrane elastic modulus
protein conformation


active fluctuations driven by protein dynamics
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Another membrane system bicontinuous
microemulsions
d 10 nm
Long-standing theoretical predictions for
dynamical behavior.
water
oil
Important in applications
e.g. unique nanostructured materials through
polymerization
templates for chemical reactions

Fluctuations at relevant wave vectors (2p/d)
too slow for NSE, too short for DLS well
suited for XPCS at NSLS-II
(G. Gompper et al., Juelich)
Numerous such nanostructured soft materials
have intrinsic dynamics in the window that
NSLS-II will fill.
Others likely include lamellar phases (smectics),
ringing gels, etc.
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Protein protein complex conformational
fluctuations
Fluctuations involving large-scale
conformational changes can occur on microseconds
to milliseconds.
t 100 ms
Potentially important for function.
e.g. enzymatic activity
Enzyme from E. coli
(H. Yang, UC Berkeley)
Potential strategies to access fluctuations
with XPCS
Time dependence of diffuse scattering around
bragg peaks of protein crystals (???)
Deviations of diffusion from rigid-body
behavior
- Demonstrated with NSE for domain-scale
fluctuations (t 10 ns)
(Z. Bu et al., PNAS 2005)
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Other interesting opportunities with XPCS at
NSLS-II

• 1) Expanding polymer research

Reptation
Highly successful phenomenological model
Motion accessible to XPCS (Lumma et al,. PRL,
2001)
Broader dynamic range will illuminate
- Specific nature of relaxation (e.g., constraint
release)
- Rouse-to-reptation crossover
Surface fluctuations
Well suited for XPCS (Kim et al., PRL, 2003)
- probe nature of fluctuations at molecular
scales Rg, entanglement length
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2) Local dynamics in glassy materials
Approach to glass transition characterized by
growing separation of time scales
b and a relaxations
fast, localized motion
slow, terminal relaxation
accessed experimentally
Eg., gelation and aging in nanocolloidal
suspensions
inferred
APS, 8-ID
NSLS-II will have dynamic range to track full
relaxation spectrum.
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Analysis beyond g2(Q,t) required.
Systems far from equilibrium characterized by
Intermittent (non-Gaussian) dynamics
Spatial and/or temporal heterogeneity
Eg., degree of correlation
dilute colloidal gels
Large, non-Gaussian fluctuations
temporal heterogeneity
Duri Cipelletti, EPL (2006)
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Other ideas from DLS for characterizing
intermittent dynamics
Higher order moments
(Lemieux and Durian, Appl. Opt. 2001)
,
etc.
(Note
)
Speckle-visibility spectroscopy
(Bandyopadhyay et al., RSI 2006)
Measure variance in speckle intensity as a
function of exposure time.
NSLS-II should make these (and other) analysis
approaches feasible for XPCS.
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Conclusion
NSLS-II will revolutionize XPCS.
But,
Realizing many of these advancements will require
a corresponding improvement in detector
technology
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• K detector efficiency
• T total experiment duration
• accumulation time
• angle subtended by Q of interest
• scattering cross section per unit volume
• W sample thickness
• 1/attenuation length
• B source brilliance
• DE/E normalized energy spread
• r factor depending on source size, pixel size,
  and slit size