CARS Microscopy of Colloidal Gels

1. CARS Microscopy of Colloidal Gels Evangelos Gatzogiannis

4. CARS (Coherent Anti-Stokes Raman)

6. CARS Microscopy M. D. Duncan, J. Reintjes, and T. J. Manuccia Optics Letters, Vol. 7, Issue 8, pp. 350- (Deuterated Onion Cells) Revived by Zubmusch, Xie in 1999.

7. CARS Advantages • Chemical selectivity without labeling. • High sensitivity. • Signal photon at higher frequency - no spectral overlap with one-photon fluorescence background. • Small excitation volume for microscopy. • Unlike fluorescence, CARS is a coherent process and signal is proportion to ~N2 where N is the number density of scatterers.

8. ~610nm ~575 nm ~610nm ~650nm ~1000cm-1 My Previous Work With CARS Elimination of Non Resonant FWM Background CARS Sample Lens pump Stokes Lens probe Strong vibrational resonance at ~1000 cm-1.

9. STAND-OFF CARS SPECTROSCOPY Detector Phaseonium (Goal) Coherent Radiating Dipoles CARS Signal Epump, Eprobe x z y EStokes SPORE (~1µm) DPA Molecule, i

10. Experimental Setup THG 1kHz/10Hz Regen OPA Stokes Tsunami UV Shaper UV CARS Evolution Millenia Stokes OPA Pump/Probe OPA Quanta Ray CARS Microscope Cost: $700,000+

11. 14 GHz 14 GHz 14 GHz Loop gain Phase shifter Delay Experimental Setup for RF LockingEssential for CARS, Many Uses in Metrology, Frequency Standards SHG Fs/Ps Laser 2 SFG Fs/Ps Laser 1 BBO SHG 100 MHz SFG Cross- Correlation Fast Sampling Oscilloscope 50 ps Phase shifter Laser 1 repetition rate control 76 MHz Loop gain

12. Stokes Laser (Master) To CARS microscope Pump/Probe Laser (Slave) 14 GHz 76 MHz Feedback Loop At the CARS Microscope, Forward vs. Epi Detection

13. Epi-CARS Good for sub-wavelength structures, Less background Forward CARS

14. APD/PMT Synchrolock-Based Setup SFG/Cross-Correlation BBO Filter Stokes Laser (Master) WP/PC WP/PC Pump/Probe Laser (Slave) 3-D scanner 14 GHz 14 GHz 80 MHz Phase Shifter 14 GHz Loop gain 76Mhz Loop Gain Phase Shifter DBM Dichroic mirror wp,ws DBM was APD/PMT

15. Simplified Setups (Improved Performance)

17. Several CARS Images

18. Maximum packing Maximum packing φRCP≈0.63 φliquid≈0.48 φxtal≈0.54 φHCP=0.74

20. Direct Imaging of Attractive and Repulsive Colloidal Glasses Repulsive Glass: Less Motion, Coop. Attractive Glass: Significant Motion J. Chem. Phys. 125, 074716 2006

24. Cluster Formations Is a Precursor To Colloidal Gelation – NOT Well Understood

25. Current Experimental System This is an SEM picture of the ASM204 ~1micron colloids I am working with.

26. Colloidal Gel Basics • A gel will not form at low volume fraction unless it is buoyancy matched. • For U/kBT << 1, hard sphere like behavior, monodisperse particles jam at Φ=0.63. • For U/kBT >> 1, irreversible aggregation, fractal clusters are formed. • Can bear stresses, have interesting mechanical properties. • Physics of formation, aging, and other question remain unresolved.

27. Most groups use fluorescence (downsides include): rapid bleaching, photo physics, alters system (in some cases, cell fixing) can’t do in vivo studies CARS: Can image for longer times (hours) depending on laser stability (without long delays frame-to-frame), Chemical specificity Resonant coherent process (better signal/background) In vivo studies, intrinsic 3d sectioning with improved spatial resolution in some cases. Noninvasive. Label-Free High Speed Imaging. No perturbation of system.

28. Colloid-Polymer Mixtures Provide Rich Phase Behavior Blue: Gel Red: Fluid of Clusters Green: Fluids

29. Topology and Structure Fromd 3D Images Shortest path between two particles (red stripes) along the gel, yellow, red, second shortest path.

30. Radial distribution function provides direct measure of fractal dimension. Consistent with Diffusion Limited Cluster Aggregation Length of chains related to fractal dimension. Dinsmore, PRL 96 185502 (2006)

32. Low Interaction Energies, U ≤ 2.6kBT No Structures U≥2.9kBT space filling networks with Changing Morphology Static over 30 min observation time, no signs of aging.

33. 3D Colloidal Gel

34. Three Days Later, After Stirring

35. Zooming In: Colloids Still Quite Small

36. Trajectories of Colloids In 20% Gel

37. Van Hove Function G(r,t)dr is the number of particles j in a region dr around a point r at time t, given a particle i at the origin at time t=0. It separates into two terms: Self part: Distinct part: Gs(r,0) = δ(r), Gd(r,0) = ρg(r)

38. Van Hove II

39. Signature of particles moving into positions occupied by other particles.

40. Measures Heterogeneity and Indicative of Cage Escape