Post on 20-Dec-2015
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Measuring Fluid Properties on a Microscopic Scale Using Optically Trapped Microprobes
Mark Cronin-Golomb
Biomedical Engineering
Tufts University
With the help of:
Boaz Nemet Yossef Shabtai Lisa Goel
at Tufts University Tayyaba Hasan Paal Selbo
at Wellman Laboratories of Photomedicine, MGH
Scanning Probe Microfluidic Analysis
Viscosity is an important indicator of biopolymer concentration. Flow analysis is important in development of microfluidic devices. Method: Confocal phase sensitive detection of optical tweezer beam
reflected from a trapped probe bead set in sinusoidal oscillation by the tweezer beam enables micrometer scale spatially resolved viscosity measurements at 10kHz data acquisition rates.
Acousto-Optic deflector
Scanning Mirrors
Confocal microscope principle
Laser
Detector
Sample Stage
Scanning Mirror
Raster display
Pinhole
Beam Expander
Prior methods to measure viscoelasticity
Video microscopy of magnetically induced fluctuations Schmidt F.G., Ziemann,F. & Sackmann,E. Eur.
Biophys. J. 24, 348 (1996). Positional and temporal statistics of trapped
bead trap strength and viscosity A. Pralle, E.L. Florin, E.H.K. Stelzer & J.K.H. Horber, Appl. Phys. A-Mat. Sci. & Proc. 66, S71 (1998).
Viscosity measurement using position sensing detector
M.T. Valentine, L.E. Dewalt & H.D. OuYang, “Forces on a colloidal particle in a polymer solution: A study using optical tweezers.” Journal of Physics-Condensed Matter 8, 9477-9482 (1996).
Experiment Details
Ti - Sapphire Laser
Lock - in Amplifier
CCD Camera
OL
DM
P APD
*
*
AOD Driver
BS
*
AOD
#
#
#
SM
•As the tweezer beam is moved back and forth, the probe bead lags behind.
•The bead is bright when the tweezer beam illuminates it.
•The confocal signal is highest when the tweezer beam is centered on the probe bead.
Theoretical Background
sin( ) ( )dx
x a t L tdt
x: trap position : viscous drag
: tweezer spring constant a: amplitude of trap oscillation
: frequency of trap oscillation L(t): Brownian forcing function
2
cos
sin
( ) 1 ( )
duu a t
dtu x a t
I t u t
Experimental Results
0 1 2 3 4 5 6 7 8 90.00
0.05
0.10
0.15
0.20
0.25
0.30
A2
2n
d Har
mo
nic
am
pli
tud
e (a
.u)
0
10
20
30
40
50
60
70
80
90
100
2
nd H
arm
on
ic p
has
e (d
eg
)
Signal to Noise Ratio
Absolute Position Detection
Relative Position Detection
01/ 22 2
0 8c c
c B
aSNR
k T
20
1/ 22 20 8
c
c B
aSNR
k T
Viscosity Image
Viscosity distribution around A. pullulans imaged by raster scanning an optically trapped probe bead.
This blastospore has a halo of the polysaccharide pullulan around it. Note the viscosity gradient.
Flow field measurement
An optically trapped microsphere is used as a probe for two-dimensional velocity field imaging using scanning optics.
A fluid viscosity map may be obtained simultaneously. Calibration is based on a single length measurement only. Applications are anticipated in the design of microfluidic
devices.
MicrofluidicsNew microfluidic devices are being constantly developed. Their fluid dynamics need to be understood.
After A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezi, H. A. Stone, G. M. Whitesides, Science 295, 647 (2002)
Flow Off
Oscillating Laser Trap
Probe Bead Probe Bead
a
r
Flow On
Probe BeadProbe Bead
aOscillating Laser Trap
0 5 10 15 20 25 30 35 400
5
10
15
20
25
30
35
40
Ex
per
imen
tal
vel
oci
ty f
rom
tw
eeze
rs (m
/sec
)
video velocity m/sec)
Note offset induced by Brownian motion of probe bead
Comparison of tweezer and video velocity measurement
Force Measurement
Flow measurement is one example of force measurement.
We can use tweezers to apply forces to probe beads and measure their response.
Bead stuck on pullulan around blastospore:
Use To Study Effects Of Photodynamic Therapy On Adhesive Properties Of Cancer Cells Photodynamic Therapy (PDT) is frequently
extremely effective in controlling the primary malignancy, but have also been associated with an increase in distant metastasis.
PDT, used as clinical cancer therapy worldwide, is a method in which photosensitizers (PS) are administered to tumor cells and are activated by light at the appropriate wavelength, where a combination of light, oxygen, and PS are toxic to tumors.
Tayyaba Hassan and Paal Selbo, Wellman Lab MGH
Conclusions
Our scanning confocal tweezers microscope can measure velocity and viscosity simultaneously.
Viscosity can be measured rapidly with microspheres on microscopic scale.
Absolute measurements are obtained in real time for the flow velocity with minimal calibration.
Results from the measurements of the flow shear in z suggest that this technique has the potential of mapping the full 3-D distribution of fluid flow and viscosity.