Biomedical Engineering Reference
In-Depth Information
10.0
200
deionized water
(a)
5.0
100
0
0
-100
-5.0
(b)
(c)
tris 0.01
tris 0.1
M
-200
-10.0
0
100
200
300
400
500
600
700
800
Distance from the surface [nm]
Fig. 1.35.
The surface force as a function of the distance from the surface
between the three forces: the surface force from the surface, gravity, and
the bouyant force on the particle. The trapping force exerted by the laser
beam and the amount of displacement
Δz
of the particle are related by
F
z
=
K
z
Δz
,where
K
z
is the spring constant of the laser trapping in the
z
direction. Considering that the gravity and the bouyant force on the particle
do not change during the measurement, the surface force is obtained from
the displacement measurement.
To measure the particle displacement, evanescent illumination is used. In
the evanescent illumination, a laser beam is incident to a prism attached to
the sample substrate under a total internal reflection condition, and it gene-
rates an evanescent field on the sample surface. The evanescent field decays
in an exponential manner with increase of the distance from the surface. The
decay length, which is defined as the distance from the surface to the point
at which the field intensity decays to 1
/
e
2
, is a fraction of the wavelength
of the laser light. If a small particle is located in the evanescent field, the
particle scatters the field and the scattered light can be detected far from
the surface. The scattered light intensity also changes as a function of the
distance between the surface and the particle in an exponential manner. The
relation between the distance and the scattered light intensity is
−
I
=
A
exp
{−
2
Δz/α
}
,
(1.4)
where
α
is the decay length of the evanescent field and
A
is a constant.
From this relation, the displacement of the particle can be obtained through
scattered light intensity measurement.
Figure 1.35 shows a measured result of a surface force on a glass sur-
face. The sample was a clean glass substrate and the probe particle was a
polystyrene latex particle of 1
m diameter, dispersed in water. The curves
in the figure show the different results taken under three different solution
conditions. Plot (a) is the case of deionized water as surroundings, plot (b)
is that of 0.01 M tris-aminomethane solution, and plot (c) is that of 0.1 M
μ
