Online proceedings - EDA Publishing Association

Time of diffusion (hr)
1200
1000
800
600
400
200
0
11-13
May, 2011, Aix-en-Provence, France
Poliovirus
TYMV
Hepatitis B virus
Adenovirus
HIV
50 100 150
Diameter of virus (nm)
are selected to 1mm, 3mm, and 475m, respectively. The
height of the cylindrical structure will be discussed in the
following paragraph. First, the fluid rotates in direction Y due
to flow difference caused by the level difference between the
channels as the fluid enters the concaved-down surface from
the inlet. Second, rotation in direction Z is created by the flow
rate difference with compression of the fluid when the fluid
flow passes the sides of the cylindrical structure. This structure
facilitates the chaotic effect generated by the rotating flow of
the fluid, enhancing the mobility of the TYMV with the chaotic
flow. The chaotic streamline, might present more opportunities
for sensing surface attachments.
inlet
Fig. 2 Time to diffuse 5 mm for viruses
In micro-fluidics, usually the behavior of small particles
like microbes or even virus particles can be understood through
the Reynolds number and Péclet number. The Reynolds
number of a particle is shown in Equation (1).
av
Re (1)
a is the diameter of a particle and v is the velocity of flow.
and are density and viscosity of fluid, respectively.
The Reynolds number for TYMV in water with a speed of
order 10 m/s is calculated where the diameter of TYMV is 31.8
3
nm. Density and viscosity of water are 1000kg m and η
=10 –3 Pa-s, respectively. The Reynolds number for the TYMV
(3.18×10 – 7 ) is negligibly small. A small Reynolds number
means that the movement of molecules was dominated by
viscosity force and that molecules stop moving immediately
when the drag force is removed. The movements of viruses or
molecules depend on diffusion or Brownian motion. However,
regardless of whether the movement relies on diffusion or
Brownian motion, the motion is rather slow in terms of virus
movement. This is attributed to poor efficiency in the reaction
zone attached to the microchannel.
Additionally, the Péclet number is shown in Equation (2).
Ul
d
Pe
(2)
a
D
d
is molecular diffusion time and
a
is typical hydrodynamic
transport time. U, D and l are flow velocity, diffusion
coefficient of molecules and depth of microfluidic channel,
respectively. Taking TYMV as an example, the diffusion coefficient
in water is 1.35×10 –11 m 2 /s. The Péclet number is equal to 74.07 when
the fluid flow is 10 m/s while depth of microfluidic channel is
100m. This shows that hydrodynamic transmission is much
more effective than molecular diffusion effect.
III. MICROFLUIDICS SIMULATION & EXPERIMENT
3.1 Simulation
Figure 3 shows the type of microenvironment adopted for
this study. The design added a cylindrical structure onto a
microchannel with a cross-section area of 300m×3000m.
The diameter and the height of cylinder (Fig. 3) are W c and h c ,
respectively. The design used W s mm ×W s mm flat sensing
surface into a h s distance that is concave down. W c , W s , and h s
wc
ws
hc
hs
Au
PDMS
Fig. 3 Microfluidic devices to produce vortex
outlet
This study simulated the height of the cylindrical structure,
and the vorticity of the sensing field was adopted as an indicator
to determine the degree of fluid rotation. The height of the
cylindrical structure, hc, was divided into heights of 300 m,
400 m, 500 m, and 600 m. COMSOL Multiphysics is used
to predict the performance of microfluidic device.during
simulation. The vorticity simulation result is shown in Fig. 4
revealing a positive relationship between the overall size of
vorticity and the height of the cylinder in the sensing field. The
results most significantly reveal the vorticity variation on two
sides of the cylinder. It is noteworthy that the vorticity in the
bottom area of the 600m cylinder actually decreases rather
than increases due to high flow resistance. The aforementioned
easons show that a cylinder structure with a height of 500 m
generates a vorticity with a wider range and higher strength and
allows the fluid to generate a rotation flow effect more easily.
After the height of the cylindrical structure is determined as
500 m, the flow rate of fluid into the microenvironment is then
discussed. The study aims to determine the minimum rate of
vortex, allowing the microenvironment to generate rotational
flow in multiple directions. Hydrodynamics explains that
vortices are most likely to be revealed in a higher rate of flow.
Moreover, the microenvironment is combined by the
microchannel with the cylindrical structure and the substrate of
the concaved-down sensing surface. When flow rate increases
in the microenvironment, the pressure generated on the
sidewall also increases. To prevent the micro channel and
substrate from collapsing due to extremely high fluid pressure,
this study aims to determine the minimum rate of flow for
generating chaotic flow.
The vorticity component and flow chart of the sensing field
of X, Y, and Z directions when the inlet flow equals 20 ml / hr are
small and have no obvious variation. The flow chart also
reveals that the chaotic flow effect was not created in the
sensing field. When the inlet flow was constantly increased to
1500 ml / hr , the fluid in the cylinder front displayed significant
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