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Homework Solution #6 MEMS – MECE E4212
Problem #1
The force balance on the beam must take into account the surface tension,
γ
, over the
circumference of the drop, and the pressure difference between the drop and the ambient air.
2
sin
2
R
P
R
F
π
α
γ
∆
+
−
=
The pressure difference is given by
∆
P = P
i
– P
o
.
To find
∆
P, analysis of the curved surface
between the droplet and the air gives the following force balance:
−
=
∆
a
R
P
1
1
where
cos
2
h
a
=
−
−
−
−
=
−
=
∆
6
1
30
cos
2
6
5
.
1
1
3
72
cos
2
1
E
E
E
h
R
P
Pa
P
282
,
118
−
=
∆
A more simplified method of balancing the forces between the droplet and the air gives
∆
P as the
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 Fall '11
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