Micromanipulation Using Squeeze EffectTetsuyou Watanabe, Nobuhiro Fujino, and Zhongwei Jiang
Department of Mechanical Engineering
Yamaguchi University
Ube, 755-8611, Japan
Email: te-watanabe@ieee.org & z075fb, jiang@yamaguchi-u.ac.jp

Abstract— In this paper, we proposed a novel strategy forpick and place operation in a micro range, by using a squeeze
effect. In a micro range, the attracting forces such as the vander Waals, capillary, and electrostatic forces are dominate
due to the scaling effect. The attracting forces make a releaseof an object difficult. In this paper, by vibrating the finger,
we generate the gas film (the squeeze effect) between theobject and the finger and relax the attracting forces. Someexperimental results are shown to verify our approach.

I. I NTRODUCTION
Recently, there has been a growing interest in a manipulation of a micro/nano sized object. It is the skill required
to assemble or maintain microcomputers, micro electronics
parts, a micro medical equipment, and so on. Different
from a manipulation in a macro range, we cannot neglect
the attracting force between a micro object and end effectors. In the macro range, the van der Waals, capillary, andelectrostatic forces (proportional to surface area) become
more significant than the inertial and gravitational forces(proportional to volume), because of the scale effect [1],[2]. The attracting force is resulted from the van der Waals,
capillary, and electrostatic forces. Then, even in a basic
operation such as pick and place, a release of an object isvery difficult.
Many researchers have discussed how to release a micro
object [3]–[9]. Arai et. al. [3] proposed an adhesiontype micro endeffector. There are micro holes on theendeffector. By controlling the pressure inside the holesby temperature, we can absorb and release a micro object.
But it is hard to control the temperature because thetemperature is influenced by an environment. Also treatableobjects depend on the size of the holes. Zesch et. al. [4]developed a vacuum gripping tool consisting of a glass
pipette and a computer controlled vacuum supply. But
treatable objects depend on the size of the hole of thepipette. Rollot et. al. [5] proposed a method for pick andplace of a micro spherical object when the endeffector
has higher surface energy than the table (substrate). The
problem is the release of the object. The release was doneby slopping the endeffector. But the strategy can be applied
to limited objects. Then, Haliyo et. al. [6], [7] proposed a
strategy for the release, which is to vibrate the endeffector
and give the micro object enough acceleration to remove
from the endeffector. However, it is hard to control the
motion of the object after the release and to preciously
position the object. Saito et. al. [8] proposed a method
for pick and place of a micro object under an SEM.

But, treatable objects are limited to a sphere. Saito et. al.
[9] proposed a way for detachment of an adhering micro
particle from a probe by controlling the electrostatic force.
But, It is hard to control the motion of the object and to
preciously position the object.
In this paper, we propose a novel way for pick andplace operation in a micro range. The strategy is basedon a squeeze effect [10], [11]. The squeeze effect is aphenomenon of tribology/lubrication. When the distance
between the two surfaces is very small and one/both of
the surfaces moves vertically to the surfaces, a pressurecauses between the surfaces. The pressure can relax theattracting forces. By using this phenomenon, we propose anovel method for manipulation. This method can provide
a precious operation. Also, the strategy can be applied toany arbitrary shaped object and can be simply constructed.
This paper is organized as follows. At first, the target
system is shown. Next, we describe about the reduction of
attracting forces. Then, we propose a novel way for pick
and place in a micro world. Finally experimental results arepresented in order to show the validity of our approach.
II. TARGET S YSTEM
The target system is shown in Fig.1. In this paper, weconsider a pick and place operation of a micro object in aplanner space (a gravity force doesn’t work). The operation
is done by a gripper constructed by a pushing finger and a
support finger. The pushing finger plays a role of pushing
the mico object toward the support finger/a substrate. The
support finger plays a role of supporting the mico object
against the pushing forces from the pushing finger. Using
these fingers, we consider picking and placing a sphericalobject on the substrate. For the simplicity, we deal with aspherical object, but the proposing strategy can be applied
to other shaped objects.
A. Experimental set up
Fig.2 shows the experimental set up. This system consists of the manipulation system, the image-capturing system, and the finger-oscillating system. The manipulation
system consists of the pushing finger, the support finger,
the substrate, and the object. The pushing and support
fingers are copper cuts in size of 45×3×0.3mm (see Fig.3).
On the pushing finger, the piezocell (FUJI CERAMICS,
Z0.2T50×50×50S-W C6) of 4×3×0.3mm is bonded as
an actuator for oscillating it. These fingers are attachedmicropushing object
fingersupportfinger(a) Case I
substrate

Fig. 4.

Fig. 1.

(b) Case II
Two cases for analysis

Target System

z
r0
θ

r

support fingerzsurface for vibrationφpushing fingerh0

manipulation systemmanipulation system

Fig. 2.

Experimental set up

45mm

Fig. 5.

r0hModel for analysis of squeeze effect

45mm

0.3mm
3mm

0.3mm
3mm

4mm

PZT

(a) The pushing finger
Fig. 3.

(b) The sopport finger

Overview of the fingers

with the three-dimensional manipulator (NARISHIGE, M152). The substrate is also a copper cut. The fingers andthe substrate are grounded for preventing an extra charge.
The object is a glass sphere (UNION, unibeads) with aradius of 100[µm]. The image-capturing system is forrecording the movie of the manipulation. The manipulation
is captured by the CCD camera (IAI, CV-S3200) throughthe microscope (MORITEX, ML-Z07545). The captured
data is send to PC through the capture board (V-STREAM,
VS-TV2800R). The oscillation of the pushing finger is generated by oscillating the piezocell by the function generator(YOKOGAWA, FG120) through the power amplifier (NF,4010).
III. R EDUCTION OF ATTRACTING F ORCES
In this section, we present a novel way for reducing theattracting forces between the object and the finger.
For the pick and place operation, we consider reducing
the attracting forces which work between the pushing finger
and the object in the two cases shown in Fig.4. By notoscillating the finger in contact with the object but making
the oscillating finger contact with the object, we reduce theattracting forces. Note that the distance between the finger
and the object is very small. In this case, we can take thefollowing advantages; (1) The squeeze effect is caused anda pressure between the finger and the object is generated.
The pressure can reduce the attracting forces between thefinger and the object. (2) The acceleration is generated at
the tip of the finger and the acceleration can counteract theattracting forces. In the following, we address the detail ofthe above two phenomena and the attracting forces in thecase where the oscillated pushing finger just comes into
contact with the object and any adhesions don’t still occur,and then we show that the two phenomena is effective for
reducing the attracting forces.
A. Squeeze Effect
In this subsection, we describe the squeeze effect [11].
Here, we consider generating a gas film between the tipof the finger and the spherical object. For the simplicity,
we regard the tip of the fingers as a surface and thesurface is assumed to be oscillated sinusoidally in a verticaldirection to the surface (see Fig.5). We make the following
assumption; (1) The flow is Newtorian, isothermal, and acompressible perfect gas, (2) The inertia effect of the flowis negligible, (3) The sphere (object) is in stationary state.
In this case, the flow is governed by the following
generalized Reynolds equation [12];
∂P
1 ∂
∂P
1 ∂
(Q̄p P H 3 R)+ 2(Q̄p P H 3
)
R ∂R
∂R
R ∂Θ
∂Θ
∂(P H)∂(P H)
= σ
+Λ
(1)
∂T
∂Θ
where R: normalized r coordinate (= r/r0 where r0
denotes the representative length (the radius of the sphere)),Θ: normalized θ coordinate, T : normalized t (time)(= ωt where ω denotes the frequency of the oscillation), P : normalized pressure (= p/pa where p denotes
the pressure and pa denotes the atmospheric pressure ),H: normalized thickness between the surface and theobject (= h/hr where hr (= r0 ) denotes the representative
length), σ: squeeze number (=12µω/pa where µ denotes
the viscosity of the gas), Λ: bearing number (=6µU/pa hrQ̄p = Qp (D, α)/Qcon (D),Qcon (D) = D/6

0.4
Load Capacity ( Fst [µN])

5
4
3
2
1
0
-1
-2
-3
-4
-5
0

π/2

where R denotes the gas constant and T denotes thetemperature. It is hard to calculate Q̄p and then a database of Q̄p was made for easy calculation [13].
Considering the symmetry of the system, Equation (1)can be reduced to
(4)

(5)

where δh denotes the amplitude of the oscillation and2
λ(p, r) = −p(x) log((r − x)2 )dx + const(6)πE 
2/E = (1 − ν12 )/E1 + (1 − ν22 )/E2
where Ei and νi (i = 1, 2), respectively, Young’s modulus
and Poisson’s rate of the surface (i = 1) and the object(i = 2). If we normalize h with respect to hr (= r0 ), thenormalized thickness of the film is expressed by
H = H0 + δH cos T + R2 /2 + Λ(P, R)(7)

where H0 = h0 /hr , δH = δh/hr , and Λ(P, R) is anormalized λ(p, r).
The boundary conditions with respect to P (R, T ) are;a) P (1, T ) = 1: The pressure at the periphery is pa at all(0,T )
times. b) ∂P∂R
= 0: The slope of the pressure profile atthe center is zero at all times.
Solving (4) for P subject to (7) and the above boundary
conditions, the pressure exerted on the surface and theobject at time T is expressed by
Ps (R, T ) = P (R, T ) − 1.

(8)

3π/2

0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
0

2π

Time [sec]π/2(a) Case I
Fig. 6.

π
3π/2Time [sec]2π(b) Case II
Transition of the load capacity

TABLE IL IST OF PARAMETERS
Young’s modulus (Cu)Young’s modulus (glass)Young’s modulus (Cu)Young’s modulus (glass)radius of the sphere
amplitude of the oscillation(Case I/Case II)
roughness (Case I/Case II)
atmospheric pressure
viscosity of the gas
frequency of the oscillation

E1
E2
ν1
ν2
r0
δh

12.3 [104 MPa]
7.5 [104 MPa]0.35 [-]
0.17 [-]
100 [µm]
0.17/0.024 [µm]

hb
pa
µ
ω/2π

0.05/0.5 [µm]0.1 [MPa]18.6[µPa s]
4.088 [kHz]

Then, the squeeze force at time T is expressed by
Fst (T )In a micro range, the elastic deformation of the materials cannot be negligible. Then, we consider the elastic
deformation of the surface and the object. In this case, thethickness of the film is given by [14]h = h0 + δh cos ωt + r 2 /(2r0 ) + λ(p, r)

tcap1 π tcap2(2)where Qp (D, α) is a Poiseuille flow rate coefficient,
Qcon (D) is the coefficient for continuous flow, D is aninverse Knudsen number, and α is a reflection coefficient.
D is expressed by√(3)
D = D0 P H, D0 = pa hr /µ 2RT∂(P H)∂P
1 ∂{Q̄p P H 3 R
}=σ
.
R ∂R
∂R
∂T

Load Capacity ( Fst [µN])

where U denotes the relative rotational velocity between
the surface and the sphere).
Note that this equation (1) is expressed by cylindricalcoordinates (Fig.5). The flow of gas is characterized by
Knudsen number Kn (= λ/h where λ denotes the molecular mean free path). The equation (1) was derived basedon the Boltzman equation in order to deal with a ultra-thingas film whose Knudsen number is large [12]. Note thatthe equation (1) is originally for large Knudsen numbers
but valid for arbitrary Knudsen numbers.
In the equation (1),=

 2π  1

Ps (R, T )R dRdΘ
 10
R(P (R, T ) − 1) dR.
= 2π
0

(9)

0

Then, the mean squeeze force is expressed by
fst

=

1
(pa r02 )=pa r02 2π

2π 0 2π  1
00

Fst (T ) dT

R(P (R, T ) − 1) dRdT. (10)Using the parameters shown in Table.I, we computethe pressure profile of the squeeze film. We measured
the displacement of the oscillating finger by the laser
displacement meter (SONY, VL10). The frequency of theoscillation is set to 4.088[kHz]. The input signal is set toa sine curve. The voltage of the amplitude of the input
signal is set to 10 [V] for Case I and 30 [V] for Case
II. We also measured the roughness (arithmetic averageroughness (Ra ) ) of the contact surface of the pushingfinger by surfcom120A (tokyo seimitsu). The values of theamplitude and the roughness in Table.I are the measured
values. Note that we set h0 = δh+hb . Namely, we consider
the case where the pushing finger just comes into contact
with the object. We applied Newton-Raphson method tofinite difference representations of (4) and (7).
The results about load capacity given by (9) are shownin Fig.6. The average of load capacity per one cycle (givenby (10)) was 0.16[µN] in Case I and 0.55[nN] in Case II.

y
l

fy

r0
θ

x

τ

θc

The vibration of the finger
4fy [mN]

5
0

-10

-4
π
3π/2
Time [sec]

2π

-60

(a) Case I
Fig. 8.

Fig. 9.

Interaction between sphere and plane

6

0
-2

π/2

plane

2

-5

h

π/2

π
3π/2
Time [sec]2π(b) Case II

5
4
3
2
1
0
0

Trajectory of fy

Capillary force (fmten [µN])

10fy [mN]

6

Capillary force (fmten [µN])

Fig. 7.
15

-150

rmen

z

π/2

π
3π/2
Time [sec](a) Case I
Fig. 10.

2π

0.6
0.5
0.4
0.3
0.2
0.1
00

π/2

π
3π/2
Time [sec]2π(b) Case II
Profile of the capillary force

B. the accelerarion of the tip of the finger
In this subsection, we cosider the accelerarion of the
tip of the pushing finger without considering the effectof the squeeze film. For the simplicity, we consider the
motion of the pushing finger in a planar space (for Case I,
see Fig.7). Let θ be the angle between the figner and thex axis. Let τ be the joint driving torque (applied by thepiezocell). Then, the equation of motion of the finger isgiven by τ = I θ̈ where I denotes the inertia moment withrespect to the proximal end of the finger. Letting l be thelengthTthe tip position of theTfinger is given
 of the finger,= l cos(θ) l sin(θ). If θ is very
by xt yt
T

T

l lθ
=
. Then, the (inertial)
small, xt yt
force equivalent to the joint torque τ , which works at the
tip of the finger in y direction, is given by
fy = I ÿ/l2 .

(11)Now, we oscillate the tip of the finger with the frequency
and the amplitude shown in Table.I. Then, yt can be written
as follows;y¨t = −(δh)ω 2 cos(ωt).

(12)

From (11) and (12), we get
fy = −I(δh)ω 2 cos(ωt)/l2 .

(13)

Using similar formulation, fy for Case II is given by
fy = −m(δh)ω 2 cos(ωt).

(14)

Substituing the values in Table.I and Fig.3 into (13) and(14), we get the trajectory of fy (See Fig.8).
C. Attracting Forces
The attracting forces cause due to the van der Waals,
capillary, and electrostatic forces [1], [2], [5], [15]. Amongthe three forces, capillary force is largest and is verylarge compared with the other forces. Then, in this paper,
we consider only capillary force. The capillary force,interacting between a sphere and a plane, is expressed by[1], [2], [5], [15]ften = (4πγr0 cos θc )/(1 + h/b)

(15)
−3

where γ denotes the surface tension (73 × 10 [N/m] forwater), r0 denotes the radius of the sphere, θc denotes thecontact angle of a liquid on the surface, h denotes the
distance between the surface of the sphere and that of theplane, b denotes the immersion height (b = 2rmen wherermen is a meniscus curvature radius. rmen =1.6 [nm] for
50% relative humidity ) (See Fig.9). Note that if h → 0
and θc → 0, equation (15) becomes ften = 4πγr.
The thickness of the liquid bridge which causes acapillary force is about 3[nm]. Then, we assume thatthe capillary force works when the distance between theobject and the finger is under 3[nm]. Let tcap1 and tcap2(tcap2 > tcap1 ) be the times when the distance between thesurface and the object hd (= h(r, t) − hb = h(1, t) − hb ) is
3[nm]. Considering the roughness of the surface and using
h given by (5), we calculate the capillary force. The results
are shown in Fig.10.
The mean capillary force per one cycle is expressed by tcap2
4πγr0 cos θcfmten = 1/2πdt.
(16)
1 + h(t)/btcap1
We obtained fmten = 0.26 [µN] in Case I and fmten =0.075 [µN] in Case II.
D. Discussion
Based on the above analysis, we consider the reductionof the attracting forces. For the simplicity, we consider thecase where the oscillated pushing finger just comes into
contact with the object and any adhesions don’t still occur.
At first, we consider Case I. In this case, the average ofthe squeeze force (0.16 [µN]) is approximately same as thatof the capillary force (0.26 [µN]). Then, we believe that thesqueeze force can almost counteract the capillary force that

works between the pushing finger and the object (ftp−o ).
In addition, from Fig.10(a), we can see that the ftp−o itself
is reduced (note that when there is no oscillation (and thepushing finger contacts the object), the magnitude of ftp−o
is always the maximum value shown in Fig.10(a)).
If we analyze from the local viewpoint, from Fig.6(a)and Fig.10(a), we can see that ftp−o is reduced by thesqueeze force in the almost all cases where ftp−o works.
When the squeeze force cannot reduce ftp−o , ftp−o itself
is reduced for an increase of the thickness (h). When
there is no oscillation, ftp−o is same as the capillary force
that works between the substrate and the object (ftsu−o )because both the roughnesses of the contact surfaces aresame (we use the same part of the copper cut as the contact
surface). Then, we think that ftp−o is smaller than ftsu−o .
On the other hand, from Fig.8(a) and Fig.10(a), we can seethat the very large (inertial) force at the tip of the pushingfinger (fy ) works compared with ftp−o when ftp−o works.
Then, we believe that ftp−o can be completely counteracted
by fy (note that ftp−o <ftsu−o ).
Next, we consider Case II. In this case, we cannot getenough large squeeze force (0.55[nN]) to reduce ftp−o(0.075 [µN]). However, from Fig.10(a), we can see that
ftp−o itself is reduced. In addition, even in the case where
there is no oscillation, ftp−o is smaller than the capillary
force that works between the support finger and the object(fts−o ) because the roughness of the contact surface of the
support finger is smaller than that of the pushing finger(see Table.I).
If we analyze from the local viewpoint, from Fig.8(b)and Fig.10(b), we can see that very large fy works compared with ftp−o when ftp−o works. Then, we believe that
ftp−o can be completely counteracted by fy (note that
ftp−o <fts−o ).
If we summarize, we believe that in the both cases, if
we make the oscillating pushing finger contact with theobject, the attracting force is always counteracted, namely
adhesions don’t occur, and it is easy to remove the finger
from the object. Note that if a decrease of h (δh) for thecontact isn’t large, we think a similar analysis can be validand similar results can be obtained (note that an increaseof h (δh) makes the squeeze force and fy increase). But
the detail analysis is our future work. Note also that if
we oscillate the pushing finger in contact with the object,
the above analysis cannot become valid because the profileof the oscillation is absolutely different and ftp−o always
works.
E. Confirmation of reduction of the attracting forces
Based on the above analysis, we do simple experiments
in order to confirm whether it is easy to release the object
or not. In this paper, we pay attention to the friction at thecontact surface between the pushing finger and the object.
If the attracting forces are reduced, the total force exerted
on the contact surface becomes smaller and the sliding
at the contact surface becomes easy to occur. Then, weinvestigate the friction at the contact surface by making thefinger slide on the object. If the sliding is smooth, it means

pushing finger

substrate(a) Initial stateFig. 11.

(b) Medium state(c) Final state

Confirmation of reduction of the attracting forces in Case I
support finger

pushing finger(a) Initial stateFig. 12.

(b) Medium state(c) Final state

Confirmation of reduction of the attracting forces in Case II

the attracting forces are enough reduced. If the sliding is
hard to occur, it means the attracting forces aren’t enough
reduced and it is hard for the finger to release the object.
At first, we placed the object on the substrate and made theoscillating finger contact with the upper side of the object(Case I). Then, we slid the finger on the object (Fig.11).
The frequency of the oscillation was set to 4.088[kHz].
This frequency corresponds to the 4th mode of the finger.
The input signal was a sine curve. We investigated the cases
where the voltage of the amplitude of the input signal is0, 10, 20, 30[V]. In the case where the voltage was 0[V],The object rotated and the finger didn’t slide on the object.
In the case where the voltage was larger than or equal
to 10[V], the finger slid on the object. Fig.11 shows thecase where the voltage is 10[V]. From these results, theattracting forces can be enough reduced in the case wherethe voltage is larger than or equal to 10[V].
We also investigated in Case II (Fig.12). At first, weplaced the object on the support finger and made theoscillating finger contact with the left side of the object.
Then, we slid the finger on the object (Fig.12). The
frequency of the oscillation was set to 4.088[kHz]. The
input signal was a sine curve. We investigated the cases
where the voltage of the amplitude of the input signal is0, 30, 35, and 40[V]. In the case where the voltage was
0[V], The object rotated and the finger didn’t slide on theobject. In the case where the voltage was larger than orequal to 30[V], the finger slid on the object. Fig.11 shows
the case where the voltage is 30[V]. From these results, theattracting forces can be enough reduced in the case wherethe voltage is larger than or equal to 30[V].
IV. S TRATEGY FOR PICK AND PLACE OPERATION IN A
MICRO RANGE

Based on the above analysis and the experimental results,
we propose a novel strategy for pick and place operation
in a micro range.
The procedure of the strategy is as follows (Fig.13):(1) This is the initial state. (2) Make the support finger
contact with the object on the substrate. (3) Make theV. C ONCLUSION
support finger

pushing finger
object (sphere)
substrate

(1)

(2)

(3)

(4)

(5)

(6)

In this paper, we proposed a novel strategy for pick andplace operation in a micro range. In a micro range, theattracting forces such as the van der Waals, capillary, andelectrostatic forces become dominate due to the scaling
effect. The attracting forces make a manipulation of an
object difficult. In this paper, by vibrating the finger, we
generated the squeeze film between the object and thefinger. We showed the squeeze film can make the attracting
forces relaxed and the manipulation of the object easy. We
proposed a strategy for pick and place operation using thesqueeze film. This strategy can be applied to any arbitrary
shaped object and give a precious manipulation. The architecture of the strategy is very simple and can be embedded
into a conventional end-effecter. The experimental results
showed the effectiveness of our approach.
R EFERENCES

(7)

(8)
Fig. 13.

(9)

Pick and place operation

oscillating pushing finger contact with the object on thesubstrate from the left side. (4) Pick the object by moving
the pushing and support fingers upward. (5) Place the object by moving the pushing and support fingers downward.
(6) Remove the pushing finger from the object. (7) Make
the oscillating pushing finger contact with the upper side of
the object. (8) Remove the support finger from the object
by moving it upward. (9) Remove the pushing finger fromthe object by moving it upward.
Note that in the procedure (6) and (9), we can easily
remove the pushing finger for the oscillation of the finger(squeeze effect), which relaxes the attracting forces.
The experimental results are shown in Fig.13. The
frequency of the oscillation of the pushing finger was set
to 4.088[kHz]. The input signal for the oscillation was asine curve and the voltage of its amplitude was set to 30[V]in the procedure (3) ∼ (6), and to 10[V] in the procedure(7) ∼ (9). This setting is based on the above analysis andthe experimental results. From Fig.13, it is clear that thepick and place operation was done successfully. We did thisoperation several times. We succeeded every time. We also
did this procedure without the oscillation of the pushingfinger. In this case, the attracting forces interfered and theoperation resulted in fail (for example, in the procedure (9),the object removed from the substrate for the attracting
forces). Also, we tried to take this strategy with othermaterials: aluminium for the two fingers and polystyrene
for the substrate (the object is the same material). The
operation was also done successfully in this case.

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