HIGH SPEED MICROFLUIDIC DOUBLET FLOW IN OPEN POOLS
DRIVEN BY NON-CONTACT MICROMACHINED THERMAL SOURCES
Amar S. Basu 1, and Yogesh B. Gianchandani
Department of Electrical Engineering and Computer Science
University of Michigan, Ann Arbor, USA
ABSTRACTWe report a phenomenon in which a micromachined
heat source placed less than 50 µm above the surface of aliquid drives a high-speed doublet flow pattern with linear
velocities reaching nearly 5 mm/sec and rotational velocities
up to 1200 rpm. Tests were performed on a 50-100 µmthick layer of water containing 3 µm polystyrene beads forflow visualization. The thermal source is a polyimide
cantilever with an integrated heater near the tip, operated
with input powers ranging from 0-32 mW. It has no moving
parts and does not contact the liquid. The speed of thedoublet flow scales with input power as well as liquid
temperature, and is inversely related to the air gap between
the heater and liquid surface. The orientation of the doublet
flow can be reversed by changing the angle of the cantilever.
A one-dimensional array of probes used in the same manner
generates a linear flow pattern.

I. INTRODUCTION
The ability to generate high speed micro-flow patterns
plays a critical role in the mixing, pumping, and preconcentration of particles, particularly for cellular andbiomolecular manipulations. There have been several
attempts in the past to generate vortex flow in microfluidicenvironments.
An opto-electrostatically driven vortex
pattern generated by a focused 50 mW laser spot in
combination with a 2 kV/cm electric field was shown tohave a maximum particle velocity of 120 µm/sec in
conductive liquids [1]. Vortices driven electrokinetically inpolymer channels with patterned surface charges [2] operate
on less electric field (100 V/cm), but produce slower
velocities and require ionic solutions. More recently, a laser
cavitation pump was shown to generate flows up to 1 mm/s
[3]. It required that an optical fiber to be immersed in theliquid and coupled to a high-power pulsed laser. Bubbles
generated in cavitation pumps can pose challenges for avariety of applications. Approaches using moving parts
include a 600 rpm magnetic micro-stirrer [4].
This effort reports a thermally driven phenomenon inwhich a micro-scale heat source suspended above water
drives a high-speed doublet flow patterns at the liquidsurface. Particle velocities of 5 mm/sec and rotational
velocities of up to 1200 rpm have been achieved, making it
potentially useful for high speed pumping and mixing ofliquids and suspended particles.
The heat source, amicromachined thermal probe, is separated from the liquidsurface by a small air gap and has no moving parts, thus
1Cantilever
Microheater
Substrate
Air Gap

Liquid Reservoir
Fig 1: Schematic of device operation. A heated micro-cantileversuspended above a liquid induces a high-speed doublet flow
pattern at the surface.

eliminating common problems of mechanical wear,
electrodecontamination,andbubble
generation.
Furthermore, the technique does not require ionic or
conductive liquids. In addition to generating single doublet
flow patterns, a linear array of probes is shown to generate alinear channel-like flow pattern that would be useful forpumping applications.

II. DEVICE CONCEPTA fluidic doublet is a two-dimensional flowcharacterized by two adjacent vortices of opposing rotational
directions, and resulting linear streamlines between them.
The flow pattern for a single doublet located at (x0 , y0) on a
Cartesian plane is described by the following stream
function [5]:( x, y ) =µ( y  y0 )2 ( x  x 0 ) 2 + ( y  y 0 ) 2(1)where µ is a constant reflecting the strength of the doublet.
A contour plot (Fig. 2a) illustrates the streamlines resulting
from a single doublet. The direction of flow can be inferredfrom the gradient of the  function, which in this case is left
to right.
To determine more complex flow patterns resulting
from multiple doublets, the stream equations for eachdoublet may be added up by applying the principle ofsuperposition. The flow pattern generated by an array of 8
doublets with 85 µm spacing is shown in Fig. 2b. Thisparticular geometry was chosen to model the structure of themultiprobe array. Flow patterns resulting from single andmultiple doublets present a number of opportunities for flow
manipulation; namely, the linear streamlines can be used todrive high speed particle flow, while the adjacent eddies canbe used for mixing or particle trapping.

Address: 1301 Beal Avenue, Ann Arbor MI 48109-2122, USA; Tel: (734) 709-6688; Fax: (734) 647-1781; E-mail: basua@umich.edu

300y ( m)

200

100

0100

200x ( m)

300

400

1000y ( m)

800
600(b)

0500x ( m)

1000

1500

Fig. 2: Theoretical flow patterns generated by single and multiple
doublets. (a) Streamlines for a single doublet located at (200 µm,150 µm) on a Cartesian plane. The contours of the stream function(Eq. 1) were plotted using a value of 1 for the constant µ. (b)Streamlines generated by an array of 8 doublets with 85 µm
spacing obtained by summing the respective stream functions.

In this effort, high-speed doublet flow at the surface ofwater is driven by a micromachined heat source suspended
at varying heights and angles above the surface of areservoir (Fig. 1). When the source is brought close to thesurface of the water (<50 µm), the thin air gap allows
transfer of heat to the liquid surface. The mechanism of how
the heated region drives the doublet flow is not yet clear;
however, initial data suggests that localized evaporation
plays a role.
The heat source is a micromachined thermal probe
reported in [6], consisting of a joule heater integrated near
the tip of a cantilever. Thin film metal forming the heaterand leads are embedded within a 3 µm-thick polyimide
cantilever. The excellent thermal isolation provided by apolymer-based cantilever allows the probe tips to be heatedto temperatures up to 250 ºC with <20 mW input power.
The length and width of the cantilever as well as proberesistance vary depending on the type of probe. In thiswork, two probe geometries are used: R01 (length 360 µm,width 42 µm, resistance 25-40 ), and R02 (length 360 µm,width 120 µm, resistance 20-35 ). In addition to the single

R2 = 0.971

2000
1000
0400200(b)3000Particle Velocity( m/sec)(a)Probe: RO2 (25Gap: 25 m
Angle: 150102030
Input Power (mW))

40Fig. 3: (a) High-speed flow is illustrated in 4 sequential
micrographs taken at 1/30 second intervals. 3 µm beads are used
to visualize the flow. (e) Particle velocities scale linearly with theinput power to the heat source. Angle and air gap are fixed at 25µm and 15°.

probes, an 8-probe array reported in [7] is also used to drive
multi-doublet flow patterns as shown in Fig. 2. In thisdevice, the cantilevers have 85 µm pitch and can be heatedindividually.

III. EXPERIMENTAL RESULTS
Experiments were carried out on a 50-150 µm thicklayer of water placed on a glass slide. Polystyrene beads
with 3 µm diameter were immersed in the liquid for flowvisualization. The micromachined cantilever was held at a
fixed angle and lowered towards the surface of the liquidusing a motorized micromanipulator. Micrographs andvideo were recorded using a CCD camera, and speed wasdetermined by measuring distance covered by sequential
frames spaced 1/30 second apart. In accordance withdoublet flow model, the velocity of a traveling particle
increases as it approaches the center of the doublet, which inthis case appears to be directly beneath the heated cantilevertip. In all experiments, the velocities indicated are themaximum values measured.
In order to show that the doublet flow is in fact
thermally driven, the flow and rotational velocities werecharacterized as a function of input power (Fig. 3). The
cantilever was held at a fixed angle and air gap above theliquid surface, and the input power was ramped from 0-32mW.
Particle velocities scale approximately linearly,

Angular Velocity(rpm)

Particle Velocity( m/sec)

Particle Velocity( m/sec)

4000
3000
2000 Probe: RO2 (35 )1000 Power: 15.4 mW0(a)

(b)

1000

5000

Angle: 150150020Air gap ( m)40

Probe: RO2 (35 )Power: 15.4 mW
Angle: 151000
5000020
Air gap ( m)40

Fig. 4: The doublet speed can be controlled by modulating the airgap. For a fixed 15 mW input power to a 35 ohm probe, particle
velocities increase nearly 5000 µm/sec as the air gap is reduced to<10 µm. Rotational rates in the adjacent vortices approach 1300rpm.

increasing 90 µm/sec for every 1 mW applied. These results
imply that the flow is proportional to tip temperature, since itis well known that tip temperature increases linearly withinput power [6].
Reduction of the air gap permits more efficient heattransfer, also resulting in increased velocities. A probe wasbiased at a fixed input power and lowered at 10 µm intervals
until it came into contact with the liquid. Figure 4 showsthat with a 15 mW input power, particle velocities of nearly
5000 µm/sec are achieved at air gaps of <10 µm, along withrotational velocities of 1200 rpm in the adjacent eddies.
Particle velocities decrease as 1/x with increasing air gap,indicating that heat transmission through the gap is inversely
proportional to the gap thickness.
The dependence of particle velocities on liquid
temperature (Fig. 5) provides further evidence supporting
thermally driven flow. In this experiment, the glass slide
below the water sample was biased at 13, 27, and 41 °C
using a circulating heating/cooling plate.
At eachtemperature, particle velocities were measured as a functionof air gap while holding the probe at constant power andangle. Trends of faster velocities with smaller air gaps holdtrue as before, but it can also be seen that higher liquid
temperatures shift the entire trend upwards. For example, ata ~10 µm air gap, the particle velocity at 13 ºC is only 165µm/sec, compared to nearly 900 µm/sec at 41 ºC. Increased
liquid temperatures, therefore, enhance the speed of doublet
flow. Overall, particle velocities in this experiment wereslower than the results shown in Fig. 4 due to lower input
powers, and possibly due to the fact that a thinner probe(R01) was used.

800600Probe: R01 (27 )Power: 10.4 mW
Angle: 30

41 C
27 C13 C40020001020Air gap ( m)

30Fig. 5: Effect of liquid temperature on doublet flow. Particle
velocities are plotted as a function of air gap while biasing theglass slide at 3 different temperatures.
Increased liquidtemperatures and reduced air gaps enhance the speed of thedoublet flow. A 27 ohm probe fixed at 30° angle and 10.4 mW
input power was used in this experiment.

The geometry of the heat source plays a significant rolein the doublet pattern. For example, a probe (R02)
suspended at a 15° angle with respect to the liquid plane
results in doublet flow from left to right; however, when theprobe angle is doubled to 30° (keeping all other parametersconstant), the direction of flow is reversed, and the locationof the adjacent vortices shifts to the left as shown in Fig. 6.
The flow can also be manipulated into a linear channellike pattern by generating multiple doublets with arrayedheat sources. The 8-probe array was biased at 2.3V,
dissipating 92 mW total power in six probes (probes 1 and 4
were nonfunctional). The resulting flow pattern (Fig. 7) has
a linear flow region with adjacent rotational regions aspredicted by simulations (Fig. 2b). Distortions in flow areobserved in several regions, such as the trajectory marked
with a triangle, and can be attributed to the difference in air
gap between the various probes in the array in addition to thefact that two probes were not operational. The noticeably
smaller velocities (190 µm/sec) compared to single probes
may be attributed to the reduced temperatues of the heaters
in the multiprobe array. These results illustrate that flow isFig. 6: The direction of the flow can be reversed by changing theangle of the heater. For example, using probe R02 at a 15° tilt, 15mW input power, and a ~15 µm air gap, the doublet flows left toright, and the rotation in the vortices is counter-clockwise (a).
Increasing the angle to 30° while keeping all other parametersconstant reverses the direction of the flow and rotation and shifts
the location of the vortices to the left of the cantilever (b).

Fig 7: Linear flow generation using an 8-probe array, numbered 18 from the top down. The array was placed ~15-20 µm above theliquid surface, held at a 15° tilt, and biased with a total power of 92
mW. Probes 1 and 4 were nonfunctional. Trajectories for 3
particles are marked with a square, triangle, and circle on
micrographs taken at 3 second intervals.

highly dependent on the geometry of the heat transmitted tothe liquid surface. Therefore, it is possible to obtain custom
flow patterns by arranging the heat sources in various
configurations.
Subsurface particle flow, visualized by focusing themicroscope at the bottom of an 80 µm thick layer of water,differs significantly from the doublet flow patterns observedat the surface. Particles flow radially inward over time,
converging on a point directly below the tip of thecantilever. Once at this point, the particles are accelerated
upwards towards the surface of the liquid film (Fig. 8). The
column of vertically-directed current may be due toconvection and/or localized evaporation driven by the highliquid temperatures beneath the probe tip. Further modelingand experimentation is needed to clarify the mechanism
driving both the surface and subsurface flow patterns
observed.

IV. CONCLUSIONSIn summary, we have shown that high speed doublet
flow can be thermally generated by a micromachined heatsource operating without contact to the liquid, and a linear
flow profile can be achieved using by arraying the heatsources. Characterization of the flow patterns indicate that itis thermally driven by heat transmitted through the thin airgap. The high-speed particle velocities and rotational
velocities have potential applications in pumping andmixing.

ACKNOWLEDGEMENTS
Partial support for this effort was provided by TheWhitaker Foundation (AB). The authors also acknowledge
Dr. Shamus McNamara for his assistance in probefabrication.

Fig 8: (a) Schematic of subsurface particle flow (80 µm below thesurface). Particles flow radially inward towards the area underneaththe microheater tip. Upon reaching this point, they are immediately
propelled upwards to the surface. (b) Sequential micrographs show
the particles (marked with a square, triangle, and circle) converge
towards the center and then disappear from the field of view as they
are propelled upwards.

REFERENCES[1] A. Mizuno, M. Nishioka, Y. Ohno, and L. Dascalescu,“Liquid Micro-Vortex Generated around a Laser Focal Pointin an Intense High-frequency Electric Field,” Proc. Industry
Applications Society Annual Meeting, 1993, pp. 1774-1778.
[2] W.L.W. Hau, L.M. Lee, Y.K. Lee, M. Wong, and Y.
Zohar, “Experimental Investigation of ElectrokineticallyGenerated In-Plane Vorticity in a Microchannel,” Proc. Intl.
Conf. on Solid State Sensors, Actuators, and Microsystems,
June 2003, pp. 651-654.
[3] G.R. Wang, J.G. Santiago, M.G. Mungal, B. Young, andS. Papademetriou, “A Laser Induced Cavitation Pump,” J.
Micromech. Microeng., 14(7), July 2004, pp. 1037-1046.
[4] L. Lu, K. Ryu, and C. Liu, “A Magnetic Micro-Stirrer
and Array for Microfluidic Mixing,” J. Microelectromech.
Syst., 11(5), Oct. 2002, pp. 462-469.
[5] D. Young, B. Munson, and T. Okiishi, A Brief
Introduction to Fluid Mechanics, Wiley, NY, 1997.
[6] M.H. Li and Y.B. Gianchandani, "Applications of a Low
Contact Force Polyimide Shank Bolometer Probe for
Chemical and Biological Diagnostics," Sensors andActuators A: (Physical), 104(3), May 2003, pp. 236-245.
[7] S. McNamara, A. Basu, and Y. B. Gianchandani,“Ultracompliant Thermal Probe Arrays: Large Area
Mapping of Non-Planar Surfaces without Force Feedback,”
Proc. Intl. Conf. on Micro Electro Mechanical Sys,
Maastrich, The Netherlands, January 2004, pp. 825-828.