A 128-PIXEL DIGITALLY-PROGRAMMABLE MICROFLUIDIC PLATFORM FOR
NON-CONTACT DROPLET ACTUATION USING MARANGONI FLOWSAmar S. Basu and Yogesh B. Gianchandani
Department of Electrical Engineering, University of Michigan, Ann Arbor, USA
Abstract: A contactless programmable fluidic processor utilizing Marangoni flows is presented. Twodimensional control of microdroplets suspended in oil is achieved using a 128-pixel heater arraysuspended 100-500 m above the oil layer. The heaters create surface temperature perturbations (<7°Csimulated), resulting in local recirculating Marangoni flows which can move droplets in either a push orpull mode. Programmed movement is achieved by the sequential activation of the heaters. Addressible
control of each heater is provided by digital circuitry and a graphical interface. Droplets with diameters300-1000 m can be manipulated and merged at speeds up to 140 m/s. This system provides a platform
for high-throughput droplet-based assays where the reagents do not contact any solid surface.
Keywords: Marangoni flow, programmable fluidic processor, droplet, microfluidics, micro-TAS.
I. INTRODUCTION
Microdroplet-based
programmablefluidic
processors (PFPs) provide a versatile platform foruser-defined, high-throughput biochemical assays[1]. Droplets are manipulated on a 2-dimensional
grid using methods such as electrowetting-ondielectric (EWOD) [2], dielectrophoresis (DEP)[1], and magnetic fields [3].
All of theseapproaches, however, require a substrate patternedwith electrodes or coils, and all allow some degreeof contact between the droplet and the substrate,
both of which raise concerns for device
contamination, sample loss, and reusability.
This paper describes a scalable system forprogrammable droplet manipulation which does
not require patterned substrates or droplet
interactions with solid surfaces. Droplet actuationis accomplished using Marangoni flows, generated
by a 128-pixel array of heaters held above the oillayer. Previously we have shown that a small heatsource, when placed <500 m above a layer of oil,creates a local temperature increase on the liquidsurface, accompanied by a local decrease insurface tension. This results in Marangoni floworiented outward (away from the heat source) on
the fluid surface, and inward below the surface [45]. The recirculating flows can trap, filter, andpump aqueous droplets in oil, and can also movethe droplets if the heat source translated laterallywith a scanning stage [5-7]. The present effort
incorporates these concepts into a programmablesystem for non-contact actuation of microdroplets(Fig. 1) where the droplets are moved by theFig. 1: A complete system for programmable, noncontact droplet actuation based on Marangoni flows,
including a 128-pixel array of resistive heaters
suspended above the oil layer, control circuitry, anda graphical user interface.

sequential activation of the heaters rather thantranslation of the heaters. Physical structures suchas microchannels or electrodes are avoided, thus
simplifying the fluidic substrate and reducing
contamination. The favorable scaling of theMarangoni effect at small dimensions provides anopportunity for further miniaturization.
II. DEVICE CONCEPT AND DESIGNA 128-pixel array of heaters is suspended <500
m above an oil layer. Each heater, when
activated, creates a temperature gradient on theliquid surface which generates surface andsubsurface Marangoni flows (see simulation, Fig.

Latching Signal

VRAddress

ShiftRegister
Clock

8-Bit Shift Register

Data Aquisition Card

Serial Input

7On/offLatch
Selection
Logic

RH=100Ω

EnableDMOS
Driver

D D-Latch Q
Reset

Asynchronous Clear

Fig. 4: Schematic of logic and driver circuitry.

Fig. 2: CFD simulation (FLUENT 6.0). (a)Temperature contours generated by the heater andvelocity vectors of the surface and subsurface
Marangoni flows. (b) Flow velocity vs. heatertemperature for a 100 m, 200 m, and 500 m gap
between the heater and liquid.

2). The heater can either push a droplet away with
its surface flow (push mode actuation), or pull adroplet toward it via its subsurface flows (pullmode actuation) depending on the conditions
discussed in section III. Droplet movement can be
programmably controlled through the sequential
activation of individual heaters.
The 16x8 heater array is implemented as #0603
surface-mount resistors (1x0.8x0.3 mm) placed at
1.9 mm pitch on a printed circuit board (Fig. 3).
Heaters are individually set and reset via agraphical interface and control circuitry (Fig. 4).
D-latches maintaining the state of each heater areset by serial commands from the DAQ card. The
refresh rate is ~105 heaters/second, therefore, aFig 3: Micrograph of heater array.

128 pixel array can be set in <5 ms. The control
interface includes 3 signals for setting/resetting
individual pixels along with an asynchronous clear
to deactivate the entire array. The heaters have
100  resistance and are driven by power DMOS
transistors tied to an isolated supply and ground.
Figure 5 shows the experimental setup. The oillayer is placed on a glass slide so that droplet
motion can be imaged from below with a CCDcamera. A high-density oil (specific gravity=1.07)
is chosen so that droplets float on the oil surface
with no contact to the glass slide below. Droplet
samples are deposited directly into the oil using amicropipette. The heater array, with the chip
resistors facing the sample, is then lowered towithin 500 m above the oil surface using a
micromanipulator. Two ribbon cables provide
electrical interconnect to the control circuitry.
No customized microfabrication is required inthe present millimeter-scale implementation.
Further miniaturization has potential to improvepumping efficiency because the Marangoni effect,
like other surface phenomena, scales favorably tosmall dimensions. Miniaturization will, however,require additional attention to issues such as heatdissipation and thermal crosstalk.

Fig. 5: Experimental setup.

III. EXPERIMENTAL RESULTS
Individual heaters were characterized forthermal efficiency and capability to produce flow.
The thermal isolation of each heater is 100 K/W,
and a maximum temperature of 160 °C is obtainedon 1.3 W power (Fig. 6a). (Micromachinedstructures, by contrast, can achieve thermal
isolation >104 K/W.) The parasitic temperature
increase on the adjacent heater is 80 °C, roughly
50 % of the increase on the first heater. The bulk
substrate remains close to room temperature. Heatsinks and other thermal isolation were shown toreduce thermal crosstalk. Surface flow velocities
up to 1700 m/s are achieved using 25 m pollen
particles as tracers.
The simulated surface
temperature change corresponding to thesevelocities is <7 °C. (A consequence of further
miniaturization is that even lower temperature
perturbations can achieve the same flow velocities[4]). The velocity of droplets is less compared topollen due to their larger mass and drag (Fig. 6b).
A =300 m droplet, for example, moves atspeeds up to 73 m/s in the push-mode.
The push-mode of actuation occurs in single

Fig 7: Push-mode actuation. (a) Surface Marangoni
flows create a droplet force (FD) oriented away fromthe active heater. (b-e) A =900 m droplet is
moved along a square path by activating multiheater configurations sequentially. Active heaters
are shaded. PH=1 W/heater, gap400 m.

Fig 6: (a) Temperature of an active heater (TH) vs.
input power. Also shown is the temperature increase
one pixel away (T1), two pixels away (T2), and on thesubstrate 2 cm away (TS). (b) Flow velocity (25 mpollen) and droplet velocity (=300 m droplet) vs.
input power and heater temperature. Gap300 m.

layers of oil with depth >500 m, and when thedroplets are very small (<100 m). In thismode, surface Marangoni flows drive the droplet
away from the active heaters (Fig. 7). Multiheater configurations may be used to direct adroplet in the desired direction. For example, two
adjacent heaters next to a droplet push the droplet
in a straight line. A right-angle turn can beaccomplished using three heaters in an L-shape.
Arbitrary movements can be generated withsequential activation of such configurations.
The pull-mode of actuation utilizes subsurface
flows which bring the droplet toward the activeheater at speeds up to 140 m/s (Fig. 8). Thismode, which occurs in relatively thin layers of oil(<500 m) and multilayer oil systems, is moreeasily controlled because only one activated pixel
is necessary to trap a droplet. The sequential
activation of adjacent pixels can translate thedroplet along a virtual grid. Figure 9 shows thesimultaneous actuation of multiple droplets.
While a surface temperature rise of <7 °C is
necessary for actuation, higher temperature can be
encountered in various operations.
In suchFig 10: Preventing droplet evaporation. Droplet sizeis plotted vs. time (log scale) for a =650 m
droplet in single and double layer oil systems.

Evaporation times are on the order of hours, 100x
higher than the single layer oil system where thedroplet is exposed (Fig. 10), thus allowing forassays which require longer incubation times.
ACKNOWLEDGEMENTS
Fig 8: Pull-mode actuation. (a) Subsurface
Marangoni flows create a force on the droplet (FD)toward the active pixel. (b-e) Programmed
movement of a =600 m droplet. Active heaters
are shaded. PH=1 W/heater, gap400 m.

Fig 9: Droplet merging with pull-mode actuation.
Two droplets (=600 m) are merged throughsequential activation of the heaters. Active heaters
are shaded. PH=1 W/heater, gap300 m.

circumstances, droplet evaporation can present achallenge. For example, 1 mm droplets floating
on top of the oil layer tend to evaporate in <2 min.
To address this issue, two miscible oils werecombined, one with density less than water(mineral oil), and the other with density greaterthan water (Dow Corning 550 Fluid). Aqueous
droplets remain suspended between the twolayers, unexposed to both the air and the glass
surface below.
The suspended droplets aremanipulated using subsurface flows (pull mode).

The authors thank Ms. Seow Yuen Yee for help
with experiments, and funding from the Whitaker
Foundation, NSF, and the University of Michigan.
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