Supplementary Information



Electrostatic manipulation of freely suspended
droplets for liquid-liquid microfluidics


Orlin D. Velev*, Brian G. Prevo and Ketan H. Bhatt


Department of Chemical Engineering, North Carolina State University

Raleigh, NC 27695



* E-mail:


Submitted to Nature, 06/07/2003






Figure 1. Example of the design of an electrode pattern where droplets are moved along tracks, mixed, and can be switched to different paths. The red and the green leads are situated on the top and bottom part of the chip, respectively, and are connected through the holes.



Figure 2. Droplet speed plotted as a function of the field intensity squared. The data are for 750 nL aqueous droplets submersed a 1.15 mm deep PFMD layer. The speed was measured by the smallest time required for the droplet to traverse an automated 8-electrode sequence forwards and backwards. The field was estimated by dividing the voltage applied by the electrode pitch (1.54 mm). Frequency was 200 Hz.



Table 1. Estimate for the energy required to move a 500 nL water droplet 1 cm at 2 mm/s by the liquid-liquid microfluidics method described here, by moving of droplets on surfaces, and by conventional microfluidics with channels.



Droplet moved in F-oil

Hemispherical droplet dragged on solid surface

Viscous flow in microfluidic channel

Assumptions and approximations

         Stokes sphere in bulk liquid

         qAdvancing = 90 deg

         qReceding = 80 deg

         No viscous dissipation

         Circular channel of diameter 20 m

         Poiseuille flow

Type of estimation




Energy required / J

≤ 9.410-10

≥ 1.610-7

≥ 1.410-4

Energy ratio








Description of the supplementary movies (click on image to play)




Movie 1: Four droplets moving synchronously on parallel tracks

Four 750 nL droplets of aqueous suspensions are moved synchronously. The droplets contain (top to bottom) gold nanoparticles, 2 % white polystyrene latex, 2 % pink polystyrene latex, and 0.2 % white polystyrene latex. The voltage applied was 300 V/300 Hz. The top two rows of electrodes are 1 mm circles, and the lower two rows are 1 mm squares. Note that all latex microspheres in the lower three droplets accumulate on their top surfaces.




Movie 2: Driving water droplets with DC voltage

DC voltages allow moving water droplets faster than AC ones of same magnitude. These two 750 nL droplets contain dilute suspensions of white and red latex. The electrodes with a pitch of 1.54 mm were energized with -500 V. Dodecane droplets respond to DC fields in a similar manner, but more sluggishly.




Movie 3: Controlled mixing of droplets on a matrix

750 nL droplets containing white polystyrene latex and gold nanoparticles merge at the convergence of the two tracks of electrodes, and the mixed droplet moves on the single track.





Movie 4: Controlled mixing with chemical reactions

Two separate precipitation reactions are performed by synchronous movement of two pairs of droplets. On the top track solutions of CaCl2 and K2HPO4 are combined to form the white precipitate, Ca3(PO4)2. On the lower track drops of FeSO4 and NaOH are mixed to form the green precipitate, Fe(OH)2. The latter part shows the growth of the crystalline solids with time, and that the crystal shell particles can still be moved by dielectrophoresis because of the water core within. All droplets are 750 nL in volume and driven by voltages of 400 V/200 Hz.




Movie 5: Multistage process - mixing and encapsulation

Droplets of aqueous suspensions of gold nanoparticles and of white polystyrene latex (750 nL each) were mixed and subsequently encapsulated inside a 1000 nL dodecane droplet transported separately. A 200 nL droplet of Na-dodecyl sulfate (SDS) solution within the dodecane droplet facilitated its dielectrophoretic control. Encapsulation is also facilitated by small amount of SDS inside the aqueous droplets. The columns of electrodes are energized at 400V/200Hz.