Connected Droplet Shape Analysis for Nanoflow Quantification in Thin Electroosmotic Micropumps and a Tunable Convex Lens Application

Abstract

Thin electroosmotic flow (EOF) micropumps can generate flow in confined spaces such as lab-on-a-chip microsystems and implantable drug delivery devices. However, status quo methods for quantifying flow and other important parameters in EOF micropumps depend on microfluidic interconnects or fluorescent particle tracking: methods that can be complex and error-prone. Here, we present a novel connected droplet shape analysis (CDSA) technique that simplifies flow rate and zeta potential quantification in thin EOF micropumps. We also show that a pair of droplets connected by an EOF pump can function as a tunable convex lens system (TCLS). We developed a biocompatible and all polymer EOF micropump with an SU-8 substrate and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrodes. We microdrilled a channel through the electrode/SU-8/electrode layers to realize a monolithic EOF micropump. Then, we deposited a pinned droplet on each end of the microchannel so that it connected them. By controlling the EOF between the droplets and measuring the corresponding change in their shape, we quantified the nanoliter EOF rate and zeta potential at the interface of SU-8 with two liquids (deionized water and a l-glutamate neurotransmitter solution). When the droplet pair and pump were used as a TCLS, CDSA successfully predicted how the focal length would change when the pump drove fluid from one droplet to another. In summary, CDSA is a simple low-cost technique for EOF rate and zeta potential measurement, and a pair of droplets connected by an EOF micropump can function as a TCLS without any moving parts.

Figure 1

Schematical sequence showing the fabrication process of a complete EOF device used for CDSA. (a) SU-8 negative photoresist was spin-coated on a 2 in. glass wafer coated with a PET sheet and exposed using a laser writing system to crosslink an area of 8 × 8 mm dimensions and 155 μm thickness. (b) The exposed SU-8 was developed and mechanically released from the PET sheet using a razor blade to obtain the free standing SU-8 microstructures. (c) The SU-8 microstructures were held in self-closing forceps and dip-coated in the PEDOT:PSS liquid electrode polymer. The excess liquid was removed by spinning the microstructure on a spin coater to obtain a uniform electrode coating. The coated devices were baked for ∼6 h, and PEDOT:PSS layers from the sides of the SU-8 structure were removed by gently sanding the sides of the device. This step prevented shorting of the two electrodes. (d) The 50 μm diameter microchannel was drilled through the device using a 50 μm drill bit. (e) Spray-on hydrophobic coating was used to conformally coat the device to create a 1 mm diameter island for pinning the SD and PD. A part of the device was left uncoated to allow electrical contact between the pincers and the EOF device electrodes. Kapton tape was used as a mask where the coating was not required. (f) Schematic showing the complete EOF device with the location of the droplets and the dielectric coating.

Figure 2

Schematic of the experimental setup and close-up view of the connected SD and PD. (a) Schematic showing the experimental setup for CDSA. The EOF device consisted of an SU-8 microstructure (155 μm thick) sandwiched between two PEDOT:PSS electrode layers (1 μm thick) held in custom pincers with integrated copper electrodes for supplying voltage. The SD and PD were deposited near the openings of the 50 μm diameter microchannel. Voltage application was controlled by a LabVIEW program operating the sourcemeter. The EOF device and the pincers were housed inside a humidifying chamber to reduce droplet evaporation. Side view of the real-time SD and PD deflection was recorded using a 5 MP CMOS camera. (b) The SD and PD arrangement on an EOF device. The EOF device consisted of a passive SU-8 layer sandwiched between two PEDOT:PSS polymeric electrodes.

Figure 3

Deflection of the droplet versus applied voltage and flow quantification methodology. (a) Frames showing the deflection of the SD and PD when 10 V is applied to the PEDOT:PSS electrodes for 15 s. The device outline is shown by the dotted orange line and the microchannel outline is shown by the red dotted line. A green LED is visible when the voltage is non-zero. (b) SD height vs time when 10 V is applied. (c) SD volume transient calculated from the height transient. The inset shows the initial portion of the transient where it is approximately linear (i.e., the flow rate is approximately constant).

Figure 4

Effect of droplet volume on SD height transient. (a) Transient response for HV (0.75 μL) and LV (0.5 μL) scenarios (N = 4 devices, n = 24 trials). (b) Box plot of droplet deflection at the end of voltage application at t = 15 s. The red line is the median, the box edges are the 25th and 75th percentiles, and the whiskers span the data points that do not appear to be outliers. (c) Side view of the representative HV droplet and (d) LV droplet. Both side view images were acquired at t = 0 s.

Figure 5

Effect of voltage on the flow rate generated by the EOF device. (a) Representative figure showing the transient variation of SD height for different applied voltages (5, 10, and 15 V). (b) Flow rate generated at different voltages of 5, 10, and 15 V (N = 4 devices, n = 12 measurements per voltage). The features of the box plot are defined as described in the caption for Figure 4b.

Figure 6

Numerical simulation of droplet dynamics. Transient SD height characteristics found experimentally and by COMSOL simulation for device 1 in the (a) HV case and (b) LV case. The thin multicolor lines represent the droplet deflection predicted by COMSOL for zeta potential values ranging monotonically in steps of 0.005 V between the values stated on the charts, whereas the thick dashed line represents the averaged experimental data (n = 3 each) for HV and LV cases. (c) Flow rate comparison between experiments and numerical simulations shows similar transient flow rate characteristics when 10 V is applied for 15 s to drive the flow from the SD to PD. Flow rate from experimental trials (n = 3 each) from HV and LV cases are averaged and presented as solid red and black lines, respectively. (d) Flow profile in the microchannel at various times for an HV scenario.

Figure 7

Comparison of SU-8/glutamate zeta potential measured using CDSA and the CMM. Zeta potential increases with decreasing glutamate concentration (for CDSA, N = 4 devices, n = 12 trials; for CMM, n ≥ 13). Red and blue solid lines are plotted through the averages for CDSA and CMM measurements, respectively.

Figure 8

Optical application of CDSA for changing the magnification and sharpness of microscopic objects. (a) Montage of images showing side views of the SD and PD droplets when 0, −50, and 50 V are applied to the EOF device. [a(i)] SD and PD droplets at the equilibrium. [a(ii)] Fluid from the PD flows up into the SD when a −50 V is applied to the EOF device. [a(iii)] Reversing the EOF of fluid from the SD to PD. (b) Montage of images corresponding to the sequence in subplot (a) when an 80 μm diameter sphere (indicated by the white arrow) is viewed through the droplets from the top. [b(ii)] lower sharpness of the microsphere can be observed when −50 V is applied, whereas higher magnification can be observed in plot [b(iii)]. (c) Montage showing images of a PCB label (∼1 mm in height) being brought in and out of focus by controlling EOF voltage. (d) True size geometry of the droplet system showing the range of droplet deflection (solid lines) and a corresponding change in the focal point (+mark). The focal point is at the lowest (lightest gray color) when +50 V is applied, whereas the highest focal point (black color) is achieved when −50 V is applied. (e) Plot showing the measured dimensions of the SD and PD and the corresponding focus distances.