| dc.description.abstract | Droplet manipulation on microfluidic platform has gained significant importance
due to its applicability to various healthcare technologies, where larger
equipment can be reduced to portable handheld systems based on microfluidic
devices. Electrowetting-On-Dielectric (EWOD) has emerged as one of the most
promising techniques for droplet manipulation in microfluidic devices because it
is an easily programmable, cost effective, reconfigurable and reversible
technique. Droplet creation, actuation, merging, mixing, and splitting are the
fundamental operations which enable biochemical assays on EWOD based
microfluidic platform. In conventional EWOD the droplet is sandwiched between
two substrates. This however reduces the accessibility of the droplet to the
device edges only. Recently, it has been demonstrated that EWOD based droplet
actuation is also possible on single sided electrodes. Enhanced droplet
accessibility on such an open-chip microfluidic platform holds promise for
development of complex platforms with better integration of external sensors
and actuators. In the quest to improve the fundamental EWOD operations for its
open-chip version, we have studied the role of droplet interface oscillation in
enhancing mixing and enabling localized sensing.
The thesis work presented here focuses on two main aspects of interfacial
oscillations: i) non-axisymmetric modes of droplet which appear due to the
parametric coupling during oscillations. We have demonstrated the use of these
modes for applications in mixing; and ii) localized electrowetting where only a
part of droplet interface is actuated instead of full droplet by patterning the
actuation electrodes. We have demonstrated the use of localized interface
actuation for localized sensing application.
In non-axisymmetric oscillations, droplets contact line loses its symmetrical
shape during spreading and expands to form asymmetrical lobes. The number of
lobes represent different mode shapes. These mode shapes were analyzed using
image analysis. The extracted interface was fitted to a Fourier series to extract
amplitudes of different modes. Analysis of the extracted mode amplitudes
indicated that above a certain actuation voltage (force), the non-axisymmetric
modes grew at the expense of the axisymmetric modes. This indicated a coupling
between the axisymmetric and non-axisymmetric modes. Investigations revealed
parametric coupling leads to manifestation of the large non-axisymmetric mode
amplitudes. Further, the non-axisymmetric modes were identified to be
degenerate modes as given by the spherical harmonic functions. These nonaxisymmetric
parametric oscillations were modelled using the Mathieu equation
to identify the regime of actuation parameters where the parametric coupling is
obtained. These non-axisymmetric oscillations were applied to enhance mixing
(i.e. reduce mixing time) of reagents on an open-chip. In comparison to mixing by
pure diffusion, using non-axisymmetric modes leads to 37 times faster mixing of
droplets.
Manipulation of droplets containing biological samples is often hindered by
biofouling. To apply these oscillations to biological samples, an oil surrounding
was required. So, we propose use of compound droplets in open-chip
microfluidic platforms. Compound droplets are formed with sample (bio) as the
core and silicone oil as the surrounding shell medium. In this work, we studied
interface oscillations for different compound droplet configurations. For low
actuation frequencies, the aqueous-core responds to the actuation voltage
whereas the oil-shell is actuated by the oscillating core. Effect of varying oil-shell
volume on the oscillation of compound droplet was studied. The resonance
frequency of compound droplets decreased with increase in the oil-shell volume.
This reduction has been attributed to the increased mass loading and damping of
the increased oil volume. The regime of actuation parameters for attaining nonaxisymmetric
modes also changes with oil volume. These dynamics of compound
droplets were modelled using mass-spring-damper model and the Mathieu
equation. The mixing efficiency of these oscillations was also studied for
biological fluids (i.e. red blood cells (RBC) containing phosphate buffer saline
(PBS) solution). We observed enhanced droplet mixing using the non
axisymmetric modes in comparison to the mixing by pure diffusion. This
provides a technique for achieving faster mixing in biochemical assays on digital
chip. This mixing reduces the required chip space by removing the need of
external pumping or numerous electrodes.
Another interesting phenomenon pertaining to coalescence was observed while
studying mixing of oscillating compound droplets. For certain actuation
parameters prolonged non-coalescence was observed between the two core
droplets. Different regimes of coalescence and non-coalescence were obtained
based on amplitude and frequency of the core oscillations. The transition from
coalescing to non-coalescing regime was explained based on oscillation mode
amplitudes which led to periodic modulation of the entrapped oil bridge
between cores. We found that the role of electrostatic repulsion was limited to
the contact line and did not prevent droplet coalescence away from the contact
line. The capillary pushing of cores with time-period faster than the normal oil
bridge drainage time caused continuous modulation of the oil bridge width,
which was proposed as the reason for the observed non-coalescence of droplets
for certain range of frequencies and voltages. This study can be used to maintain
stable non-merging of droplets on substrate required for various applications
like compound lenses.
The last part of the thesis investigates droplet contact line as a micro-mechanical
resonator. Here, we reduced the dimensions of oscillating droplet interface by
actuating a small portion of droplet contact line using patterned line electrodes
of 50-450 μm width. By reducing the actuated interface length, its resonance
frequency given by
∝ ⁄ was expected to increase. We, however,
obtained completely damped oscillations in our experiments. This indicated the
dominant role of viscous forces. We used this damped localized electrowetting as
a sensing technique to study the liquid properties. The relaxation time of the
actuated interface was used as a measure of viscosity and surface tension of
liquid. The change in these relaxation dynamics during an on-going chemical
process in a droplet or a microfluidic chip, can tell us about the dynamic state of
reaction. This was demonstrated by monitoring the process of sugar dissolution
in water. This technique offers great potential to sense particles and determine
progress of fluid reactions in both droplet-based platforms and microfluidic
channels at different time instants and positions. | en_US |
