We provide here visual summaries of some of our projects. For additional information, see our publications page.
Electroosmotic flow enhancement over superhydrophobic surfaces
Electroosmotic flow is a well-established and efficient method for driving microchannel flows that relies on the interaction of an externally applied electric field with charge arising at the interface between the liquid and the channel walls. However, its relatively low velocities together with its dependence on the pH of the liquid severely limit its utility. Here, we experimentally demonstrate fast electroosmotic flow over microstructured superhydrophobic surfaces. By suspending the electrolyte in a Cassie-Baxter state over hierarchical surfaces, we create stable gas-liquid interfaces on which we induce charge through a gate electrode. We provide a detailed investigation and characterization of the electroosmotic velocity as a function of the surface geometry by utilizing particle tracking velocimetry in a microfluidic device, and show that the resulting electroosmotic velocity scales with the ratio of slip length to double-layer thickness. Compared to no-slip surfaces, we demonstrate an order of magnitude enhancement in velocity, and complete pH independency, enabling wider utility of EOF in manipulation of microscale flows.
Intermediate states of wetting on hierarchical superhydrophobic surfaces
Wetting transition on superhydrophobic surfaces is commonly described as an abrupt jump between two stable states - either from Cassie and Wenzel for non-hierarchical surfaces or from Cassie to nano-Cassie on hierarchical surfaces. We here experimentally study the electrowetting of hierarchical superhydrophobic surfaces composed of multiple length scales, by imaging the light reflections from the gas-liquid interface. We present the existence of a continuous set of intermediate states of wetting through which the gas-liquid interface transitions under a continuously increasing external forcing. This transition is partially reversible and is limited only by localized Cassie to Wenzel transitions at nano defects in the structure. In addition, we show that even a surface containing many localized wetted regions can still exhibit extremely low contact angle hysteresis, thus remaining useful for many heat transfer and self-cleaning applications. Expanding the classical definition of the Cassie state in the context of hierarchical surfaces, from a single state to a continuum of meta-stable states ranging from the centimeter to the nanometer scale, is important for a better description of the slip properties of superhydrophobic surfaces, and provides new considerations for their effective design.
Tunable bidirectional electroosmotic flow for diffusion-based separation
We present a new concept for on-chip separation that leverages bidirectional flow, to tune the dispersion regime of molecules and particles. The system can be configured so that low diffusivity species experience a ballistic transport regime and are advected through the chamber, whereas high diffusivity species experience a diffusion dominated regime with zero average velocity and are retained in the chamber. We detail the means of achieving bidirectional electroosmotic flow using an array of alternate-current (AC) field-effect electrodes, experimentally demonstrate the separation of particles and antibodies from dyes, and present a theoretical analysis of the system, providing engineering guidelines for its design and operation.
Nonuniform Electro-osmotic Flow Drives Fluid-Structure Instability
We demonstrate the existence of a fluid–structure instability arising from the interaction of electro-osmotic flow with an elastic substrate. Considering the case of flow within a soft fluidic chamber, we show that above a certain electric field threshold, negative gauge pressure induced by electro-osmotic flow causes the collapse of its elastic walls. We combine experiments and theoretical analysis to elucidate the underlying mechanism for instability and identify several distinct dynamic regimes. The understanding of this instability is important for the design of electrokinetic systems containing soft elements.
Electrokinetic Scanning Probe
We present theoretical analysis and experimental demonstration of a new concept for a non-contact scanning probe, in which transport of fluid and molecules is controlled by electric fields. The electrokinetic scanning probe (ESP) enables local chemical and biochemical interactions with surfaces in liquid environments. We present the physical mechanism and design criteria for such probe, and demonstrate its compatibility with a wide range of processing solutions and pH values. We show the applicability of the probe for surface patterning in conjunction with localized heating and 250-fold analyte stacking.
Spatially resolved genetic analysis of tissue sections enabled by microscale flow confinement retrieval and isotachophoretic purification
We have developed a method for spatially resolved genetic analysis of formalin-fixed paraffin-embedded (FFPE) cell block and tissue sections. This method involves local sampling using hydrodynamic flow confinement of a lysis buffer, followed by electrokinetic purification of nucleic acids from the sampled lysate. We characterized the method by locally sampling an array of points with a circa 200 mm diameter footprint, enabling the detection of single KRAS and BRAF point mutations in small populations of RKO and MCF-7 FFPE cell blocks. To illustrate the utility of this approach for genetic analysis, we demonstrate spatially resolved genotyping of FFPE sections of human breast invasive ductal carcinoma.
Dipolar thermocapillary motor and swimmer
The study of thermocapillary driven flows is typically restricted to ‘open’ systems, i.e. ones where a liquid film is bounded on one side solely by another fluid. However, a large number of natural and engineered fluidic systems are comprised of solid boundaries with only small open regions exposed to the surrounding. In this work we study the flow generated by the thermocapillary effect in a liquid film overlaid by a discontinuous solid surface. If the openings in the solid are subjected to a temperature gradient, the resulting thermocapillary flow will lead to a non-uniform pressure distribution in the film, driving flow in the rest of the system. For an infinite solid surface containing circular openings, we show that the resulting pressure distribution yields dipole flows which can be superposed to create complex flow patterns, and demonstrate how a confined dipole can act as a thermocapillary motor for driving fluids in closed microfluidic circuits. For a mobile, finite-size surface, we show that an inner temperature gradient, which can be activated by simple illumination, results in the propulsion of the surface, creating a thermocapillary surface swimmer.
Electroosmotic flow dipole: experimental observation and flow field patterning
We experimentally demonstrate the phenomenon of electroosmotic dipole flow that occurs around a localized surface charge region under the application of an external electric field in a Hele-Shaw cell. We use localized deposition of polyelectrolytes to create well-controlled surface charge variations, and show that for a disk-shaped spot, the internal pressure distribution that arises, results in uniform flow within the spot and dipole flow around it. We further demonstrate the superposition of surface charge spots to create complex flow patterns, without the use of physical walls.
Dynamic microscale flow patterning using electrical modulation of zeta potential
The ability to move fluids at the microscale is at the core of many scientific and technological advancements. Despite its importance, microscale flow control remains highly limited by the use of discrete channels and mechanical valves, and relies on fixed geometries. Here we present an alternative mechanism that leverages localized fieldeffect electroosmosis to create dynamic flow patterns, allowing fluid manipulation without the use of physical walls. We control a set of gate electrodes embedded in the floor of a fluidic chamber using an ac voltage in sync with an external electric field, creating nonuniform electroosmotic flow distributions. These give rise to a pressure field that drives the flow throughout the chamber. We demonstrate a range of unique flow patterns that can be achieved, including regions of recirculating flow surrounded by quiescent fluid and volumes of complete stagnation within a moving fluid. We also demonstrate the interaction of multiple gate electrodes with an externally generated flow field, allowing spatial modulation of streamlines in real time.
Dynamic control of capillary flow in porous media by electroosmotic pumping
Microfluidic paper-based analytical devices (μPADs) rely on capillary flow to achieve filling, mixing and delivery of liquids. We investigate the use of electroosmotic (EO) pumping as a mechanism for dynamic control of capillary flow in paper-based devices. The applied voltage can accelerate or decelerate the baseline capillary-driven velocity, as well as be used to create a tunable valve that reversibly switches the flow on and off in an electrically controlled manner. The method relies on simple fabrication and allows repeated actuation, providing a high degree of flexibility for automation of liquid delivery. We adapt the Lucas–Washburn model to account for EO pumping and provide an experimentally validated analytical model for the distance penetrated by the liquid as a function of time and the applied voltage. We show that the EO-pump can reduce filling time by 6.5-fold for channels spanning several cm in length, relative to capillary filling alone. We demonstrate the utilization of the EO-pump for a tunable and dynamic flow control that accelerates, decelerate and stop the flow on demand. Finally, we present the use of EO-pump for fluid flow sequencing on a paper-based device.
Real-Time Monitoring of Fluorescence in Situ Hybridization Kinetics
We present a novel method for real-time monitoring and kinetic analysis of fluorescence in situ hybridization (FISH). We implement the method using a vertical microfluidic probe containing a microstructure designed for rapid switching between a probe solution and a non-fluorescent imaging buffer. The FISH signal is monitored in real time during the imaging buffer wash, during which signal associated with unbound probes is removed. We provide a theoretical description of the method as well as a demonstration of its applicability using a model system of centromeric probes (Cen17). We demonstrate the applicability of the method for the characterization of FISH kinetics under conditions of varying probe concentration, destabilizing agent (formamide) content, volume exclusion agent (dextran sulfate) content, and ionic strength. We show that our method can be used to investigate the effect of each of these variables and provide insight into processes affecting in situ hybridization, facilitating the design of new assays.