Understanding tree frog adhesion



Our ongoing effort is aimed at understanding the role of the tree frogs structured toe pads in promoting adhesion under flooded conditions. The locomotion mechanisms employed by tree frogs under flooded conditions could offer the ultimate solution for the need of strong, reversible, reusable, tunable, and water tolerant adhesives. Central to the adhesion and locomotion of tree frogs are their structured toe pads, which consist of an array interconnected channels that end in mucus secreting glands. On dry surfaces, the toe pads enhance adhesion via deformation of the soft epithelial cells to improve foot-surface conformity and secretion of watery-mucus into the area of contact to promote capillary or hydrodynamic interactions. Under flooded conditions, however, capillary interactions are expected to be negligible given the absence of any free fluid interface. The mechanisms for tree frog adhesion under flooded conditions, and by extension the role played by the structured toe pads, has been the subject of speculations. It is suspected that tree frogs can climb and grip on wet surfaces (and prevent hydroplaning) by reducing the hydrodynamic forces through drainage of the fluid in the channels present on their toe pads. While hypothesized, the mechanism for tree frog adhesion, and more specifically the role played by hydrodynamics and elastic deformation, has not been clearly demonstrated. Moreover, the potential of biomimetic materials based on the tree frog toe pads has yet to be realized. 


Measurement and scaling of hydrodynamic interactions in the presence of draining channels (Langmuir)


Assembly of nanoparticles at fluid interfaces


We investigate the mechanism driving the assembly of gold nanoparticles to the oil-water interface. More specifically we investigate how electrostatic interactions between the particles and between the particles and the interface can be employed to control interfacial assembly. The interface between two immiscible liquids, such as oil and water, holds promise as an assembly ground for organizing and exploiting the unique optoelectrical properties offered by nanoparticle arrays. The optical transparency of oil and water, combined with the dynamic liquid-liquid interface, makes colloidal crystals formed at fluid interfaces especially attractive. Moreover, the mechanical flexibility of the interface is amenable to forming a wide range of geometries - from planar sheets, to curved lenses, and three-dimensional objects- coated with nanoparticles. In nanoparticle-based plasmonic materials, for instance, the optoelectrical properties of the interfacial films can be tuned by interparticle separation, leading to rich behaviors at liquid-liquid interfaces such as the Stark effect, insulator-to-metal transition, and metal liquid-like films (MLLFs). A better understanding of the complex driving forces that direct nanoparticles to the liquid-liquid interface could allow for the external modulation of the interfacial assembly of nanoparticles. 


Electrostatic interactions to modulate the reflective assembly of nanoparticles at the oil-water interface (Soft Matter)


Capillary bridges across heterogeneous surfaces 


We investigate the role of partial contact line pinning in the morphological evolution of capillary bridges. Capillary bridges between solid substrates are critical to a multitude of industrial and natural processes. For example, they provide the cohesion force necessary to give structure to wet granular materials such as sand castles. In confined geometry fluid bridges formed via capillary condensation are responsible for unwanted adhesion in microelectromechanical systems (MEMS). With the advent of patterning and lithography, surfaces can be created with heterogeneous physical or chemical boundaries that pin the triple contact line of fluids. Pinning of the triple contact line on a surface alters the shape of a capillary bridge and allows for the design of surface patterns that lead to pre-determined capillary forces or bridge morphology. While the number and complexity of surface patterns that can bound capillary bridges is virtually unlimited, even simple cases that rely on high levels of symmetry have shown promise in micro & nanoscale systems. Our experiments and numerical simulations aimed at characterizing the morphology of capillary bridges in slit pore geometry have led to the counterintuitive observation that Laplace pressure goes from negative to positive as the height of the capillary bridge is increased. 


From Concave to Convex: Capillary Bridges in Slit Pore Geometry (Langmuir)


Responsive surfaces


Responsive or “smart” materials are materials designed to respond to an external stimuli such as a change in pH, voltage, light, or solvent conditions. These materials have potential applications for separation, drug delivery, and as means to direct fluid flow in microfluidic devices. We are using electrical potential as a stimulus as it can be easily integrated in micro- and nanoscale devices. We are developed smart surfaces that can change their wetting and adhesive properties due to a change in applied potential. We are using a wide variety of techniques such as x-ray spectroscopy, contact angle measurements, electrochemical impedance spectroscopy, and direct surface forces measurements to characterize and optimize the dynamic response of these materials.



Role of Solution and Surface Coverage on Voltage-Induced Response of Low-Density Self-Assembled Monolayers (Journal of Physical Chemistry C)

Supramolecular Ion-Pair Interactions to Control Monolayer Assembly (Langmuir)


Macroscopic models for microfluidic separation systems


In collaboration with German Drazer, we investigate the mechanism behind deterministic microfluidic separation methods such as pinch flow fractionation and deterministic lateral displacement. Our approach involves scaling up the techniques by building macroscale models (often using LEGOs). 


Force driven separation of drops by deterministic lateral displacement (Lab on a Chip)

Directional locking and the role of irreversible interactions in deterministic hydrodynamics separations in microfluidic devices (Physical Review Letters)

Irreversibility and pinching in deterministic particle separation (Applied Physics Letters)



Nanoscale capillarity


As device dimensions shrink into the nanometer range, interfacial forces become increasingly important. At the same time, traditional continuum theories of interfacial forces become inadequate, and fundamentally new phenomena can appear. We seek to determine the limits of traditional theories, identify new interfacial phenomena, and explore processes that may enable new active nanodevices. We study interfacial forces to obtain information about field-induced changes in the charge distribution and forces at solid/liquid and fluid interfaces that are important in electrowetting on dielectric (EWOD). Electrowetting on dielectric (EWOD) is a popular method to move fluid in a microchannel and to vary the focal length of a fluidic lens, and it is being envisioned as a means for oil-water separation. While the phenomenon is relatively well-understood for large (mm) drops, the limits of traditional theories for a fluid drop in a nanoscale channel had not been explored. Moreover, the mechanism at play during electrowetting is not fully understood and the origin of anomalous features, such as contact angle saturation, remains debated. Using the surface force apparatus (SFA) we designed experiments to determine the mechanism driving electrowetting. We have modified the instrument to allow for external potential control of both interacting surfaces and used capillary condensation to generate nanoscale water droplets. Our experiments allowed us to probe contact angle changes within the first tens of nanometers of a drop, and are not limited by possible issues caused by contact angle hysteresis. Using this approach, we unequivocally demonstrated that the real contact angle does not change in electrowetting experiments. Our results show that there is no measurable change in the solid-liquid surface energy in EWOD and that the mechanism at play is electromechanical in nature. One key implication of our work is the demonstration that electrowetting is not a viable mechanism to control surface for micro-/nanoscale devices.



Invariance of the Solid-Liquid Interfacial Energy in Electrowetting Probed via Capillary Condensation (Langmuir)