Microfluidic Chips Based on Patterned Thermocapillary Flow
Research involving DNA sequencing and protein analysis is being facilitated by the development of new miniaturized diagnostic platforms called labs-on-a-chip. The development of these small biofluidic chips has fostered interesting partnerships between researchers in academia and industry interested in microarray and microfluidic technologies. Our laboratory is devoted to the study of a number of these technologies based on free surface flow where liquid samples are manipulated on the open surface of a glass or silicon substrate. The ability to tune and shape the speed of liquid samples is not only useful for bioengineering applications but also hodl tremendous potential for optofluidic studies where liquid elements can be used for tunable filters, gratings, waveguides and photonic structures.
 
For bioarray applications, we are investigating a number of dynamical printing techniques for selective deposition of micron or nanoscale liquid samples on surfaces of mixed wettability. A larger part of our program is devoted to the development of a microfluidic device for droplet actuation using micropatterned thermocapillary flow. Glass embedded microelectrode arrays provide control over the substrate temperature distribution with high spatial resolution. The applied thermal gradients locally alter the surface tension of a liquid sample thereby propelling the liquid toward colder regions of the surface, an effect known as thermocapillary flow. The liquid surface temperature can also be modulated by radiative heating of the air-liquid interface using an overhead laser spot array with modulated intensity. With either method, the liquid flow speed and direction are electronically tuned and the trajectories further refined by chemical patterning of the supporting substrate. We have used this device as a miniature automated platform for such functions as droplet routing (for polar and organic liquids), site specific droplet trapping and release, droplet scission, mixing of two samples, and chemical reactions. What makes this technique particularly attractive for microfluidic applications is the variety of tasks possible solely by control of the liquid surface temperature. These experimental studies are also complemented by an extensive theoretical program which includes hydrodynamic modeling as well as molecular dynamics simulations.
 
Microfluidic Detection and Analysis by Integration of Evanescent Wave Sensing With Thermocapillary Actuation
Our laboratory has previously demonstrated two methods for droplet detection and sensing for microfluidic devices which makes use of the coplanar microelectrode arrays used for thermocapillary actuation. The first method monitors the rise time of embedded microheaters, from which can be extracted droplet location, volume or composition due to changes in thermal conductivity induced by the presence of overlying liquid film. The second method records the capacitance change induced by an overlying droplet. The rapid response for electrode widths comparable to the liquid film thickness allows detection of droplet position, volume, composition and evaporative loss for nanoliter liquid samples. These sensing techniques, which probe samples by thermal or electric fields, are not suitable for many applications. Integrated optical and spectroscopic probes such as evanescent wave sensing offer a number of advantages in this respect.
 
In a recent set of experiments, we have demonstrated a non-intrusive optical method for microfluidic detection and analysis based on evanescent wave sensing. The device consists of a planar thin film waveguide integrated with a microfluidic chip based on thermocapillary flow. Microliter droplets are electronically transported and positioned over the waveguide surface by actuation of a glass-embedded microelectrode array. The attenuated intensity of propagating modes is used to detect droplet location, to monitor dye concentration in aqueous solutions, and to measure the increase in chemical reaction rates as a function of increasing substrate temperature for a chromogenic biochemical assay. This study illustrates just a few of the capabilities possible by direct integration of optical sensing with surface directed fluidic devices. Our design also offers high sensitivity with few additional fabrication steps and is especially well suited to any fluidic device based on droplet manipulation by modulation of surface tension.
 
Boundary Conditions for Liquid-on-Solid Flows
The celebrated no-slip condition used to calculate velocity and shear stress profiles in hydrodynamic flows dictates that a liquid element adjacent to a solid surface assume the velocity of the surface. Despite its simplicity, this boundary condition has proven remarkably successful in reproducing most commonplace flows. There exist, however, several notable examples for which the no-slip condition leads either to singular or unrealistic behavior. Examples include the spreading of a liquid drop on a solid substrate and the extrusion of polymer melts from a capillary tube. It is now well accepted that suitably constructed boundary conditions which allow fluid elements to slip past the adjacent solid surface can regularize such flows and reproduce realistic behavior. Unfortunately, these slip models are phenomenological in origin and provide no universal understanding of the nature of momentum transport at the liquid/solid interface. Using molecular dynamics simulations of liquids in planar shear, we have shown there exists a general non-linear function relating the slip length to the local shear rate at a smooth liquid-solid interface. This boundary condition stems from the potential energy corrugation established by the liquid-solid interactions. We have also investigated how the in-plane structure factor and in-plane diffusion coefficient affect the degree of slip between the first liquid layer and the adjacent wall. We are currently extending these studies to various nanofluidic geometries, more complex flow fields and polymeric liquids. Our recent studies of slip include the effects of sinusoidal wall roughness and surfaces with mixed boundary conditions to mimic the effects of nanobubble attachment on hydrophobic substrates immersed in water. These studies are particularly useful in the context of drag reduction for laminar flow.
 
Rivulet Instabilities in Driven Spreading Films
When a thin liquid film is forced to wet and coat a solid surface by the application of a body or surface force, the advancing front can develop a narrow capillary ridge at the leading edge of the film. This ridge is highly unstable and easily separates into numerous parallel rivulets which act as channels for the flow. Examples of this instability include liquid paint streaming down a vertical wall, the spin coating of a liquid film on a rotating substrate, the thermocapillary migration of a liquid film on a differentially heated substrate, or the forced spreading of a viscous film by an overhead gas stream. We have investigated this instability via modal and transient growth analysis. For the thermocapillary driven system, the optimal perturbations which give rise to the fingering behavior rapidly asymptote to the most unstable modes determined from linear stability of the base state traveling wave solution. This holds true even if the base state equation includes van der Waals interactions. Our work provides the basis for understanding the excellent agreement between experiment and linear stability theory despite the non-normal character of the relevant disturbance matrix. We are extending our studies to other coating flows and to surfaces of mixed wettability obtained by chemical micropatterning.
 
Onset and Evolution of Digitated Structures in Spreading Surfactant Drops and Films
The spreading of a surfactant droplet on a thin liquid film of higher surface tension is known to produce an unusual fingering instability near the initial deposition edge. The spreading front undergoes repeated branching and tip-splitting forming arterial or dendritic patterns characterized by a fractal dimension similar to processes governed by Laplacian growth. Calculations based on linear stability and transient growth analysis suggest that the coupling of Marangoni and capillary stresses causes dramatic variations in film thickness and surfactant concentration. We are conducting experimental and theoretical studies to identify the characteristics of the base state profiles and conditions leading to instability to identify the source of unstable flow in sufficiently thin films. This problem of a surfactant monolayer spreading on a thin liquid film is of relevance to the study of certain respiratory diseases in newborn infants, which are alleviated by the exogenous delivery of lung surfactant. While many studies have focused on the equilibrium properties of the formulations used in clinical applications, we are exploring the dynamics of the spreading process in order to prevent non-uniform coverage of the surfactant concentration.
 
Most recently, we have used refracted Moire topography to reconstruct the spatial and time dependent waveform of a spreading monolayer of oleic acid (surfactant) on a thin film of aqueous glycerol. This system closely mimics the behavior of an insoluble surfactant driven to spread on a thin viscous layer under the action of Marangoni stresses induced by variations in surfactant concentration. The film thickness profiles exhibit a strong surface depression ahead of the surfactant source capped by an elevated rim at the surfactant leading edge. The surface slope and shape as well as the propagation speed of the advancing rim are directly compared with numerical solutions of a lubrication model based on Marangoni driven spreading of a surfactant monolayer. Comparison between the theoretical and measured profles reveals the importance of the initial shear stress in determining the evolution in the film thickness and surfactant distribution. This initial stress appears to thin the underlying liquid support so drastically that the surfactant droplet behaves as a finite and not an infinite source even though there is always present an excess of surfactant at the origin.
 
Slip Behavior and Foam Stabilization in Polymer-Surfactant Films
Stable aqueous foams are required in many engineering processes including foam bed reactors and separators, rheology control in enhanced oil recovery, and fire fighting liquid systems. The formulation, time to rupture and taste of foams is also of considerable interest to the beer and champagne industries. Traditionally, foams used in such applications are stabilized against drainage, thinning, and rupture by the addition of co-surfactants like alcohols. Such additives enhance the stability of the underlying soap films by increasing the surface viscosity and elasticity and decreasing gas permeability. In our laboratory, we are investigating the addition of hydrosoluble polymers to surfactant solutions for promoting film stability. Hydrosoluble additives like these can form surfactant micelle-polymer complexes which increase normal stresses and elongational viscosity thereby delaying film rupture. Even at low concentrations, such aggregrates appear to cause deviations from Frankel's law above a critical molecular weight and concentration. While the literature suggests that these deviations might be caused by viscoelastic effects, our measurements based on laser interferometry and light reflectivity indicate some additional mechanism for film thinning which causes more rapid drainage but longer film lifetimes.
 
Our experimental studies have revealed that these deviations can be quantified through extension of the original Frankel analysis by replacing the surface rigid condition by an effective Navier-like slip condition. Interestingly, the extracted slip correlates well with the polymer radius of gyration including excluded volume effects. This correlation is suggestive of the Tolstoi-Larsen prediction developed for fluid flow against a solid surface that the slip length increases linearly with the fluid molecular size. The surprising feature is that this effective slip behavior in our system occurs at an air-liquid and not a liquid-solid boundary. Additional studies with pure surfactant solutions suggests that the origin of this interfacial slip behavior lies with Marangoni stresses caused by variations in surfactant concentration at the gas-liquid boundary.
 
High Resolution Lithography by Microscale Contact Printing
Large area electronics such as light emitting displays require far less stringent resolution limits than conventional photolithography. The demand for applications with lower resolution has triggered the development of patterning and fabrication methods which are large area, high throughput and low cost. We have demonstrated that contact printing methods like offset and letterpress can be used to produce micron scale wet and dry etch resists of arbitrary shape and thickness on flat or spherical surfaces. These structures can be made cheaply and rapidly in wide area formats containing disparate sizes and shapes. More recently, we have demonstrated that polymer etch masks printed with a microscale letterpress stamp can be used to fabricate amorphous thin film transistors with IV curves comparable to those fabricated by photolithography. This experimental program on flexible electronics is complemented by modeling efforts based on energy minimization and fluid dynamical studies designed to parametrize and optimize the flow and surface conditions during various stages of the printing process.