Bahador Farshchian

3D molding of hierarchical micro- and nanostructures

We show a simple and effective process to produce large area, hierarchical 3D micro- and nanostructures via a modified hot embossing process, which we name 3D molding. The 3D molding process takes advantage of both a hard mold from hot embossing and the flexibility of a thin, elastomeric intermediate stamp from soft lithography. Using this method, we have demonstrated the formation of various micro- and nanostructures in non-planar microscale structures including microchannels and step surfaces. The ability to produce micro/nanopatterns within microchannels will have potential applications in bioanalytic micro/nanofluidic devices by allowing for the manipulation of a broader range of surface properties and thus control over surface interactions with biomaterials flowing through the microfluidic channels.

Deformation behavior in 3D molding: experimental and simulation studies

Three-dimensional (3D) molding is a simple and effective technique using a modified hot embossing process to produce large area, hierarchical 3D micro/nanostructures in polymer substrates. However, the use of a thin intermediate polydimethylsiloxane (PDMS) stamp inevitably causes dimensional changes in the 3D molded channel, with respect to those in the brass mold protrusion and the intermediate PDMS stamp structures. Here we investigate the deformation behavior of the 3D molded poly(methyl methacrylate) (PMMA) substrate and the intermediate PDMS stamp in 3D molding through both experimentation and numerical simulation. Depending on the height, period and aspect ratio of the brass mold protrusions and the thickness of the intermediate PDMS stamp, strain contours of the intermediate PDMS stamp layer along the periphery of the 3D molded channels are varying, which leads to a nonuniform elongation of the imprinted structures in the 3D molded channel. Increasing the height and decreasing the period of brass mold protrusions leads to higher total strain of the intermediate PDMS stamp. It was found that for high aspect ratio brass mold protrusions the maximum strain of the intermediate layer occurs in the bottom center of the 3D channels. However, with decreasing aspect ratio of the brass mold protrusion the highest elongation occurs at the bottom corners of the channel causing less elongation of the intermediate PDMS stamp and imprinted structures on the bottom surface of the 3D channel. These experimental results are in good agreement with the results obtained from the numerical simulation performed with a simple 2D model.

3-D Integration of Micro-Gratings Into Bio-Analytical Devices

The ability to produce three-dimensional micro- and nanoscale features at low cost is desirable for many applications such as microfluidic devices, micro and nanomechanical systems, photonic crystals and diffractive optics. For example, micro and nanostructures patterned on the sidewalls of microfluidic devices allow better control over the wetting behavior of fluids flowing through the microchannel. In this study we report on a simple and effective process that allows direct integration of microstructures into a microfluidic device via a modified molding process. The key for the process is to use a thin poly(dimethylsiloxane) layer having microgratings as an intermediate stamp which was placed between a brass mold insert with microfluidic features and a PMMA sheet, which was followed by hot embossing. Using this method, we have demonstrated the formation of micropatterns on non-planar surfaces and at the sidewalls of microfluidic devices, as confirmed using scanning electron microscopy. The designed process will fill the gap in current micro- and nanofabrication technologies in that most of the technologies allow for patterning only on planar substrates.Copyright

Nanostructuring Curved Surfaces Using a Flexible Stamp

We report on a simple and effective process that allows direct imprinting of micro- and nanostructures on non-flat surfaces. A thin polydimethylsiloxane (PDMS) stamp having micro/nanogratings was placed between a metallic bar with a trapezoidal cross section or a metallic pellet and a flat polymethyl methacrylate (PMMA) substrate, followed by hot embossing at 200°C. During the hot embossing process, the metallic bar/pellet is pushed into the PMMA sheet forming a millimeter scale channel or a curved surface. Due to the presence of the PDMS stamp between the metallic object and the substrate, micro/nanostructures are produced into the channel or over the curved surface. With this method, we have successfully demonstrated micro- and nanostructures down to 300 nm wide gratings on non-flat substrates, as confirmed by scanning electron microscopy and atomic force microscopy. The process so developed will fill the gap in current micro- and nanofabrication technologies in that most of the technologies allow for patterning only on planar substrates.Copyright

Droplet impinging behavior on surfaces with wettability contrasts

Heterogeneous substrates with moderate and extreme wettability contrasts were fabricated by comprising of superhydrophobic/hydrophilic and superhydrophobic/extremely hydrophilic surfaces, respectively. The interactions of water droplets impinging on the surfaces with sharp wettability contrasts were investigated experimentally. The impinging droplets that slightly touch the hydrophilic or extremely hydrophilic areas on each substrate exhibit a directional rebounding towards the more wetting surfaces, i.e., hydrophilic or extremely hydrophilic surface. The trajectory and landing distance of the rebounded droplets were tailored by controlling the releasing height of the droplet, wetting contrast across the border, and portion of the droplet touching the more wetting surface of the substrates with wettability contrasts. The landing distance of the droplet increases with the increased releasing height and higher wettability contrast across the border. Increasing the portion of the impinging droplet touching the more wetting surface of the heterogeneous substrates leads to the shorter landing distance of rebounded droplets.

Hydrothermal Growth of ZnO Nanowires on UV-Nanoimprinted Polymer Structures

Integration of zinc oxide (ZnO) nanowires on miniaturized polymer structures can broaden its application in multi-functional polymer devices by taking advantages of unique physical properties of ZnO nanowires and recent development of polymer microstructures in analytical systems. In this paper, we demonstrate the hydrothermal growth of ZnO nanowires on polymer microstructures fabricated by UV nanoimprinting lithography (NIL) using a polyurethane acrylate (PUA). Since PUA is a siloxane-urethane-acrylate compound containing the alpha-hydroxyl ketone, UV-cured PUA include carboxyl groups, which inhibit and suppress the nucleation and growth of ZnO nanowires on polymer structures. The presence of carboxyl groups in UV-cured PUA was substantiated by Fourier transform infrared spectroscopy (FTIR), and a Ag thin film was deposited on the nanoimprinted polymer structures to limit their inhibitive influence on the growth of ZnO nanowires. Furthermore, the naturally oxidized Ag layer (Ag2O) reduced crystalline lattice mismatches at the interface between ZnO-Ag during the seed annealing process. The ZnO nanowires grown on the Ag-deposited PUA microstructures were found to have comparable morphological characteristics with ZnO nanowires grown on a Si wafer.

Fabrication of Perforated Conical Nanopores in Freestanding Polymer Membranes Using Nanoimprint Lithography and Pressed Self-Perfection Method

Nanopores have proven to be an important sensing element in biosensors to detect and analyze single biomolecules such as DNAs, RNAs, or proteins. The charged biomolecules are driven by an electric field and detected as transient current blocks associated with their translocation through the pores. While protein nanopores, such as alpha-hemolysin and MspA protein nanopores embedded within a lipid bilayer membrane [1], promise to be a rapid, sensitive and label-free sensing paradigm, their duration of usage is too short to perform repetitive experiments due to the mechanical instability of the lipid bilayer. A variety of methods have been developed to prepare synthetic nanopores, which can substrate for protein nanopores, including a direct milling with a focused high-energy electron or ion beam in insulating substrates, an ion track etching in polymer substrates, and an anodizing in aluminum substrates. However, those methods do not allow for control over both the size and location of pores and the high yield of production.Copyright

Novel, Gasketless, Interconnect Using Parallel Superhydrophobic Surfaces for Modular Microfluidic Systems

A novel, modular, microfluidic interconnect was developed using parallel superhydrophobic interfaces to facilitate the transport of fluids between component chips in modular microfluidic systems. A static analytical model, derived from the Laplace equation [1], approximates the maximum steady-state pressure of the liquid at the liquid bridge which forms across the gap between the chips. Preliminary experiments using parallel superhydrophobic surfaces on PMMA validated the concept. Additional experiments controlled the gap distance, measured contact angles of the superhydrophobic surfaces, gradually increased the pressure of the novel, gasketless, interconnect until rupture to find the maximum pressure across the liquid bridge and verify the model. The measured pressures were on the same order of magnitude (1–10 kPa) as estimated using the model for gap distances of 25 μm and 100 μm.© 2011 ASME

The Influence of Ratchets Dimension and Shape on the Motion of Leidenfrost Droplet

The influence of ratchet aspect ratio and the shape of ratchet ridges on the motion of film boiling water droplet were investigated. Continuous rebounding behavior of the droplet was observed on micro ratchets with lower depths and round ridges. The residence time and flying time of rebounding droplets are independent of surface temperature. The terminal velocity increases but the number of rebounds decreases as the surface temperature increases in the range of 192–248°C.Copyright