Amar S. Basu

Mechanical behavior and interface design of MoSi2-based alloys and composites

The mechanical behavior of hot pressed MoSi2-based composites containing M05Si3, SiO2, CaO and TiC as reinforcing second phases was investigated in the temperature regime 1000-1300 °C. The effects of strain rate on the flow stress for M05Si~-, SiO2- and CaO-containing composites are presented. Effects of several processing routes and microstructural modifications on the mechanical behavior of MoSi2-M05Si ~ composites are given. Of these four composite additions, M05Si 3 and CaO produce strengthening of MoSi 2 in the temperature range investigated. SiO 2 greatly reduces the strength, consistent with the formation of a glassy phase at interface and interphase boundaries. TiC reduces the flow stress of MoSi 2 in a manner that suggests dislocation pumping into the MoSi 2 matrix. The strain rate effects indicate that dislocation creep (glide and climb) processes operate over the temperature range investigated, with some contribution from diffusional processes at the higher temperatures and lower strain rates. Erbium is found to be very effective in refining the microstructures and in increasing the hardness and fracture properties of MoSi2-MosSi 3 eutectics prepared by arc melting. Initial results on microstructural modeling of the deformation and fracture of MoSi2-based composites are also reported.

Virtual microfluidic traps, filters, channels and pumps using Marangoni flows

This paper describes how Marangoni flows of various forms can be generated in thin liquid films for the purposes of microfluidic manipulation. Several microfluidic components, including traps, channels, filters and pumps, for manipulating aqueous droplets suspended in a film of oil on blank, unpatterned substrates are demonstrated. These are ‘virtual’ devices because they have no physical structure; they accomplish their function entirely by localized variations in surface tension (Marangoni flows) created in a non-contact manner by heat sources suspended just above the liquid surface. Various flow patterns can be engineered through the geometric design of the heat sources on size scales ranging from 10 to 1000 μm. A point source generates toroidal flows which can be used for droplet merging and mixing. Virtual channels and traps, emulated by linear and annular heat fluxes, respectively, demonstrate nearly 100% size selectivity for droplets ranging from 300 to 1000 μm. A source of heat flux that is parallel to the surface and is triangular with a 10 ◦ taper serves as a linear pump, translating droplets of about the same size at speeds up to 200 μ ms −1 . The paper includes simulations that illuminate the working principle of the devices. Models show that Marangoni flows scale favorably to small length scales. By using microscale thermal devices delivering sharp temperature gradients, it is possible to generate mm s −1 flow velocities with only small increases (<1 ◦ ) in liquid temperature. (Some figures in this article are in colour only in the electronic version)

Field-free particle focusing in microfluidic plugs.

Particle concentration is a key unit operation in biochemical assays. Although there are many techniques for particle concentration in continuous-phase microfluidics, relatively few are available in multiphase (plug-based) microfluidics. Existing approaches generally require external electric or magnetic fields together with charged or magnetized particles. This paper reports a passive technique for particle concentration in water-in-oil plugs which relies on the interaction between particle sedimentation and the recirculating vortices inherent to plug flow in a cylindrical capillary. This interaction can be quantified using the Shields parameter ([Formula: see text]), a dimensionless ratio of a particles drag force to its gravitational force, which scales with plug velocity. Three regimes of particle behavior are identified. When [Formula: see text] is less than the movement threshold (region I), particles sediment to the bottom of the plug where the internal vortices subsequently concentrate the particles at the rear of the plug. We demonstrate highly efficient concentration (∼100%) of 38 μm glass beads in 500 μm diameter plugs traveling at velocities up to 5 mm/s. As [Formula: see text] is increased beyond the movement threshold (region II), particles are suspended in well-defined circulation zones which begin at the rear of the plug. The length of the zone scales linearly with plug velocity, and at sufficiently large [Formula: see text], it spans the length of the plug (region III). A second effect, attributed to the co-rotating vortices at the rear cap, causes particle aggregation in the cap, regardless of flow velocity. Region I is useful for concentrating/collecting particles, while the latter two are useful for mixing the beads with the solution. Therefore, the two key steps of a bead-based assay, concentration and resuspension, can be achieved simply by changing the plug velocity. By exploiting an interaction of sedimentation and recirculation unique to multiphase flow, this simple technique achieves particle concentration without on-chip components, and could therefore be applied to a range of heterogeneous screening assays in discrete nl plugs.

Scanning thermal lithography: Maskless, submicron thermochemical patterning of photoresist by ultracompliant probes

This article introduces a scanning probe lithography technique in which ultracompliant thermal probes are used in the selective thermochemical patterning of commercially available photoresist. The micromachined single-probe and multiprobe arrays include a thin-film metal resistive heater and sensor sandwiched between two layers of polyimide. The low spring constant (<0.1N∕m) and high thermal isolation provided by the polyimide shank is suitable for contact mode scanning across soft resists without force feedback control. The probes provide what is effectively a spatially localized postexposure bake that crosslinks the photoresist in the desired pattern, rendering it insoluble in developer. For 450-nm–1400-nm-thick AZ5214E (Clariant Corp.), line and dot features with sizes of 450–1800nm can be printed using probe powers of 13.5–18mW, and durations of 1–60s per pixel. Variation of feature sizes with process parameters is described.

Shaping high-speed Marangoni flow in liquid films by microscale perturbations in surface temperature

The authors show that a variety of controlled flow patterns, including toroidal cells and surface doublets, can be generated in 80–400μm thick liquid films by placing scanning microscopy probes with integrated heaters just above the surface ( 1000μm∕s can be accomplished with surface temperature perturbations <1°C. This technique enables microfluidic manipulation on unpatterned substrates.

Optical microplates for high-throughput screening of photosynthesis in lipid-producing algae.

It is well known that biological systems respond to chemical signals as well as physical stimuli. The workhorses of high throughput screening, microplates and pipetting robots, are well suited for screening chemical stimuli; however, there are fewer options for screening physical stimuli, particularly those which involve temporal patterns. This paper presents an optical microplate for photonic high-throughput screening. The system provides addressable intensity and temporal control of LED light emission in each well, and operates on standard black-wall clear-bottom 96-well microplates, which prevent light spillover. Light intensity can be controlled to 7-bit resolution (128 levels), with a maximum intensity of 120 mE cm(-2). The temporal resolution, useful for studying dynamics of light-driven bioprocesses, can be as low as 10 μs. The microplate is used for high-throughput studies of light-dependent growth rates and photosynthetic efficiency in the model organism Dunaliella tertiolecta, a lipid-producing algae of interest in 2(nd) generation biofuels. By conducting 96 experiments in parallel, photoirradiance studies, which would require 2 years using conventional tools, can be completed in <2 weeks. In a 12 day culture, algal growth rates increase with total photon flux, as expected. Interestingly, the lipid production efficiency, defined as lipid production per unit photon flux per capita, increases nearly 5 fold at low light intensity (constant light) and at low duty cycle (pulsed light). High throughput protocols enabled by this system are conducive to systematic studies and discovery in the fields of photobiology and photochemistry.

A Programmable Array for Contact-Free Manipulation of Floating Droplets on Featureless Substrates by the Modulation of Surface Tension

This paper presents a contactless droplet manipulation system that relies on thermally generated Marangoni flows. Programmable 2-D control of aqueous microdroplets suspended in an oil film on a plain featureless glass substrate is achieved using a 128-pixel heater array suspended 100-500 mum above the oil layer. The heaters generate surface temperature perturbations (<25degC), resulting in local Marangoni flows that can move droplets in either a push or a pull mode. Programmed movement is achieved by the sequential activation of the heaters, with digital control circuitry and a graphical interface providing addressable control of each heater. Droplets with diameters of 300-1000 mum are manipulated and merged at speeds up to 140 mum/s. Evaporation rates can be reduced by almost two orders of magnitude by using a two-layer-oil medium, and the choice of an optimum carrier fluid can achieve fluid velocities over 17 000 mum/s. The system provides a contactless platform for parallel droplet-based assays. As such, it circumvents the challenges of sample contamination and loss that occur when a droplet interacts with a solid surface.

Programming Microbes Using Pulse Width Modulation of Optical Signals

Cells transmit and receive information via signalling pathways. A number of studies have revealed that information is encoded in the temporal dynamics of these pathways and has highlighted how pathway architecture can influence the propagation of signals in time and space. The functional properties of pathway architecture can also be exploited by synthetic biologists to enable precise control of cellular physiology. Here, we characterised the response of a bacterial light-responsive, two-component system to oscillating signals of varying frequencies. We found that the system acted as a low-pass filter, able to respond to low-frequency oscillations and unable to respond to high-frequency oscillations. We then demonstrate that the low-pass filtering behavior can be exploited to enable precise control of gene expression using a strategy termed pulse width modulation (PWM). PWM is a common strategy used in electronics for information encoding that converts a series of digital input signals to an analog response. We further show how the PWM strategy extends the utility of bacterial optogenetic control, allowing the fine-tuning of expression levels, programming of temporal dynamics, and control of microbial physiology via manipulation of a metabolic enzyme.

Ultracompliant thermal probe array for scanning non-planar surfaces without force feedback

This paper describes an array of micromachined thermal probes for scanning thermal microscopy for which the structural design and choice of materials virtually eliminate the need for z-axis mechanical feedback in contact mode scans. The high mechanical compliance accommodates significant topographical variation in the sample surface and prevents damage to soft samples. Thin film metal bolometers are molded into tips at the end of each cantilever in the array, and are sandwiched between two layers of polyimide that serves as the structural material. The probes overhang a Si substrate on which they are fabricated. Since integrated actuators and accompanying circuitry are no longer required, the prospect of scaling from the present eight-probe version to large numbers of probes for high speed, high resolution thermal mapping of large areas with simple detection circuitry is enhanced. The scalability and performance of the eight-probe prototype are evaluated, addressing issues of speed versus resolution, and thermal and mechanical decoupling. The results demonstrate that contact mode scans can provide better than 2 µm spatial resolution at speeds greater than 200 µm s−1 and show a 6.5 bit topographical resolution over a 7 µm dynamic range. Line scans obtained with a single-shank probe suggest that there are good prospects of obtaining images showing a lateral spatial resolution of less than 50 nm.

Trapping and manipulation of particles and droplets using micro-toroidal convection currents

This paper introduces a technique for generating toroid-shaped convective vortices in thin (80-400 /spl mu/m) layers of oil. Driven by micromachined heat sources suspended 5-250 /spl mu/m above the oil surface, the vortices are used to trap weed pollen with 25 /spl mu/m diameter and microdroplets of water with diameters ranging from 2-50 /spl mu/m. The speed of the convective flow pattern can be controlled by the input power to the heat source and the air gap between the heat source and the oil surface, while the radius of the collection region is determined primarily by the thickness of the oil layer. Flow velocities up to 1700 /spl mu/m/sec have been achieved in toroidal flows with radii ranging from 80-1120 /spl mu/m. When the heat source is connected to an XY scanning stage, the vortex can be swept across the liquid, effectively collecting particles in the areas over which it travels.