Investigating low salinity waterflooding via glass micromodels with triangular pore-throat architectures.

Glass micromodels have been extensively used to simulate and investigate crude oil, brine, and surface interactions due to their homogeneous wettability, rigidity, and ability to precisely capture a reservoir's areal heterogeneity. Most micromodels are fabricated via two-dimensional patterning, implying that feature depths are constant despite varying width, which sub-optimally describes a three-dimensional porous architecture. We have successfully fabricated micromodels with arbitrary triangular cross sections via femtosecond pulsed laser direct writing resulting in depth-dependent channel width. As such, we have achieved arbitrary geometric control over device fabrication and thus a more accurate recapitulation of a geological porous media. With this fabrication technique, we are now able to directly observe pore-level, depth-dependent multiphase flow phenomena. This platform was used to study the low salinity effect (LSE) by simulating waterflooding processes using various brine solutions that differ in cation type and salinity. Patterned pore-throat structures were created to investigate displacement behavior during waterflooding. Real-time monitoring of the displacement processes, combined with a comparison of the brine chemistry before and after waterflooding provides an insight into realistic interactions occurring between crude oil and brine. The results indicate that produced emulsions were prone to coalesce in the presence of lower salinity brine. Combined with previous work, the LSE was interpreted as favored coalescence and resisted breakup that resulting in a more continuous aqueous phase during waterflooding therefore improving the displacement efficiency.

Fabrication of sealed sapphire microfluidic devices using femtosecond laser micromachining.

Due to its hardness, strength, and transparency, sapphire is an attractive material for the construction of microfluidic devices intended for high-pressure applications, but its physiochemical properties resist traditional microfabrication and bonding techniques. Here a femtosecond pulsed laser was used to directly machine fluidic channels within sapphire substrates and to form bonds between machined and flat sapphire windows, resulting in the creation of sealed microfluidic devices. Sapphire-sapphire bond strength was determined by destructive mechanical testing, and the integrity of the bond was verified by the capillary filling of the channel with air and ethanol. This combination of optical micromachining and bonding establishes a fully integrated approach to the fabrication of sapphire-based microfluidic systems.

Plasmon-less surface enhanced Raman spectra induced by self-organized networks of silica nanoparticles produced by femtosecond lasers.

Raman spectroscopy is the workhorse for label-free analysis of molecules. It relies on the inelastic scattering of incoming monochromatic light impinging molecules of interest. This effect leads to a very weak emission of light spectrum that provides a signature of the molecules being observed. Considerable efforts have been made over the last decades, in particular with the development of Surface Enhanced Raman Spectroscopy (SERS), to enhance the intensity of the emitted signal so that ultimately, traces of molecules can be detected. Here, we show that dense self-organized networks of quasi-monodisperse nanoparticles redepositing during femtosecond laser ablation of trenches in fused silica can lead to a significant field enhancement effect, enabling the Raman detection of a single-molecule layer deposited on the surface (so called monolayer). Unlike previously reported for SERS experiments, here, there is no metal layer promoting plasmonics effects causing localized field enhancement. The method for producing SERS substrates is therefore quite straightforward and low cost.

Simultaneous spatial frequency modulation imaging and micromachining with a femtosecond laser.

A Ti:Al2O3 chirped-pulse amplification system is used to simultaneously image and machine. By combining simultaneous spatial and temporal focusing (SSTF) with spatial frequency modulation for imaging (SPIFI), we are able to decouple the imaging and cutting beams to attain a resolution and a field-of-view that is independent of the cutting beam, while maintaining single-element detection. This setup allows for real-time feedback with the potential for simultaneous nonlinear imaging and imaging through scattering media. The novel SSTF machining platform uses refractive optics that, in general, are prohibitive for energetic, amplified pulses that might otherwise compromise the integrity of the focus as a result of nonlinear effects.

Spatial-spectral characterization of focused spatially chirped broadband laser beams.

Proper alignment is critical to obtain the desired performance from focused spatially chirped beams, for example in simultaneous spatial and temporal focusing (SSTF). We present a simple technique for inspecting the beam paths and focusing conditions for the spectral components of a broadband beam. We spectrally resolve the light transmitted past a knife edge as it was scanned across the beam at several axial positions. The measurement yields information about spot size, M2, and the propagation paths of different frequency components. We also present calculations to illustrate the effects of defocus aberration on SSTF beams.

Integrated single grating compressor for variable pulse front tilt in simultaneously spatially and temporally focused systems.

A Ti:Al(3)O(2) multipass chirped pulse amplification system is outfitted with a single-grating, simultaneous spatial and temporal focusing (SSTF) compressor platform. For the first time, this novel design has the ability to easily vary the beam aspect ratio of an SSTF beam, and thus the degree of pulse-front tilt at focus, while maintaining a net zero-dispersion system. Accessible variation of pulse front tilt gives full spatiotemporal control over the intensity distribution at the focus and could lead to better understanding of effects such as nonreciprocal writing and SSTF-material interactions.

Intuitive analysis of space-time focusing with double-ABCD calculation.

We analyze the structure of space-time focusing of spatially-chirped pulses using a technique where each frequency component of the beam follows its own Gaussian beamlet that in turn travels as a ray through the system. The approach leads to analytic expressions for the axially-varying pulse duration, pulse-front tilt, and the longitudinal intensity profile. We find that an important contribution to the intensity localization obtained with spatial-chirp focusing arises from the evolution of the geometric phase of the beamlets.

Spatio-temporally focused femtosecond laser pulses for nonreciprocal writing in optically transparent materials.

Simultaneous spatial and temporal focusing (SSTF) provides precise control of the pulse front tilt (PFT) necessary to achieve nonreciprocal writing in glass wherein the material modification depends on the sample scanning direction with respect to the PFT. The PFT may be adjusted over several orders of magnitude. Using SSTF nonreciprocal writing is observed for a large range of axial focal positions within the sample, and nonreciprocal ablation patterns on the surface of the sample are revealed. Further, the lower numerical aperture (0.03 NA) utilized with SSTF increases the rate of writing.

Two-phase displacements in microchannels of triangular cross-section.

Varying microfluidic channel cross-sectional geometry can dramatically alter fluid flow behavior, particularly for capillary-driven flow. Most fabrication techniques, however, are planar and therefore incapable of providing depth-dependent variations in width. We introduce an ultrafast laser ablation technique that enables the fabrication of microchannels with arbitrary triangular cross sectional geometry. Triangular channels were fabricated with widths ranging from 45 to 116µm and aspect ratios between 0.7 and 1.9. This experimental platform was utilized to observe two-phase flow and evaluate the capillary pressures required to initiate flow within triangular capillaries. Applying Mayer, Stowe and Princen (MS-P) theory, critical drainage capillary pressures were predicted for varying cross sections and compared to experimental observations. Results indicate the capability to predict capillary pressures inside triangular channels with perfectly water wet surfaces, providing the first instance of experimental validation of the theory for arbitrary triangular cross sections. This work was extended to intermediate wet conditions, which provides an insight into the prediction of capillary pressure under more realistic conditions. The fabrication techniques and validation of predictive frameworks presented here provide an approach to microfluidic experimental design that will impact a wide range of fundamental and applied technology areas.

Simultaneous spatial and temporal focusing for tissue ablation.

Simultaneous spatial temporal focusing (SSTF) is used to deliver microjoule femtosecond pulses with low numerical aperture geometries (<0.05 NA) with characteristics that are significantly improved compared to standard focusing paradigms. Nonlinear effects that would normally result in focal plane shifts and focal spot distortion are mitigated when SSTF is employed. As a result, it is shown that SSTF will enable surgical implementations that are presently inhibited.