Liguo Jiang

Visualizing millisecond chaotic mixing dynamics in microdroplets: A direct comparison of experiment and simulation

In order to fully explore and utilize the advantages of droplet-based microfluidics, fast, sensitive, and quantitative measurements are indispensable for the diagnosis of biochemical reactions in microdroplets. Here, we report an optical detection technique using two-photon fluorescence lifetime imaging microscopy, with an aligning-summing and non-fitting division method, to depict two-dimensional (2D) maps of mixing dynamics by chaotic advection in microdroplets with high temporal and spatial resolution. The mixing patterns of two dye solutions inside droplets were quantitatively and accurately measured. The mixing efficiency in a serpentine droplet mixer was also quantified and compared with the simulation data. The mapped chaotic mixing dynamics agree well with the numerical simulation and theoretical prediction. This quantitative characterization is potentially applicable to the real-time kinetic study of biological and chemical reactions in droplet-based microfluidic systems.

Quantitative imaging of mixing dynamics in microfluidic droplets using two-photon fluorescence lifetime imaging

Droplet-based microfluidic systems enable miniaturization of chemical reactions in femtoliter to picoliter volume compartments. Quantifying mixing dynamics of the reagents in droplets is critical to determine the system performance. In this Letter, we developed a two-photon excitation fluorescence lifetime imaging technique to quantitatively image the mixing dynamics in microfluidic droplets. A cross/autocorrelation method was used to reconstruct a high-quality fluorescence lifetime image of the droplet. The fluorescence decay was analyzed for accurate determination of the mixing ratio at each pixel of the image.

Real-time Monitoring of Hydrophobic Aggregation Reveals a Critical Role of Cooperativity in Hydrophobic Effect

The hydrophobic interaction drives nonpolar solutes to aggregate in aqueous solution, and hence plays a critical role in many fundamental processes in nature. An important property intrinsic to hydrophobic interaction is its cooperative nature, which is originated from the collective motions of water hydrogen bond networks surrounding hydrophobic solutes. This property is widely believed to enhance the formation of hydrophobic core in proteins. However, cooperativity in hydrophobic interactions has not been successfully characterized by experiments. Here, we quantify cooperativity in hydrophobic interactions by real-time monitoring the aggregation of hydrophobic solute (hexaphenylsilole, HPS) in a microfluidic mixer. We show that association of a HPS molecule to its aggregate in water occurs at sub-microsecond, and the free energy change is −5.8 to −13.6 kcal mol−1. Most strikingly, we discover that cooperativity constitutes up to 40% of this free energy. Our results provide quantitative evidence for the critical role of cooperativity in hydrophobic interactions.

Microsecond Protein Folding Events Revealed by Time-Resolved Fluorescence Resonance Energy Transfer in a Microfluidic Mixer

We demonstrate the combination of the time-resolved fluorescence resonance energy transfer (tr-FRET) measurement and the ultrarapid hydrodynamic focusing microfluidic mixer. The combined technique is capable of probing the intermolecular distance change with temporal resolution at microsecond level and structural resolution at Angstrom level, and the use of two-photon excitation enables a broader exploration of FRET with spectrum from near-ultraviolet to visible wavelength. As a proof of principle, we used the coupled microfluidic laminar flow and time-resolved two-photon excitation microscopy to investigate the early folding states of Cytochrome c (cyt c) by monitoring the distance between the tryptophan (Trp-59)-heme donor-acceptor (D-A) pair. The transformation of folding states of cyt c in the early 500 μs of refolding was revealed on the microsecond time scale. For the first time, we clearly resolved the early transient state of cyt c, which is populated within the dead time of the mixer (<10 μs) and has a characteristic Trp-59-heme distance of ∼31 Å. We believe this tool can find more applications in studying the early stages of biological processes with FRET as the probe.

Controllable formation of aromatic nanoparticles in a three-dimensional hydrodynamic flow focusing microfluidic device

Herein we investigate the formation of aromatic organic nanoparticles in a three-dimensional (3D) hydrodynamic flow focusing microfluidic device. We demonstrate a microfluidic based solvent/non-solvent exchange technique that enables controllable formation of aromatic nanoparticles with tunable size and size distribution. The 3D focusing is achieved by hydrodynamically focusing the sample stream with sheathed streams in both horizontal and vertical directions, and solvent/non-solvent exchange happens between the solvent contained sample stream and non-solvent contained horizontally sheathed streams. The 3D focusing effect was visualized using florescence confocal microscopy and the dynamics of solvent (DMF) depletion in the focused stream was calculated using a finite element computation software package COMSOL Multiphysics. By analyzing the results of self-assembled aromatic nanoparticles in the microfluidic device, we find that the speed of DMF depletion strongly influences the size and size distribution of self-assembled aromatic nanoparticles, and a rapid depletion of DMF is critical for achieving small aromatic nanoparticles with narrow size distribution. This work suggests that our 3D hydrodynamic flow focusing microfluidic device with the ability to precisely control the convective-diffusion process, and continuously vary the fluid flow conditions is promising for studying the formation of nanomaterials.

A Three-Dimensional Flow Focusing Microsecond Mixer for Dynamic Assessment of Nanoparticle Formation

This paper reports the design, fabrication, and characterization of a three-dimensional (3D) flow focusing microfluidic mixer. By hydrodynamically focusing the sample into a very confined stream in both vertical and horizontal directions, the mixer can achieve ultrarapid mixing of the sample stream with the reagent streams within ~3 ± 1 μs, which allows us to study the reaction kinetics on the microsecond timescale. Using the mixer, we investigated the formation of the hexaphenylsilole (HPS) nanoparticles. The HPS nanoparticles formed in the mixer have smaller size with narrower size distribution, compared to those formed by bulk nanoprecipitation. We also demonstrated the mixer can resolve the microsecond kinetics of nanoparticle self-assembly in two distinct steps, suggesting the nanoparticle formation follows the classical nucleation and growth theory.

A three-dimensional flow focusing microsecond mixer for dynamic assessment of nanoparticle formation

This paper reports the design, fabrication, and characterization of a three-dimensional flow focusing microfluidic mixer. By hydrodynamically focusing the sample stream into a very confined jet in both vertical and horizontal directions, the mixer can achieve ultrafast mixing of reagents within ~3 ± 1 μs. Such rapid mixing allows us to dynamically assess the formation of nanomaterial and study their assembly kinetics on microsecond timescale. Using the mixer, we investigated the formation of the hexaphenylsilole (HPS) nanoparticles. Compared to those formed by bulk nanoprecipitation, the HPS nanoparticles formed in the mixer have smaller size with narrower size distribution. We also kinetically resolved that the microsecond molecular self-assembly of HPS molecules displays two distinct steps. These results suggest that the formation of HPS nanoparticles follows the classical nucleation and growth theory.