Session J13: Nano Flows: General (8:00am - 8:45am CST)

Effect of Charge Inversion on Electroosmotic Transport in Nanochannels

The employment of electroosmosis in nanofluidic pores holds great potential for biotechnological applications. Hence, a complete understanding of the transport properties of nanoconfined multivalent electrolytes is key to enable electrokinetics as driving mechanism in nanodevices. Here, Non-equilibrium Molecular Dynamics (NEMD) simulations are conduced to study phenomena related to the presence of charge inversion in electrolytes confined in nanopores. In particular, we perform NEMD simulations of electroosmotic transport of multivalent electrolyte solutions in silica nanochannels. The solutions consist of water, chlorine as co-ion and different amounts of counter-ions i.e. sodium, magnesium and aluminum. The electroosmotic velocities are computed for different applied electric fields and magnitudes of inverted charge. Furthermore, we compute friction coefficient, zeta potential, water ordering and interfacial and bulk diffusivities and viscosities. We find that overscreening related to interfacial charge inversion modifies the electrokinetic driving force and shear stress near the walls. Our results suggest that due to charge inversion, zeta potential and water ordering are altered which induces flow reversal.   

Effect of Underlying Substrate on Interfacial Heat Transfer in Graphene Channels

Graphene is a 2D monoatomic-thick sheet of carbon atoms with exceptional thermal, electrical and mechanical properties. In addition to these properties, graphene exhibits ultra-low friction to water flow making graphene a promissory material to be used in nanofluidic conduits. Transport of fluids in nanochannels is substantially governed by interfacial phenomena therefore interfacial thermal resistance is an important parameter for the design of efficient nanofluidic devices. In this work, we employ atomistic simulations to study the role of the underlying substrate on interfacial heat transport in graphene channels. In particular, we conduct non-equilibrium molecular dynamics simulations of Poiseuille-like flow of water in pristine graphene channels and in channels with walls consisting of graphene supported on slabs of hexagonal boron nitride, silica and polyamide, respectively. For different imposed pressure gradients, we compute velocity and temperature profiles across the channels. Moreover, in order to analyze the relation between heat transfer and water structuring at the solid-liquid interface, for each graphene channel, we compute water ordering, interfacial viscosity and energy landscapes.   

Nanomechanical Measurement of the Brownian Force Noise in a Viscous Liquid

We study the spectral properties of the thermal noise force giving rise to Brownian fluctuations of a continuous mechanical system --- namely, a doubly clamped nanomechanical beam resonator --- immersed in a viscous liquid. To this end, we perform two separate sets of experiments. First, we measure the power spectral density (PSD) of the Brownian fluctuations of the resonator around its fundamental mode at its center by detecting the thermal fluctuations. Then, we measure the frequency-dependent linear response of the resonator, again at its center, by driving it with a harmonic force, via an electrothermal transducer, that couples well to the fundamental mode. These two separate measurements allow us to determine the PSD of the Brownian force acting on the structure in its fundamental mode. The PSD of the force noise extracted from multiple resonators with varied lengths spanning a broad frequency range displays a "colored spectrum" and follows the viscous dissipation of a cylinder oscillating in a viscous liquid by virtue of the fluctuation-dissipation theorem. The data are compared with a single-mode theory, with the deviations providing insight into the nature of the Brownian force acting on a multi-degree-of-freedom continuous system.   

Measuring the DNA Cargo of Viruses Using Nanofluidics

Adeno-associated viruses (AAVs) are engineered to deliver therapeutic DNA for gene therapy. However, AAV manufacturing is far from perfect, producing only a small portion of full viruses with the therapeutic gene. Real-time quality control in continuous AAV manufacturing requires characterizing the ratio of full to empty viruses. Here, we developed a nanofluidic approach for distinguishing full from empty viruses by measuring mass. Our approach uses nanochannel resonators, which measure nanoparticle mass from a proportional change in the device's resonant frequency. Single AAVs weigh only a few attograms, producing too low of a signal-to-noise ratio; we thus measure the average mass of AAV populations. We theoretically derived the relationship between the average AAV mass of a tested solution and the variance in resonant frequency. Using this relationship, we experimentally measured AAV mass, producing results consistent with standard, yet slower, biochemical methods. Using Monte-Carlo simulations of AAVs advecting and diffusing within the nanochannel, we gained additional insight into our measurements. With our approach, we aim to offer a real-time, high resolution characterization of AAV mass to enable quality control in continuous AAV manufacturing.   

Green recovery of lithium from seawater by hydrophobic interaction of poly($N$-isopropylacrylamide)/alginate composite

The demand of lithium, which is known as a crucial strategic element, has been largely increased in various applications, including lithium ion batteries. Herein, we propose the technique in a green recovery of Li$^{\mathrm{+}}$ ions from seawater. Poly($N$-isopropylacrylamide)(PNIPAAm)/alginate(Alg) composite crosslinked with Al$^{\mathrm{3+}}$ ions selectively adsorbs Li$^{\mathrm{+}}$ ions from seawater. Strong repulsion force by Al$^{\mathrm{3+}}$ ions rejects cations with a high adsorption affinity, while less rejecting Li$^{\mathrm{+}}$ ions with a low adsorption affinity. Structural characteristics of PNIPAAm/Alg composite were analyzed by using \textit{in situ} TEM and \textit{in situ} FTIR techniques. PNIPAAm/Alg composite is rapidly hydrated within 60 min, which enables rapid adsorption of Li$^{\mathrm{+}}$ ions. In addition, the incorporation of thermoresponsive PNIPAAm polymer induces a green recovery of Li$^{\mathrm{+}}$ ions by a hydrophobic interaction with applying a small thermal energy without the acidic treatment. 7.3{\%} of Li$^{\mathrm{+}}$ ions can be recovered from Li$^{\mathrm{+}}$-spiked seawater containing an extremely high concentration of impurities. The present study will provide an efficient Li$^{\mathrm{+}}$ recovery method with a novel interaction between ions and polymeric network.