Capillary Condensation in 8 nm Deep Channels.

Condensation on the nanoscale is essential to understand many natural and synthetic systems relevant to water, air, and energy. Despite its importance, the underlying physics of condensation initiation and propagation remain largely unknown at sub-10 nm, mainly due to the challenges of controlling and probing such small systems. Here we study the condensation of n-propane down to 8 nm confinement in a nanofluidic system, distinct from previous studies at ∼100 nm.

Direct visualization of fluid dynamics in sub-10 nm nanochannels.

Optical microscopy is the most direct method to probe fluid dynamics at small scales. However, contrast between fluid phases vanishes at ∼10 nm lengthscales, limiting direct optical interrogation to larger systems. Here, we present a method for direct, high-contrast and label-free visualization of fluid dynamics in sub-10 nm channels, and apply this method to study capillary filling dynamics at this scale.

Bubble nucleation and growth in nanochannels.

We apply micro- and nanofluidics to study fundamental phase change behaviour at nanoscales, as relevant to shale gas/oil production. We investigate hydrocarbon phase transition in sub-100 nm channels under conditions that mimic the pressure drawdown process. Measured cavitation pressures are compared with those predicted from the nucleation theory.

Condensation in One-Dimensional Dead-End Nanochannels.

Phase change at the nanoscale is at the heart of many biological and geological phenomena. The recent emergence and global implications of unconventional oil and gas production from nanoporous shale further necessitate a higher understanding of phase behavior at these scales. Here, we directly observe condensation and condensate growth of a light hydrocarbon (propane) in discrete sub-100 nm (∼70 nm) channels.

Vapour adsorption kinetics: statistical rate theory and zeta adsorption isotherm approach.

The equilibrium zeta adsorption isotherm for vapours indicates the amount adsorbed is finite for vapour-phase pressures approaching the saturation value, and is strongly supported by experimental measurements for a number of different vapour-solid surface systems. This isotherm assumes the adsorbate consists of differently sized molecular clusters in local equilibrium rather than the adsorbate being in layers. We use the local-equilibrium approximation and develop a method to determine the expression for chemical potential of the adsorbate in terms of the amount adsorbed, n(t).

From adsorption to condensation: the role of adsorbed molecular clusters.

The adsorption of heptane vapour on a smooth silicon substrate with a lower temperature than the vapour is examined analytically and experimentally. An expression for the amount adsorbed under steady state conditions is derived from the molecular cluster model of the adsorbate that is similar to the one used to derive the equilibrium Zeta adsorption isotherm. The amount adsorbed in each of a series of steady experiments is measured using a UV-vis interferometer, and gives strong support to the amount predicted to be adsorbed.

Nucleation and growth of condensate in nanoporous materials.

We consider the adsorption-desorption cycles of water and of three hydrocarbons on MCM-41 and on SBA-15. We show that during the desorption portion of a cycle, when the condensate is still at the mouth of the pores, in equilibrium, and the pressure, P, is the minimum value reached before pore-emptying begins, the contact angle is zero. This value of the contact angle is used with the Kelvin equation to calculate the pore radius of each of the mesoporous silicas considered.

Prediction of the wetting condition from the Zeta adsorption isotherm.

We use the Zeta adsorption isotherm and propose a method for determining the conditions at which an adsorbed vapour becomes an adsorbed liquid. This isotherm does not have a singularity when vapour phase pressure, P(V), is equal to the saturation-vapour pressure, Ps, and is empirically supported by earlier studies for P(V) < Ps. We illustrate the method using water and three hydrocarbon vapours adsorbing on silica.

Clusters in the adsorbates of vapours and gases: zeta isotherm approach.

A procedure for determining the structure of vapour and gas adsorbates that is based on the Zeta adsorption isotherm is reported. For vapours and for gases, this isotherm supposes that an adsorbate consists of particle clusters with the number of particles in a cluster denoted ζ, where ζ can be 1, 2, 3… up to a maximum of ζm, and predicts the isotherm constants are independent of pressure. For vapours, this allows the isotherm constants to be determined from isotherm measurements made at pressures less than the saturation-vapour pressure, Ps, but applied at pressures greater than Ps.