Fluid release pressure for nanochannels: the Young-Laplace equation using the effective contact angle

Releasing fluids from nanochannels is quite challenging, yet crucial for the application of nanofluidic systems, e.g. drug delivery and nanoprinting. Previous work suggests that the pressure required to activate the releasing is enormously high (50 to above 300 MPa), while its underlying mechanism still remains unclear. In this work, through molecular dynamics simulations, we have identified a critical tilt angle of the hydrophilic nanochannel, below which spontaneous release of water is achieved. A significant increase in the contact angle is observed during the fluid releasing process due to the transition from the solid fluid contact to the fluid vapor contact. Such transition in nanoscale channels can significantly raise the release pressure by at most ∼30 MPa depending on the channel height and surface property, which makes the classical Young-Laplace equation underestimate the release pressure. By incorporating the derived formula for the largest effective contact angle, a modified Young-Laplace equation is developed, which predicts the release pressure well for both hydrophobic and hydrophilic channels down to the nanoscale. Furthermore, it is discovered that for nanoscale channels, the decreased rate of the normalized release pressure as a function of the contact angle becomes fast when the surface energy of the channel grows strong. The fast decreased rate is mainly caused by the adsorption of water molecules at the exit when the surface becomes highly hydrophilic.