Three-dimensional printed liquid diodes with tunable velocity: Design guidelines and applications for liquid collection and transport

Abstract

Directional and self-propelled flow in open channels has a variety of applications, including microfluidic and medical devices, industrial filtration processes, fog-harvesting, and condensing apparatuses. Here, we present versatile three-dimensional-printed liquid diodes that enable spontaneous unidirectional flow over long distances for a wide range of liquid contact angles (CAs). Typically, we can achieve average flow velocities of several millimeters per second over a distance of tens to hundreds millimeters. The diodes have two key design principles. First, a sudden widening in the channels' width, in combination with a small bump, the pitch, ensure pinning of the liquid in the backward direction. Second, an adjustable reservoir with differing expansion angles, the bulga, is introduced to manipulate the liquid velocity. Using a combination of experiments and lattice Boltzmann simulations, we provide a comprehensive analysis of the flow behavior and speed within the channels depending on CAs, pitch heights, and bulga angles. This provides guidelines for the fabrication of bespoke liquid diodes with optimal design for their potential applications. As a feasibility investigation, we test our design for condensation of water from fog and subsequent transport uphill.

Percolation in Networks of Liquid Diodes

“…Experiments were recorded from above using a Panasonic DC-S1 camera with Sigma 70 mm F2.8 DG Macro lens. Screenshots of videos and the fraction of occupied sites were obtained using a MATLAB script, adjusted from the one used in our previous work . The algorithm separates the final frame of the video into three RGB channels and subtracts the green from the red, highlighting only the liquid front with respect to the sample background.…”

“…The liquid diodes design is based on our previous work . The percentage of the pitch height was marked next to each unit cell for better visualization of the final sample.…”

“…In addition, controlling the percolation threshold and being able to tune the permeability of a network opens new horizons for designing microfluidic complex networks for mixing and separation, 19,35 heat transfer 36 and actuation. 37 Figure 1 shows the liquid diodes used in this work 33 and how they are implemented to form 2D disordered isotropically directed networks. Each diode features an asymmetric geometry comprising four main components (Figure 1a,i): the entrance channel (hilla), a central area (bulga), an exit channel (orifice), and, in blue, an ellipsoid-shaped bump (pitch), shown in more detail in Figure 1a,ii.…”

“…The liquid diodes comprise 3D surface structures that promote spontaneous unidirectional liquid flow in the capillary regime . We design a 2D network made of millimeter-size open channels , that set the bonds between neighboring sites. By tuning the geometric features of the liquid diodes and the contact angle (CA) that the flowing liquid creates with the surface, we control the diodicity of the bonds.…”

“…By lowering the CA (i.e., increasing the liquid wettability), the diodes start exhibiting flow in the backward direction and the diodicity breaks (Figure c, left). The dependence of the diodes’ performance on the pitch height is associated with the local reduction and following expansion of the channel’s depth, caused by the pitch, when the liquid flows in the backward direction . For example, with a pitch height of 40%, the channel depth is locally reduced to 60% of its original value.…”