Phase-change thermal diodes rectify heat transport much more effectively than solid-state ones, but are limited by either the gravitational orientation or one-dimensional configuration. Here, we report a planar phase-change diode scalable to large areas with an orientation-independent diodicity of over 100, in which water/vapor is enclosed by parallel superhydrophobic and superhydrophilic plates. The thermal rectification is enabled by spontaneously jumping dropwise condensate which only occurs when the superhydrophobic surface is colder than the superhydrophilic surface.Analogous to the electronic diode, a thermal diode transports heat with a strong directional preference. Thermal diodes are useful in a variety of applications, such as heat distribution in spacecrafts, 1 thermal regulation during energy harvesting, 2 thermally adaptive building materials, 3 and potentially phononic computing. 4 The effectiveness of a thermal diode is measured by the rectification coefficient (diodicity),

This paper describes a concept of concentration and binary separation of particles and its experimental confirmations for digital microfluidics where droplets are driven by the mechanism of electrowetting-on-dielectric (EWOD). As a fundamental separation unit, a binary separation scheme is developed, separating two different types of particles in one droplet into two droplets, one type each. The separation scheme consists of three distinctive steps, each with their own challenges: (1) isolate two different types of particles by electrophoresis into two regions inside a mother droplet, (2) physically split the mother droplet into two daughter droplets by EWOD actuation so that each type of particle is concentrated in each daughter droplet, and (3) free the daughter droplets from the separation site by EWOD to ready them for follow-up microfluidic operations. By applying a similar procedure to a droplet containing only one type of particle, two daughter droplets of different particle concentrations can be created. Using negatively charged carboxylate modified latex (CML) particles, 83% of the total particles are concentrated in a daughter droplet. Successful binary separation is also demonstrated using negatively charged CML particles and no-charge-treated polystyrene particles. Despite the undesired vortex developed inside the mother droplet, about 70% of the total CML particles are concentrated in one daughter droplet while about 70% of the total polystyrene particles are concentrated in the other daughter droplet.

This paper describes various manipulations of micro air bubbles using electrowetting on dielectric (EWOD): transporting, splitting, merging and eliminating. First, in order to understand the response of bubbles to EWOD, the contact angle modulation is measured in a capped air bubble and confirmed to be in good agreement with the Lippmann-Young equation until saturation. Based on the contact angle measurement, testing devices for the bubble manipulations are designed and fabricated. Sequential activations of patterned electrodes generate continuous bubble transportations. Bubble splitting is successfully realized by activating a single electrode positioned in the middle of bubble base. However, it is found that there are criteria that make splitting possible only in certain conditions. For successful splitting, smaller channel gap, larger bubble size, wider splitting electrode and/or larger contact angle changes by EWOD are preferred. These criteria are verified by a series of experiments as well as a static analysis. Bubble merging is achieved by moving bubbles towards each other in two different channel configurations: (1) channel I, where bubbles are in contact with the bottom channel plate only, and (2) channel II, where bubbles in contact with the top as well as bottom channel plates. Furthermore, eliminating a bubble to the ambient air is accomplished. All the bubble manipulation techniques may provide a versatile integrated platform not only to manipulate micro objects by utilizing micro bubbles as micro carriers, but also to enable a discrete bubble-based gas analysis system.

This paper describes a new microparticle sampler where particles can be efficiently swept from a solid surface and sampled into a liquid medium using moving droplets actuated by the electrowetting principle. We successfully demonstrate that super hydrophilic (2 microm and 7.9 microm diameter glass beads of about 14 degrees contact angle), intermediate hydrophilic (7.5 microm diameter polystyrene beads of about 70 degrees contact angle), and super hydrophobic (7.9 microm diameter Teflon-coated glass beads and 3 microm size PTFE particles of over 110 degrees contact angles) particles on a solid surface are picked up by electrowetting-actuated moving droplets. For the glass beads as well as the polystyrene beads, the sampling efficiencies are over 93%, in particular over 98% for the 7.9 microm glass beads. For the PTFE particles, however, the sampling efficiency is measured at around 70%, relatively lower than that of the glass and polystyrene beads. This is due mainly to the non-uniformity in particle size and the particle hydrophobicity. In this case, the collected particles staying (adsorbing) on the air-to-water interface hinder the droplet from advancing. This particle sampler requires an extremely small amount of liquid volume (about 500 nanoliters) and will thus be highly compatible and easily integrated with lab-on-a-chip systems for follow-up biological/chemical analyses.

This paper describes a new efficient in-droplet magnetic particle concentration and separation method, where magnetic particles are concentrated and separated into a split droplet by using a permanent magnet and EWOD (electrowetting on dielectric) droplet manipulation. To evaluate the method, testing devices are fabricated by the micro fabrication technology. First, this method is examined for magnetic particle concentration, showing that over 91% of magnetic particles can be concentrated into a split daughter droplet. Then, separation between magnetic and non-magnetic particles is examined for two different cases of particle mixture, showing in both cases that over 91% of the magnetic particles can be concentrated into split daughter droplets. However, a significant number of the non-magnetic particles (over 35%) co-exist with the magnetic particles in the same daughter droplets. This problem is circumvented by adding a droplet-merging step prior to applying the magnetic field. Finally, over 94% of the total magnetic particles are separated into a one split daughter droplet while 92% of the non-magnetic particles into the other split daughter droplet. This integrated in-droplet separation method may bridge many existing magnetic particle assays to digital microfluidics and extend their application scope.