Electrokinetic framework of dielectrophoretic deposition devices

Abstract

Numerical modeling and experiments are performed investigating the properties of a dielectrophoresis-based deposition device, in order to establish the electrokinetic framework required to understand the effects of applied inhomogeneous electric fields while moving particles to desired locations. By capacitively coupling electrodes to a conductive substrate, the controlled large-scale parallel dielectrophoretic assembly of nanostructures in individually accessible devices at a high integration density is accomplished. Thermal gradients in the solution, which give rise to local permittivity and conductivity changes, and velocity fields are solved by coupling electric, thermal, and fluid-mechanical equations. The induced electrothermal flow ETF causes vortices above the electrode gap, attracting particles, such as single-walled carbon nanotubes SWNTs, before they are trapped by the dielectrophoretic force and deposit across the electrodes. Long-range carbon nanotube transport is governed by hydrodynamic effects, while local trapping is dominated by dielectrophoretic forces in low concentration SWNT dispersions. Results show that by decreasing the ac frequency ac electroosmosis on the metallic electrodes occurs due to the emergence of an electric double layer, disturbing the initial flow pattern of the system. By superimposing a dc potential offset, a generated tangential electroosmotic fluid flow in the dielectric electrode gap also disrupts the ETF. Capacitive coupling is most efficient in the high frequency regime where it is the dominating impedance contribution. Understanding the occurrence and interaction of these different effects, including a self-limiting integration mechanism for individual nanostructures, allows an increased deposition yield at overall lower electric field strengths through a prudent choice of electric field parameters. The findings provide important avenues toward gentler particle handling, without direct current throughput, a relevant aspect for limiting process effects during device fabrication, all while increasing dielectrophoretic deposition efficiency in nanostructured networks

Joule heating effects on electroosmotic flow in insulator‐based dielectrophoresis

“…Similar to those previously studied in eDEP [34][35][36][37][38][39][40][41][42][43][44][45][46][47], electrothermal flows in iDEP devices also arise from the action of the electric field (both DC and AC) on fluid inhomogeneities (predominantly electrical properties including conductivity and permittivity) formed in the constriction region due to Joule heating-induced temperature gradients. Figure 4 shows the numerically predicted temperature and electric field contours in the constriction region at 100 V DC/500 V AC.…”

“…It was reported that a pair of counter-rotating fluid circulations could form near the microelectrodes [39]. This so-called electrothermal flow is a consequence of the interactions between the applied AC electric field and the Joule heating-induced gradients of fluid conductivity and permittivity [40,41]. Its magnitude is proportional to the fourth power of the local electric field.…”

The Influence of the Asymmetric Metal Electrodes on the Self-assembly of Single-walled Carbon Nanotubes by AC Dielectrophoresis

“…The total-assembly yield rate (P) is the ratio of the number of electrode pairs assembled by SWCNTs to 360, and the single-assembly yield rate (Ps) is the ratio of the number of electrode pairs bridged by only one bundle of SWCNTs to 360 and the multi-assembly yield rate (Pm) is that of electrode pairs bridged by more than one bundles of SWCNTs. Both Figure 3(a) and 3(c) show that the P increased with both of voltage magnitude and DEP assembly time because more SWCNTs can be transported to the gap and assembled then, which is in agreement with other experiments and theoretical work [4][5][6][7][8][9]. As we know, in the range of 1 to 10 MHz, the total number of SWCNTs assembled depends very little on the frequency [6].…”

“…There are mainly three forces acting on the SWCNTs during this procedure, such as DEP force, hydrodynamic forces of electro-thermal flow and AC electro-osmosis flow, and adhesive forces as shown in Figure 1. The first two are long-range forces, which determine the moving direction and speed of the SWCNTs in the suspension [4][5][6][7][8]. The z-direction of the forces push the SWCNTs toward the tip of the electrodes, and the x-and y-direction make the SWCNT align with the gap direction.…”

“…DEP is advantageous over other post-processed techniques because it allows controlling the position and density of the assembled SWCNTs between pre-fabricated electrodes and has no post-etching or transfer printing processes, which may introduce defects in the SWCNTs and degrade their electrical properties. A lot of research have investigated the dependence of the assembly yield of SWCNT depends its conditions, including AC bias voltage and frequency [5][6], assembly time [7], electrode geometry [8] and solution concentration [9]. In our SWCNT assembly experiments for asymmetric-work-function metal electrodes, different electrode-metal materials result in greatly differing assembly yields.…”