Predicting and eliminating Joule heating constraints in large dielectrophoretic IDE separators

Highlights

• Millimeter-scale dielectrophoretic (DEP) channel for high-throughput.

• Prediction of particle DEP velocity with electrothermal disturbance was validated.

• Joule heating disturbance minimized by tailored channel and electrode geometries.

Abstract

Dielectrophoresis (DEP), a measure to manipulate motion trajectories of suspended particles, has a high potential for solving difficult particle–liquid separation problems. Applications of DEP so far have been limited to micro-channels and lab-on-a-chip devices. However, for designing DEP separators with sufficiently high throughput to reach preparative scale, an understanding of the interplay of channel geometry and electrode concept with respect to induced particle velocity is required. The objective of tailored design is a control of particle motion trajectories predominantly by DEP while avoiding electrothermal interference. It is Joule heating, which gives rise to temperature gradients in the liquid phase and, thus, induces thermal convection. In this work we demonstrate that a solution of this Joule heating problem in large scale DEP systems is a tailored ratio of electrode diameter, electrode distance, and channel height. Based on model calculations we predicted the influence of both DEP force and drag force through thermal convection on particle trajectories for a case study, a channel with rectangular cross section and an array of cylindrical interdigitated electrodes (IDE) at the bottom. These theoretical results were verified experimentally by measured velocities of polyelectrolytic resin microparticles located at the subsurface of demineralized water. This allowed for a qualitative sensitivity analysis of the impact of voltage input, particle size and medium properties on the critical design parameter. From this, design criteria were deduced for the IDE–DEP system that allow for minimizing the influence of Joule heating. The findings demonstrate that, even if high voltages are applied, Joule heating problems can be effectively suppressed in DEP system scale-up.

Introduction

Effective manipulation, separation and concentration of microparticles and nanoparticles are essential in many pharmaceutical and biological production routes as well as in diagnostic and clinical applications (Cetin and Li, 2011). Electrical fields provide attractive electrophoretic or dielectrophoretic forces for particle manipulations as they tend to scale favorably in microfluidic devices (Voldman, 2006). Different from electrophoresis (EP), dielectrophoresis (DEP) is an electrokinetic phenomenon allowing for manipulation of neutral or charged particles due to dielectric polarization in inhomogeneous electric field. A net force (DEP force) gives arise due to the difference of local electric field on both sides of polarized particle, and thereby initiating particle translational motion (Jones, 1995, Pethig and Markx, 1997, Neculae et al., 2012). Therefore, particle need not to carry a net electric charge for inducing motion, and hence even alternating current (ac) can be applied to superimpose an electric force field on particle (Pethig, 2013). The strength of DEP force depends mainly upon the size of particle, frequency dependent particle effective polarizability to suspending medium, as well as the inhomogeneity of electric field (Morgan and Green, 2003, Thöming et al., 2006, Hemmatifar et al., 2013).

As a promising technique, DEP is now widely used in microfluidic systems for continuously fractionating neutral and charged particles due to its demonstrated high selectivity (Krupke et al., 2003a, Chuang et al., 2014). Nevertheless, reliable application of DEP is still limited to microsystems with flow rates in the range of microliters per minute and electrode distances in the micrometer scale (Han and Frazier, 2008, Li et al., 2010, Srivastava et al., 2011, Malekshahi et al., 2013). In case of continuous separation of particles from suspensions or dispersions using DEP, the term ‘high-throughput’ has not yet been defined: flow rates of 10 µL min⁻¹ (Cheng et al., 2009), 100 µL min⁻¹ (Gadish and Voldman, 2006), 150 µL min⁻¹ (Yan et al., 2014), 1000 µL min⁻¹ (Pesch et al., 2014) and even 140 mL min⁻¹ (Du et al., 2008) were considered high-throughput. While the µL min⁻¹ range was suggested to bridge the gap between preparative sample collectors and microscale detectors (Gadish and Voldman, 2006), the flow rate of 140 mL min⁻¹ obtained by Du et al. (2008) is to the best of our knowledge the highest one reported in literature so far and the only one with potential for production scale. It was achieved in a system used for separating gold particles from an aqueous suspension of heavy minerals and limited to discontinuous operation due to Joule heating. It was assumed that this problem was attributed to the low volumetric surface of the DEP channel. This assumption is supported by the analysis of Lewpiriyawong et al. (2010), who believed Joule heating to be negligible for flow rates in the µL min⁻¹ range only when a medium of low conductivity and a glass substitute with good thermal conductivity are applied. For scaling up DEP separation Sano et al. (2012) proposed a dielectrophoretic particle separator by using three-dimensional mesh stacked electrodes to separate tungsten carbide particles from a mixture with diatomite. The simplest way to enlarge throughput per unit is to increase the width of the channel. Obviously this is the lower cost option to a massively parallelized numbering-up of single units, but it might be associated to manufacturing challenges. However, Cetin and Li (2011) pointed out that, in order to make DEP-based systems competitive with conventional separators, research is required on the improvement of DEP-system׳s throughput. A DEP system with a large distance between electrodes and, definitely requires high electric potential and accordingly high energy input (Du et al., 2013). This in turn causes unavoidable energy consumption and undesirable thermally induced medium flow by Joule heating disturbing the particle DEP motion, which is a general DEP problem. It is a problem that can be solved in parts by using interdigitated electrodes (IDE), which were suggested to reduce the energy demand (Wang et al., 2014).

The aforementioned general DEP problem raises the question whether an optimal design of DEP channel geometries and electrode arrangement exist for a specific application, which provides optimal distribution of electric field gradient and, hence, maximized separation efficiency at minimized energy demand. Obviously it is not possible to outwit physics, in particular the limitations in creating the electric field gradients. Consequently these limitations act as constraints when optimizing the trade-off of conflicting requirements, both dielectrophoretic and electrothermal. It can be expected that in continuous separation systems, i.e. with imposed flow, the relative disturbance of DEP particle motion by electrothermal induced fluid motion reduces with volumetric flow rates of the imposed flow. Hence Joule heating disturbances are largest at zero flow rates, i.e. in a batch process which is therefore chosen for a worst case study to demonstrate the possibility of eliminating Joule heating disturbances.

Here we solve this general problem by firstly describing the interplay of channel geometry, size and position and secondly, based on these findings, by identifying optimality. By this means, we want to demonstrate that it is possible to avoid Joule heating disturbances in DEP systems even in a worst case, i.e. a batch process.