Preparation of Janus Particles and Alternating Current Electrokinetic Measurements with a Rapidly Fabricated Indium Tin Oxide Electrode Array

Janus particles, named after the Roman god with a double face, are asymmetric particles whose two sides have physically or chemically different surface properties^1,2. Due to this asymmetric feature, Janus particles exhibit special responses under electric fields, such as DEP^3,4,5,6, EROT², and induced-charge electrophoresis (ICEP)^7,8,9. Recently, several methods to prepare Janus particles have been reported, including the Pickering emulsion method¹⁰, the electrohydrodynamic co-jetting method¹¹, and the microfluidic photopolymerization method¹². However, these methods require a series of complicated apparatus and procedures. This article introduces a simple method to prepare Janus particles and fully coated metallic particles. A monolayer of micro-scaled silica particles is prepared in a drying process and is put into a sputtering device to be coated with Au. One hemisphere of the particle is shaded, and only the other hemisphere is coated with Au^2,13. The monolayer of the Janus particle is stamped with a polydimethylsiloxane (PDMS) stamp and then treated with a second coating process to prepare fully coated metallic particles¹⁴.

To characterize the electrical properties of a Janus particle, different AC electrokinetic responses, such as DEP, EROT, and electro-orientation, are widely used^{9,15,16,17,18,19}. For example, EROT is the steady-state rotational response of a particle under an externally imposed rotating electric field^2,9,15,16. By measuring the EROT, the interaction between the induced dipole of the particles and the electric fields can be obtained. DEP, which arises from the interaction between the induced dipoles and a non-uniform electric field, is capable of leading to particle movement^3,4,5,9,15. Different kinds of particles can be attracted to (positive DEP) or repelled from (negative DEP) the electrode edges, which serves as a general method for manipulating and characterizing particles in the microfluidic device. The translational (DEP) and rotational (EROT) characteristics of the particle under the electric field are dominated by the real and imaginary part of the Clausius-Mossotti (CM) factor, respectively. The CM factor depends on the electrical properties of the particles and the surrounding liquid, which are revealed from the characteristic frequency, ω_c = 2σ / aC_DL, of DEP and EROT, where σ is the liquid conductivity, a is the particle radius, and C_DL is the capacitance of the electrical double layer^15,16. To measure the EROT and DEP of particles, specially designed electrode array patterns are needed. Traditionally, a photolithographic technique is used to create electrode arrays and requires a series of complicated procedures, including photoresist spin-coating, mask alignment, exposure, and development^15,18,19,20.

In this article, the rapid fabrication of electrode arrays is demonstrated by direct optical patterning. A transparent thin-film ITO layer, which is coated on the glass substrate, is partially removed by a fiber laser marking machine (1,064 nm, 20 W, 90 to 120 ns pulse width, and 20 to 80 kHz pulse repetition frequency) to form a four-phase electrode array. The distance between the diagonal electrodes is 150-800 µm, which can be adjusted to suit the experiments. The four-phase electrode array can be used to characterize and concentrate particles in different microfluidic devices^15,16,18. To generate the four-phase electric field, the electrode array is connected to a 2-channel function generator and to two invertors. The phase shift between the adjacent electrodes is set at either 90° (for EROT) or 180° (for DEP)¹⁵. The AC signal is applied at a 0.5 to 4 V_pp voltage amplitude, and the frequency ranges from 100 Hz to 5 MHz during the operation process. Janus particles, metallic particles, and silica particles are used as samples to measure their AC electrokinetic properties. Suspensions of the particles are placed on the center region of the electrode array and are observed under an inverted optical microscope with a 40X, NA 0.6 objective. Particle motion and rotation are recorded with a digital camera. The DEP movement is recorded at the annular region, between 40 and 65 µm radially away from the array center, and EROT is recorded at the circular region, 65 µm radially away from the array center. Particle velocity and angular velocity are measured by the particle-tracking method. The particle centroids are distinguished by gray scale or geometry of particles using software. The particle velocity and angular velocity are obtained by measuring the movements of the particle centroids.

This article provides a simple method to rapidly fabricate arbitrarily patterned electrode arrays. It introduces the preparation of fully or partially coated metallic particles, which can be used in different fields, with uses ranging from biology to industry applications.