Colloidal Physics: Study on reversible assembly mechanism of micro-nano particles 

Here, we systematically study the effects of ionic surfactants on the colloidal particle assembly process subject to a fixed OEF, by using an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (cetyltrimethylammonium bromide).

In the concentration range below the critical micelle concentration (CMC), the particles tend to form three different aggregate structures, including randomly close-packed, hexagonally close-packed crystals, and randomly non close-packed structures. In the concentration range at CMC and above, the particles cannot assemble into any aggregate structures under the given OEF. With increasing surfactant concentration, the average interparticle separation distance within the aggregate can be regulated in a wide range.We qualitatively interpret the observed variation of particle assembly behaviors in different surfactant solutions based on the particle/particle interaction energy, implying that the repulsive electric double-layer interaction is significantly enhanced with increasing ionic surfactant concentration and hinders the particles approaching close to the high-energy barrier. These results suggest that the particle aggregate structure and interparticle separation distance of the two-dimensional colloidal assembly can be manipulated by controlling the ionic surfactant concentration according to the requirements of practical applications.

Figure1.  (a) Schematic illustration of the surfactant molecules absorption on PS particles. (b) Schematic of the experimental apparatus, inset: force vectors for the EHD drag force, van der Waals force, electric double layer force and depletion force induced by surfactant micelles (not to scale). The particle assemblies formed on the top electrode surface are not drawn for simplicity.

Figure2. (a) Representative fluorescence microscopy time-lapse images of 1 µm diameter PS particles suspended in three different liquid media (DI water, 0.1 mM SDS, 0.1 mM CTAB) at different times near the bottom electrode surface. The superimposed green points mark the particle centers, and the green lines depict the connection to the nearest neighbors, as calculated via Delaunay triangulation. The electric field is applied at t = 0 s and the time-lapse images were taken for 300 s after the application of an oscillatory electric field of 1.09 × 104 V m⁻¹ and 400 Hz. All the picture scales are 3 µm. (b) Normalized interparticle separation distance (d/2a) as a function of time for three suspensions. (c) Normalized interparticle separation distance (d/2a) after applying the oscillatory electric field for 300 s in three suspensions. (d) Number of particles forming the assembly as a function of time for three suspensions. Error bars are two standard deviations of the mean of three trial replicates.

Figure3. Representative fluorescence microscopy images of 1 µm diameter PS particles suspended in SDS (a) and CTAB (b) solutions with different concentrations (0.001 mM, 0.01 mM, 0.1 mM, 1 mM, 10 mM) near the bottom electrode surface, taken at 300 s after the application of OEF of 1.09 × 104 V m⁻¹ and 400 Hz; normalized interparticle separation distance (d/2a) as a function of time and after applying the OEF for 300 s in SDS (c), (e) and CTAB (g), (i) with different concentrations; orientational bond order parameter (Ψ6) as a function of concentration of SDS (f) and CTAB (j); number of particles forming the assembly as a function of time in SDS (d) and CTAB (h). Error bars are two standard deviations of the mean of three trial replicates.

Articles:

    Lisha Luo, Zhibin Yan*, Minqi Yang, Hongjie Yin, Mingliang Jin, Huicheng Feng, Guofu Zhou, Lingling Shui*, Two-dimensional colloidal particle assembly in ionic surfactant solutions under an oscillatory electric field, Journal of Applied Physics. 2021, 54, 475302.

Point-of-care Testing：Hybrid paper-based digital microfluidic detection chip

Here, we demonstrate fabrication of paper-based EWOD devices using cellulose paper as substrate and paraffin wax as dielectric hydrophobic layer by a simple pressing treatment for the cellulose paper and melting or spray-coating of paraffin wax. 

We optimized the key fabrication parameters according to the surface morphology and hydrophobicity of the obtained wax film, including the pressing pressure, thermal annealing temperature and amount of paraffin wax. Moreover, the EWOD devices fabricated by the spray-coating method demonstrate larger contact angle changes but with smaller contact angle hysteresis under DC electric field when compared with the devices fabricated by the melting method.

Figure1. We fabricated paper-based EWOD devices using cellulose paper as substrate and paraffin wax as dielectric hydrophobic layer, which have been widely used in fabrication of continuous-flow µPAD. The cellulose paper was firstly densified via being pressed under different pressures to reduce the surface roughness and internal void volume. The paraffin wax film was fabricated via two different approaches, including melting method and spray-coating method followed by thermal annealing treatment.

Figure2. BNP quantitative detection of HPDMF chip.

Articles：

• He Li#, Jiayi Cui#, Zhibin Yan*, Mingliang Jin, Yu Zheng, Guofu Zhou, Lingling Shui*, Paper-based electrowetting devices fabricated with cellulose paper and paraffin wax, Results in Physics  2021, 31, 105042

Patent：

• “一种一次性纸基数字微流控检测芯片及其检测方法（20200104181020）” 颜智斌、崔佳易、李贺、金名亮、周国富、水玲玲

Electronics cooling：Heat management system for 5G communication base station

In this paper, we investigate the effect of temperature gradient on the particle deposition of a pressure-driven suspension flow in a microchannel. We designed a microfluidic device which can allow direct observation of the real-time process of particle deposition with single-particle resolution along the direction of applied temperature gradient. 

The experimental results show that particle deposition rate is decreased by increasing the applied temperature gradients. Based on the framework of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, we then derive a mass transport model to describe the particle deposition under different temperature gradients. The model shows that the observed reduction of particle deposition rate with temperature gradient is due to the collective effect of the temperature gradient and the bulk solution temperature in the two steps of the particle deposition process, including the particle transport and the particle attachment.

Bioinspired Droplet Routing Method for Digital Microfluidic Cooling of Hotspots in Integrated Circuits

Integrated cooling methods based on digital microfluidics (DMF) has been regarded as a promising solution to the effective thermal management of high-performance integrated circuits. This method can realize adaptive cooling by programming a droplet to a target (e.g., hot spot) along a designed path. The droplet route is crucial for the cooling performance, especially, when multiple droplets move on the chip simultaneously and the hot spot distribution is dynamically changed. However, there is hardly research investigating into the droplet routing method for hotspot cooling.

Here, we propose a droplet route optimization method, self-generated chemotaxis-inspired algorithm (SCA), based on the real-time temperature distribution of chip which aims to achieve real-time routing for multiple droplets to search and cool hot spots. The main ideal of the proposed method is to guide a droplet to move in the direction of temperature gradient which is modeled similarly to the migration behavior of chemotactic cells. 

Fig. 1 Schematic of chemotaxis and the digital microfluidic (DMF) platform for chip cooling. (a) Schematic of chemotaxis where cells are attracted into nutrient with higher concentration.

Fig. 2a-c show the distribution of five 120 W/cm² heat sources (i.e., the red cells) and droplets were allowed to be injected from three different inlets using three kinds of routes. To compare the cooling performances of SCA route, Ant colony algorithm (ACA) route and direct route, we simulated 500 droplets cooling the chip based on these three types of routing methods. Fig. 2a illustrates three representative SCA routes. Although each coolant droplet for the SCA method was not predefined to pass all the hot spots, the droplets from three inlets could successfully cover all the hot spots. Different from SCA routes, the basic idea of ACA routes is to program the shortest path that starts from inlet, passes through all the hot spots, and leaves for the outlet. Thus, if the distribution of the hot spots keeps static, an ACA route from a certain inlet will remain unchanged under normal circumstance. As shown in Fig. 2b, there were three ACA routes from three inlets respectively. These three routes had an intersection at the Hot Spot 4 (i.e., the first hot spot visited) and became the same route after confluence. Thus, the target conflicts occurred and restricted the flow flux of the droplets in the ACA case. Compared with ACA routes, SCA routes are planned in real time based on the real-time local temperature distribution rather than predefined only according to the hot spots’ positions. Thus, the SCA routes have a characteristic of diversity and each SCA route is different from each other under normal conditions although the positions of hot spots are fixed. Moreover, the target conflicts in the ACA routes are much more serious than the SCA routes because the former is bound to pass all the hot spots. Fig. 2c shows the direct routes which started from the inlets and went straight to the outlets. When multiple droplets were transported on the chip with direct routes, there would never occur any target conflict.

Fig. 2 Five hundred droplets cooling five 120 W/cm² hot spots in three-inlet configuration at a speed of 0.1 m/s based on three different kinds of routing methods. (a) Three representative droplet routes based on SCA and they successfully searched all the hot spots. (b) Three ACA routes respectively from three inlets. ACA routes are the shortest paths that start from a certain inlet, pass through all the hot spots, and leave for the outlet. (c) Three direct routes which start from inlets and go straight to the outlets.

Article:

•   Zhibin Yan*, Xiaoyang Huang, Lingling Shui and Chun Yang, Kinetics of colloidal particle deposition in microfluidic systems under temperature gradients: experiment and modelling, Soft Matter, 16, 3649-3656, 2020. Backcover封面文章
•   Zhibin Yan*, Mingliang Jin, Zhengguang Li, Guofu Zhou and Lingling Shui*, Droplet-Based Microfluidic Thermal Management Methods for High Performance Electronic Devices, Micromachines, 10, 89, 2019. doi:10.3390/mi10020089.

Patent:

• “一种芯片热点冷却装置及其使用方法（202011607765）” 颜智斌 、杨森 、李政光 、廖明 、金名亮 、周国富 、水玲玲
• “数字微流控芯片的液滴路径规划方法及装置（202110552794）”颜智斌 、陈茂华 、陈森隆 、廖明 、金名亮 、周国富 、水玲玲