A short summaryBeing experimentalists and material chemists, we synthesize and characterize colloidal particles that autonomously swim in water, i.e. active colloids or micromotors. While observing their individual and collective dynamics under a wide variety of experimental conditions, we try to understand why they do what they do, by combining our knowledge on colloidal electrokinetics, electrochemistry, sciences of interfaces, and material chemistry. Finite element simulation and acoustic levitation techniques are often handy in clarifying mechanisms. We are mostly driven by curious questions, the answers to which hopefully lead to technological innovations and scientific advancement. Occasionally, we go one step further and ask ourselves how our observations can translate into something useful.
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Current projects
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Subject of study
Our research focus on a special type of colloidal particles--with sizes ranging from ~100 nm to tens of µm--that are capable of harvesting energy stored in their environment and swim autonomously in liquid (mostly water). These colloids, with a constant energy input and thus far from thermodynamic equilibrium, are known as micromotors, synthetic microswimmers, active colloids, or untethered microbots, by various research communities with different interests. Regardless of what they are called, these unique colloids exhibit dynamics quite different from their Brownian counterparts, and under an optical microscope show life-like motility and collective behaviors that are fascinating to watch.
Significance
The study of these active colloids is closely related to the subject of material chemistry, fluid mechanics and colloidal physics for obvious reasons. When we zoom out from their individual dynamics and pay attention to their collective behaviors, we encounter many interesting scientific questions that lie in the field soft matter physics. Alternatively, active colloids are also considered as smart materials and devices capable of moving, sensing, communicating and operating, essentially robots at nano- or microscales. As a result, there are great interests in using these smart colloids in applied scenarios such as biomedicine, environmental remediation, and micro-assembly, where microrobots are envisioned to play a revolutionizing role.
Be it for pure science or applications, the study of active colloids is highly interdisciplinary, and sits at the frontier of many of the related discipline.
Be it for pure science or applications, the study of active colloids is highly interdisciplinary, and sits at the frontier of many of the related discipline.
What we do
The research projects in our lab can be divided along two dimensions: by the complexity of the system, or by the type of propulsion.
In increasing order of complexity, we are interested in the following aspects of active colloids:
-Single particle propulsion: mostly regarding the propulsion mechanism, and any unusual dynamics such as pulses, reverse motion, etc.
-Confinement: we expose active colloids in confinement of various types and degrees, and examines how their dynamics change as a result, or if anything interesting occurs.
-Many body interactions: this line of research is concerned with the scenario where more than one active colloid is present, where a combination of chemical gradient, hydrodynamics, electrostatics and physical hindrance could lead to complicated structures and dynamics, such as dynamic assembly, active turbulence, and phase separation.
In each of the above topic, the subject of study can be chosen from a library of interesting active colloids, each powered by a different way:
-Catalytic swimmers, most typically bimetallic microrods or spheres, and Janus microspheres half coated with Pt. These colloids move in H2O2 by surface catalysis.
-Photocatalytic swimmers, typically Janus microspheres made of a photosensitive semiconductor material, half coated with metal (gold or Pt).
-Oscillating swimmers. This group of active colloid is less explored but particularly interesting, because they exhibit periodic oscillation in speed, and collectively show fascinating emergent behaviors such as synchronization, waves and pattern formation. They are also closely related to the topic of nonlinear sciences.
-Externally powered swimmers, including those powered by ultrasound, magnetic fields, or electric fields (i.e. ICEP).
Simple maths tell us that a total of 12 subgroups of topics can be explored following the above two-dimensional categorization of our research, and our publications cover almost all topics.
In increasing order of complexity, we are interested in the following aspects of active colloids:
-Single particle propulsion: mostly regarding the propulsion mechanism, and any unusual dynamics such as pulses, reverse motion, etc.
-Confinement: we expose active colloids in confinement of various types and degrees, and examines how their dynamics change as a result, or if anything interesting occurs.
-Many body interactions: this line of research is concerned with the scenario where more than one active colloid is present, where a combination of chemical gradient, hydrodynamics, electrostatics and physical hindrance could lead to complicated structures and dynamics, such as dynamic assembly, active turbulence, and phase separation.
In each of the above topic, the subject of study can be chosen from a library of interesting active colloids, each powered by a different way:
-Catalytic swimmers, most typically bimetallic microrods or spheres, and Janus microspheres half coated with Pt. These colloids move in H2O2 by surface catalysis.
-Photocatalytic swimmers, typically Janus microspheres made of a photosensitive semiconductor material, half coated with metal (gold or Pt).
-Oscillating swimmers. This group of active colloid is less explored but particularly interesting, because they exhibit periodic oscillation in speed, and collectively show fascinating emergent behaviors such as synchronization, waves and pattern formation. They are also closely related to the topic of nonlinear sciences.
-Externally powered swimmers, including those powered by ultrasound, magnetic fields, or electric fields (i.e. ICEP).
Simple maths tell us that a total of 12 subgroups of topics can be explored following the above two-dimensional categorization of our research, and our publications cover almost all topics.
Tools
Our exploration of the above topics of active colloids is enabled by the following techniques and tools:
-Material synthesis: we are first and foremost a material chemistry lab (with an interest in soft matter physics). This means that we spend quite some time synthesizing and characterizing a wide variety of colloidal particles, by either or physical chemical methods. Some examples of colloidal particles we routinely synthesize are given below.
-Numerical simulation: because many of the physical properties of an active colloid system is beyond measurable or difficult to visualize, we use COMSOL multiphysics package extensively to aid our exploration, especially in the quest of understanding propulsion mechanisms and inter-particle interactions.
-Acoustic levitation. Ultrasound operating at MHz is a powerful tool in the manipulation of colloids, via either acoustic radiation forces or acoustic streaming. Although we are not experts on acoustics or acoustofluidics, we are familiar with the technique needed to levitate colloidal particles with simple acoustic setups. We are also comfortable with combining this technique with other configurations to study active colloids in a complex energy landscape.
-Material synthesis: we are first and foremost a material chemistry lab (with an interest in soft matter physics). This means that we spend quite some time synthesizing and characterizing a wide variety of colloidal particles, by either or physical chemical methods. Some examples of colloidal particles we routinely synthesize are given below.
-Numerical simulation: because many of the physical properties of an active colloid system is beyond measurable or difficult to visualize, we use COMSOL multiphysics package extensively to aid our exploration, especially in the quest of understanding propulsion mechanisms and inter-particle interactions.
-Acoustic levitation. Ultrasound operating at MHz is a powerful tool in the manipulation of colloids, via either acoustic radiation forces or acoustic streaming. Although we are not experts on acoustics or acoustofluidics, we are familiar with the technique needed to levitate colloidal particles with simple acoustic setups. We are also comfortable with combining this technique with other configurations to study active colloids in a complex energy landscape.
我们所研究的nanomotor,或者叫做nano and microscale motor,一种常见的翻译是纳米马达或者微米马达,或者叫做自驱动胶体颗粒(self-propelled colloidal particle)。它的大小在微米左右(或许几百纳米,或许数微米或者更大),能够将外界的能量转化为自身动能,在液体中自发运动。所能够转化的能量,可以是化学能(即通过化学反应),也可以是电磁能(电场、磁场)、光能、热能、声波等多种形式。对应着多种能量,也就有着不同的颗粒驱动机制(propulsion mechanism)。这些颗粒的制备方法多种多样,常见的包括利用电化学模板沉积制备的微米棒、微米管,以及化学方法合成(也可以直接购买)的微米球,还有利用光刻蚀等方法制备的形状更复杂的颗粒。
这个领域大约是10年前开始发展的。最开始是宾州州立大学的Sen和Mallouk课题组(也就是我的博士课题组)发现双金属Au-Pt微米棒在过氧化氢溶液中可以自发运动起来。几乎同时期加拿大多伦多大学的Ozin课题组也做出了类似的观测。围绕着这个发现,不同的研究组做了很多工作,并且在几年后确定了机理(自电泳),也在这个体系基础上做了很多改进和应用方面的探索。后来慢慢的涌现出了很多其他工作,各种各样不同的驱动机制也慢慢的被开发出来,包括我们在2012年开发的超声波驱动的微米颗粒。现在的研究重点之一是开发基于自驱动微颗粒的种种应用,尤其是生物医药方面的应用。
这个领域研究有两方面的重要性。首先,在基础物理层面,物体在微观下的运动方式和宏观有显著不同。例如,在宏观下,比如我们人类的运动,很大程度上依赖惯性,即动作后物体能够保持运动一段时间。但是在微观下,惯性相比环境的粘滞阻力而言非常小,所以微观物体的运动不能依赖惯性,而必须采用其他的运动机理。另一方面,微观物体的能量转化也是很有意思的问题,效率通常比较低,也涉及到一些宏观下不是很明显的能量损失机制。除此之外,微观物体的互动与活性物质问题,也是在基础物理层面本领域的一个研究热点,受到很多物理学家的关注。
在应用层面,自驱动微颗粒也有很多研究。这方面更容易理解一些。首先,自驱动微颗粒代表着一类人工合成的类微生物体,因而对于研究微生物,如细菌、细胞等的运动、群聚现象等很有帮助。其次,正是由于这种相似性,很多研究者也希望能够利用自驱动微颗粒做一些生物医药方面的应用,例如药物释放、微创手术等等。另外,这些颗粒在环境监测与污染物去除、微纳制造、骨损伤修复领域也体现出了一些潜力。总体来看,这是一个非常年轻,也非常有潜力的体系,有很多的应用机遇。我们的科研计划是深入探索基于自驱动微颗粒的智能材料体系,希望能够对超声波驱动的体系机理做更进一步研究,也在生物医药领域取得一些应用方面的突破。
*我们课题的科普性质介绍,请看B站视频:https://www.bilibili.com/video/BV1LT411g7sF/?vd_source=23b0ba9fd63595312ec4802f929facc9
我们组的B站账号是hitsz-wanglab,里面有大量和微纳米马达、活性胶体相关的视频、学习资料。
**《中国科学:化学》与《科学通报》近日联合出版了一期微纳米马达特刊,有十余篇中文综述,对于掌握本领域发展动态很有帮,推荐感兴趣的朋友阅读。这两期特刊的介绍见:http://blog.sciencenet.cn/blog-528739-1029072.html。文章pdf的链接: https://pan.baidu.com/s/1eS23VQq 密码: s6vc
***我们正在翻译UCSD的Joseph Wang教授的著作《Nanomachines》,预期将于2018年出版 (更新:已出版,请上网搜索“《纳米机器-基础与应用》”购买)。