Gan Wang – Soft Matter Lab

Nanoalignment by Critical Casimir Torques on ArXiv

Artist rendition of a disk-shaped microparticle trapped above a circular uncoated pattern within a thin gold layer coated on a glass surface. *(Image by the Authors of the manuscript.)*

Nanoalignment by Critical Casimir Torques
Gan Wang, Piotr Nowakowski, Nima Farahmand Bafi, Benjamin Midtvedt, Falko Schmidt, Ruggero Verre, Mikael Käll, S. Dietrich, Svyatoslav Kondrat, Giovanni Volpe
arXiv: 2401.06260

The manipulation of microscopic objects requires precise and controllable forces and torques. Recent advances have led to the use of critical Casimir forces as a powerful tool, which can be finely tuned through the temperature of the environment and the chemical properties of the involved objects. For example, these forces have been used to self-organize ensembles of particles and to counteract stiction caused by Casimir-Liftshitz forces. However, until now, the potential of critical Casimir torques has been largely unexplored. Here, we demonstrate that critical Casimir torques can efficiently control the alignment of microscopic objects on nanopatterned substrates. We show experimentally and corroborate with theoretical calculations and Monte Carlo simulations that circular patterns on a substrate can stabilize the position and orientation of microscopic disks. By making the patterns elliptical, such microdisks can be subject to a torque which flips them upright while simultaneously allowing for more accurate control of the microdisk position. More complex patterns can selectively trap 2D-chiral particles and generate particle motion similar to non-equilibrium Brownian ratchets. These findings provide new opportunities for nanotechnological applications requiring precise positioning and orientation of microscopic objects.

Poster by G. Wang at DINAMO 2023, Svolvær, 13 June 2023

Light-driven micromachines. *(Image by G. Wang.)*

Nanophotonic encoding of light-driven micromachines
Gan Wang, Marcel Rey, Mahdi Shanei, Kunli Xiong, Einstom Engay, Mikael Käll, and Giovanni Volpe
Date: 13 June 2023
Time: 21:00 (CEST）

On-chip micromotors hold significant application potential in various fields, including cells, microfluidic manipulation, and the micro integration of machines. .The driving mechanism plays a crucial role in the design of micromotors. Currently, various methods such as static electricity, light, magnetism, chemical energy, and mechanical conduction are utilized for this purpose. Optics, in particular, offers distinct advantages including precise control, addressability, non-contact operation, and compatibility with diverse liquid environments. However, existing optically actuated on-chip motors necessitate high energy input, resulting in phototoxicity concerns and hindrances to large-scale manipulation. Furthermore, achieving precise control over speed and direction remains challenging, along with difficulties in establishing coupling among multiple devices.

Presentation by G. Wang at ISMC 2022, Poznan, 20 September 2022

Recognize and selectively trap chiral particles by critical Casimir force. *(Image by G. Wang.)*

Nanopositioning and nanoalignment of microparticles on patterned surfaces
Gan Wang, Piotr Nowakowski, Nima Farahmand, Benjamin Midtvedt, Falko Schmidt, Mikael Käll, Svyatoslav Kondrat, Sigfried Dietrich and Giovanni Volpe
Date: 20 September 2022
Time: 14:10 (CEST）

Direct manipulation of objects in a solution can provide opportunities to investigate material properties and construct microscopic devices. However, currently available methods, such as optical tweezers and thermal tweezers, have several limitations especially to control the orientation and alignment of particles near surfaces. Here, we experimentally demonstrate that by exploiting the critical Casimir effect, emerging in the presence of a critical binary liquid, microparticles (diameter d≈2µm) can be trapped with nanometer precision. We investigated the motion of SiO2 microscopic disks above nanopatterned surfaces coated with a thin gold film immersed inside a critical mixture. By adjusting the adsorption preference of the gold film to one of the two components of the mixture liquid, we can finely tune the balance between the critical Casimir repulsion and attraction generated between different regions of the substrate and the disk. In this way, we can control the configuration of the disk and make it perform some complex motion. Furthermore, we show how this approach can be used to align particles with patterns, e.g., to sort asymmetric particles with respect to their chirality. We foresee this method can be extended to control the movement of small objects of various materials, thereby severing as a platform to study microscale physical and chemical phenomena.