Khá-Î Tô

Dr. Khá-Î Tô

Postdoctoral Scholar

Email: kha-i.to@oist.jp

I am a postdoc researching the dynamics of particle-laden interfaces, 2D triboelectric system,  mechanical/acoustic properties of exotic material and locomotion of burrowing fish in sand. I completed my master degree in high energy physics at National Taiwan University, Taiwan, in 2014 and my Ph.D. in 2022 working with Prof. Sidney Nagel at the University of Chicago. During my doctoral work, I studied the failure and cluster formation of particle rafts under expansional flow at different geometries. I am a passionate bunny lover and enjoy learning Sanshin and exploring Okinawa in my free time.

Patterns and structures are ubiquitous in our daily lives. While many share common characteristics, each one is unique and subtly different from the others.  My research focuses on the ways patterns form, how perturbations create variations in the patterns that we observe, and how microscopic structures can affect macroscopic properties.  Many of these topics are highly accessible and can be observed and analyzed on the scale of table-top experiments.

Cluster Formation in Particle Rafts

(A) Zoomed-in images show structure in the bulk of the rafts for uniaxial expansion with different velocity. (C) Snapshots of rafts under radial expansion with different velocity.
(A) Zoomed-in images show structure in the bulk of the rafts for uniaxial expansion with different velocities. (B) Snapshots of rafts under radial expansion with different velocities.
(A) Zoomed-in images show structure in the bulk of the rafts for uniaxial expansion with different velocities. (B) Snapshots of rafts under radial expansion with different velocities.

Particle rafts, also known as particle-laden interfaces, are self-assemblies of floating particles at fluid interfaces. They are a common occurrence in natural and industrial settings across a wide range of length scales, from pumices (軽石) in the Okinawan ocean, liquid marbles made of micro-/nano-scaled particles, to self-assembled nanoparticle membrane used for water filtration.

In my PhD study, I focused on the morphology emerged from particle rafts on an expanding liquid substrate. When extensional flow of different geometries, uniaxial and radial, is introduced on the liquid surface, the particle rafts exhibit interesting failure patterns that are not commonly seen in the context of material failure.

The rafts fail by creating cracks that are uniformly distributed throughout the system, and the failure morphology depends on the expansion velocity, as shown in the figure. I understood this phenomenon by considering two competing velocities: the relaxation speed of the cluster and the velocity difference between particles provided by the liquid expansion. I developed a one-dimensional chain model with an attractive lateral capillary force in an overdamped environment. This model captures this velocity and fluid property dependence of the patterns successfully.I was also able to build experimental setups to perform extensional motions on the liquid surface. In particular, I created a novel radial expansion apparatus to perform isotropic expansion in two dimensions.

This system is a model system for both material failure and cluster formation across multiple length scales. Because the particle sizes are easily observable, it can further provide insights into other systems where the morphology is determined by similar competing mechanisms but where the length scales are not readily accessible. These include the structure formation due to cosmological expansion and to the microscopic structure in porous media, which is therefore of interest to theoretical and experimental scientists in fields ranging from physics and materials science to astrophysics.

Publications

  • Tô, Khá-Î. "Cluster formation and relaxation in particle rafts under uniform radial expansion." Physical Review E 108.2 (2023): 024801.

  • Tô, Khá-Î., and Sidney R. Nagel. "Rifts in rafts." Soft Matter 19, no. 5 (2023): 905-912.

  • Tsai, J-C., C-Y. Tao, Y-C. Sun, C-Y. Lai, K-H. Huang, W-T. Juan, and J-R. Huang. "Vortex-induced morphology on a two-fluid interface and the transitions." Physical Review E 92, no. 3 (2015): 031002.

Fellowships and Awards

  • Oral Presentation Award, Taiwan Physical Society Annual Meeting, Taiwan (2023)

  • Government Scholarship to Study Abroad, Ministry of Education, Taiwan (2020-2022)

  • NSF MRSEC Graduate Fellowship, University of Chicago (2019)