arXiv subjects

Hiroki Sakuta

Publications and source records attributed to Hiroki Sakuta.

3 recordsLinked to original sources

arXiv2026

Tracer-free Contactless Acoustic Microrheometry Quantifies Viscoelastic Spectrum of Phase-separated Condensates

The rheology of phase-separated condensates plays a central role in applications spanning advanced materials design and cellular processes, yet quantitative characterization of their viscoelasticity remains challenging due to the limitations of existing microrheological methods that require tracer particles or mechanical contact. Here, we establish tracer-free and contactless acoustic microrheometry as a versatile platform for quantifying the frequency-dependent complex shear modulus of single microscale condensates over 0.01-10 Hz. Using spatiotemporally controlled acoustic radiation force generated within a micro-acoustic resonator, this method deforms condensates for creep-recovery and oscillatory viscoelastic measurements. Quantitative validation using dextran condensates in a polyethylene-glycol continuous phase successfully captures their size- and frequency-dependent mechanical responses, while application to nucleic-acid condensates reveals salt-dependent internal viscoelastic changes at single-condensate resolution. By enabling quantitative dissection of condensate mechanics without invasive probes, acoustic microrheometry provides a broadly applicable framework for investigating phase-separated condensates across materials science, soft matter physics, biology, and beyond.

cond-mat.soft↗

arXiv2025

Cation accumulation drives the preferential partitioning of DNA in an aqueous two-phase system

Mixtures of polyethylene glycol (PEG) and dextran (Dex) represent a widely used class of aqueous two-phase systems (ATPS), with applications ranging from the purification of various biomolecules such as nucleic acids to the synthesis of protocells. A key feature underlying these applications is the selective accumulation of biomolecules within Dex-rich droplets in an aqueous PEG phase, but the physical origin of this partitioning remains unclear. Entropic interactions were long assumed to be the primary driving force; however, our systematic experiments using DNA of different lengths indicate that entropy alone cannot fully explain the observed behavior. We identify an additional and previously underappreciated contribution from electrostatic interactions: Dex carries a slightly more negative charge than PEG, which drives preferential cation accumulation in the Dex-rich phase. These counterions facilitate the selective partitioning of DNA inside the Dex-rich droplets. This mechanism explains the dependency of DNA uptake in Dex-rich droplets on polymer length and salt concentration. Our findings establish that Donnan-type ion partitioning plays a crucial role in the localization of long nucleic acids in Dex-rich droplets, offering a unified explanation for this long-standing phenomenon. They lay the foundation for designing ATPS-based systems and help elucidate the physicochemical principles of biomolecular partition upon phase separation in cells.

cond-mat.soft↗

arXiv2024

Marangoni droplets of dextran in PEG solution and its motile change due to coil-globule transition of coexisting DNA

Motile droplets using Marangoni convection are attracting attention for their potential as cell-mimicking small robots. However, the motion of droplets relative to the internal and external environments that generate Marangoni convection has not been quantitatively described. This study used an aqueous two-phase system (polyethylene glycol (PEG) and dextran) in an elongated chamber to generate motile dextran droplets in a constant PEG concentration gradient. We demonstrated that dextran droplets move by Marangoni convection, resulting from the PEG concentration gradient and the active transport of PEG and dextran into and out of the droplet. Furthermore, by spontaneously incorporating long DNA into the dextran droplets, we achieved cell-like motility changes controlled by coexisting environment-sensing molecules. The DNA changes its position within the droplet and motile speed in response to external conditions. In the presence of Mg2+, the coil-globule transition of DNA inside the droplet accelerates the motile speed due to the decrease in the droplet's dynamic viscosity. Globule DNA condenses at the rear part of the droplet along the convection, while coil DNA moves away from the droplet's central axis, separating the dipole convections. These results provide a blueprint for designing autonomous small robots using phase-separated droplets, which change the mobility and molecular distribution within the droplet in reaction to the environment. It will also open unexplored areas of self-assembly mechanisms through phase separation under convections, such as intracellular phase separation.

cond-mat.soft↗