This study used halloysite, an aluminosilicate clay mineral, with the empirical formula Al2Si2O5(OH)4·nH2O. Supplementary Figure 1a shows the particle size distribution of halloysite observed using a laser scattering particle size analyzer (HELOS/KR, Sympatec GmbH, Germany). Over 50% of the halloysite had a sediment size smaller than 6.16 μm. The tubular morphologies of dried HNT particles were confirmed using SEM (SU5000, Hitachi) and TEM (JEM-3010, JEOL) (Supplementary Fig. 1b and 1c). Element mapping analysis was performed to confirm the composition of dried HNT by using JEM-ARM200CF (JEOL) equipped with energy dispersive X-ray (EDX) detectors. The Al, Si, and O were uniformly distributed in tubular HNT particles (Supplementary Fig 1d). A research-graded HNT (Merck, CAS: 1332-58-7) without further purification was used.
Preparation of clay suspensions
The HNT fabrics of suspension with high water content (mw/ms, [1,000 %]) were assessed from the settlement behavior, measured with time from the initial suspension stage until the HNT sediment reached a constant volume. A graduated cylinder (60 mm in diameter and maximum volume of 1,000 mL) was filled with 100 ml of the test fluid. Sodium chloride (NaCl) was used to prepare fluids with different salinity conditions (from 0 to 0.6 M) by dissolving 99.5% pure NaCl (CAS: 7647-14-5; Junsei Chemical Co., LTD., Japan) in deionized water. HNTs (90 g; 35 mL in volume) were poured into the cylinder with 100 mL of test fluid and mixed to form a condensed suspension. Then, additional test fluid was poured into the cylinder to reach a final volume of 935 mL. The HNT suspension was slowly mixed by shaking upside down until a uniform suspension was obtained. The prepared suspensions were rested for 24 h for full hydration. A thermoplastic film (Parafilm M, USA) capped the top of the cylinders for sealing and wrapping. Then, fully hydrated HNT suspensions were vigorously disturbed for at least 1 min by shaking and inverting each sealed cylinder upside down. The cylinders were placed on a level surface after the last inversion, and this was regarded the commencement of sedimentation (time = 0). The sediment height was tracked over time until the final sediment height reached a constant volume with a settlement rate of less than 1.0 mL/day. A single sedimentation period could last up to seven days.
After the sediment reached a constant height, the supernatant water was removed, and the cylinder was refilled to 935 mL with a fluid having a different salinity, using a pipette (WI.5.381.000; Witeg, Germany) to represent salination or desalination conditions. After each procedure, the pH and electrical conductivity of the supernatant were measured using a pH/conductivity meter (S470-USP-K; Mettler Toledo, USA).
In-situ liquid microscopy
HNT powders were fractionated to investigate the microscopic interactions using in-situ electron microscopy, with the gravitational methods developed by M. L. Jackson 45, which help filter agglomerated particles >2 µm. Then, to obtain the monodispersed suspension, filtered HNTs were dispersed in water (10 mM) using a tip-sonicator, VCS-750 (Sonics & Materials, Inc.). To differentiate the individual particles in confinement, we used a low concentration in suspension, which provides the flocculation behavior of the initial stage.
To investigate the microscopic insights into the associations of HNTs, we introduced in-situ liquid cell microscopy systems, which enabled us to record the time-series events of the flocculation formation and particle association change in response to a change in the solution environment and salinity. JEM-3010 (JEOL) equipped with the USC1000 (Gatan) camera and a Poseidon holder (Protochips) was used to observe the real-time LPTEM measurements at an acceleration voltage of 300-kV. The liquid chamber equipped in Poseidon holder consisted of SiNx membrane-based liquid cell with ~ 50 nm height and 2 × 2 mm2 space. HNTs were infused into the liquid holder system to resolve the flocculation process from a particulate point of view using the Pump 11 Elite Syringe Pumps (Harvard Apparatus), with an infusion rate of 0.5 µL/min. Sigma 300 (Carl ZEISS).
A homemade liquid-cell SEM holder was used to visualize the flocculation behavior at different salinity levels. The chamber dimension of LPSEM is ~1 μm in height and 2 × 2 mm2 space. We gathered a series of snapshots, which corresponded to the different salinity: 0.01 M, 0.1 M, 0.3 M, and 0.6 M.
For monitoring of HNT aggregates under the salinity exchange condition, we used optical microscopy (LPOM; Olympus). It was observed after suspension dropped onto the slide glass at each salinity after salinity exchange from salination (from 0.01 M to 0.6 M) to desalination (from 0.6 M to 0.01 M).
All necessary data generated or analyzed during this study are included in this published article, and other auxiliary data are available from the corresponding authors upon request.