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Journal Club for March 2022: Liquid Nanofoam: Past, Present and Future



Liquid Nanofoam: Past, Present and Future


Mingzhe Li and Weiyi Lu

Department of Civil and Environmental Engineering, Michigan State University


1. Introduction

Over the past decades, the growing interest in protecting human and infrastructures from collision, impact, and blasts has resulted in extensive studies into the design of sophisticated energy absorption materials and structures. Under an impact, the energy absorption material deforms and alleviates the transmitted energy on the protected objects. Various energy absorption materials and structures, such as cellular materials [1,2], fiber reinforced composite materials [3,4], nano- and micro-lattices [5,6], and granular matters [7,8], have been developed. However, the energy absorption performance of these systems has been limited by several issues, such as damage localization, insufficient response speed at high strain rate, and low system reusability due to permanent deformation. Therefore, it is desired to develop materials with new energy absorption mechanism to address these concerns.

Figure 1. LN system and its energy absorption mechanism [9]. (a) A typical LN sample (b) Typical mechanical response of LN system during the loading-unloading cycle.


The recently developed nanofluidics-based liquid nanofoam (LN) material has shown great potential for high-performance of protection [10–12]. LN is composed of hydrophobic nanoporous particles and a non-wetting liquid phase (Figure 1a). At ambient condition, the liquid stays outside of the nanopores due to the capillary effect. As a sufficiently high external load is applied, the surface energy barrier is overcome and the liquid molecules are driven into the hydrophobic nanopores (Figure 1b). Due to the ultra large surface area of nanoporous particles, the LN system is capable of mitigating tremendous amount of external energy (~100 J/g, 2 orders of magnitude higher than traditional energy absorption systems) [13]. As the external load is removed, the intruded liquid molecules may flow out of the nanopores, leading to a reusable energy absorption system.

Past studies on LN system have been mainly focused on the liquid infiltration behavior under quasi-static loading conditions to elaborate the energy mitigation mechanism. It has been found that the energy absorption density of the LN systems under quasi-static loading condition is expressed as UPin · Vin, where Vin is the specific volume change, and Pin is the liquid infiltration pressure. The deformability of the LN system is determines by Vin, which is related to the liquid accessible pore volume of the nanoporous materials. The working pressure of the LN system is determined by Pin, which is governed by the Laplace-Young equation as Pin = 4γ / D, where γ is the excess solid-liquid interfacial tension and D is the nanopore diameter [14]. Pin can be precisely controlled by adjusting various system factors, such as pore size of the nanoporous particle [15], nanopore surface coverage [16,17], viscosity of the liquid phase [18], cation and anion species in the liquid phase [19–21], external electric field [13,22], temperature [23], etc. More importantly, the adjustment in Pin is independent from Vin, which is distinct from conventional foams. While the quasi-static liquid infiltration behavior of LN systems has been thoroughly investigated, it is necessary to validate the system performance of the LN under dynamic loading conditions, the real case scenarios. Moreover, the liquid outflow behavior after removal of the external load has not been fully understood yet. Here, we would like to highlight our recent research progresses towards the development of reusable LN systems under dynamic loading conditions.


2. System feasibility under dynamic loadings

The strain rates of material under blunt impact (102 s-1) and blast waves (104 s-1) are much higher than that of quasi-static loading condition (10-4 s-1). To validate that the liquid infiltration mechanism can be fully activated at high strain rates, we need to address two fundamental research questions – i) Upon impact, is the liquid infiltration speed faster than the crushing of the nanoporous framework? ii) Is the liquid flow speed in the hydrophobic nanopores fast enough to accommodate the external high-speed impact? 

2.1 Liquid infiltration vs pore crushing. Our collaborative work with Prof. Baoxing Xu at the University of Virginia was aimed to explore the competition between liquid infiltration and nanopore deformation upon dynamic impact [24]. Experimentally, a silica gel (inset in Figure 2b) with relatively weak crushing strength was selected. Quasi-static compression tests on dry silica gel (without liquid) were performed first (Figure 2a) and the hysteresis in Figure 2b indicated permanent nanopore deformation occurred at different peak stresses, σmax. Pressure-induced liquid infiltration tests were then conducted on the LN system containing pre-compressed silica gel. The cyclic testing results in Figure 2c and d further demonstrated that the dry compression generated permanent deformation in the nanoporous framework. Similar to the deformation mechanism of conventional foams, due to the cell wall buckling, the average pore size and the total porosity of the silica decrease, which was validated by the increased Pin and reduced Vin of the liquid infiltration tests. However, with the presence of liquid, no further permanent deformation was introduced into the nanoscale porous framework. These experimental results revealed that liquid infiltration was faster than the cell wall buckling. The infiltrated liquid molecules strengthened the porous framework from inside and suppressed any damage on the porous structure. Comprehensive molecular dynamics (MD) simulations (Figure 2e) were also carried out and validated the experimental results. Figure 2f summarized the quantitative relationship of the competition between liquid infiltration and nanopore deformation.

Figure 2. Examining the nanoporous structural integrity of LN system upon compression [24]. (a) Schematic of the experimental setup, (b) Loading history of the dry pre-compression on empty silica gels under various peak stress σmax, (c) Sorption isotherm curves of the LN system with dry pre-compression at σmax = 4 MPa, (d) Sorption isotherm curves of the LN system with dry pre-compression at σmax = 2 MPa, (e) Snapshots of the MD simulation at different simulation times, (f) Quantitative relationship among the critical pressure for liquid infiltration Pin, axial buckling pressure Paxial and radial collapse pressure Pradial with the variation of surface wettability represented by the contact angle θ and tube length L.


2.2 Ultra-fast liquid flow. To characterize the liquid flow behavior in 3D nanopores, we conducted both quasi-static compression and dynamic impact tests on an LN system composed of spherical nanoporous silica particles and saturated lithium chloride (LiCl) aqueous solution (Figure 3) [25]. Figure 3c showed the mechanical behavior of the LN sample under dynamic impacts was exactly the same as that in quasi-static compression, indicating that the LN liquid infiltration behavior was strain rate independent. We further characterized the liquid flow speed in nanopores using a simplified model (Figure 3d) and found that the nanoscale liquid flow speed was adaptive to external impact speed. As the loading condition changed from quasi-static compression to dynamic impact, the flow speed increased from 5 ´ 105 cm/s to 3.9 cm/s, 5 orders of magnitude increase. The much-enhanced liquid flow speed demonstrated that the effectiveness of LN system under high strain rate impact. More importantly, the 3.9 cm/s flow speed was far from the nanoscale flow speed limit predicted by MD simulation (~400 m/s) [26]. With a linear estimation, the nanoscale liquid flow speed in the LN system is capable of accommodating the wave propagation speed, and thus effectively mitigate blast wave energy.

Figure 3. Evaluating the ultra-fast liquid flow speed in 3D nanopores [25]. (a) Schematic of the experimental setup for dynamic test (b) Typical SEM images of silica particles and the measurement of the cross-sectional area of surface nanopores (c) Typical sorption isotherm curves of the LN sample under dynamic tests (d) Schematic of the effective liquid flow measurement in an LN sample nanopore (e) Measured effective liquid flow speed in a single nanopore.


3. System reusability under cyclic loadings

The reusability of an energy absorber is an important factor in evaluating its energy absorption performance, especially under repetitive impact conditions. For an LN system, the reusability is determined by the spontaneous liquid outflow behavior. During the unloading process, the liquid molecules may be fully or partially expelled from the hydrophobic nanopores. If the liquid molecules are fully expelled from the nanopores, the nanopores are recovered and accessible for the second loading cycle. Therefore, the LN system is 100% reusable for multiple impacts. On the other hand, if the liquid molecules stay in the nanopore, the LN system becomes one-time use. Thus, elucidating the liquid outflow mechanism from nanopores is of great significance for developing fully reusable LN systems.

3.1 Liquid-solid interaction. In fundamental, the spontaneous liquid outflow is favorable due to the hydrophobic nature of nanopore surface. Our recent work investigated this hydrophobic nanoconfinement effect on the liquid outflow behavior in LN system [27]. The nanopore surface was grafted with different silyl chains (C1, C4, and C8 in Figure 4a), leading to a reduced nanopore diameter and increased liquid-solid interaction reflected by the Pin increase. Under single-step (Figure 4b). In addition, under multi-step loading conditions, the extent of liquid outflow was significantly promoted (Figure 4c). However, with the addition of ions into the liquid phase, the liquid outflow efficiency was much-reduced despite of the increased Pin. Full-scale MD simulations were conducted and the results (Figure 4e) were in remarkable agreement with those in experiments. Further analysis revealed that the grafted hydrophobic silyl chains on nanopore surface enhanced the hydrophobic nano-confinement of liquid molecules and facilitated the liquid outflow (Figure 4f). In contrast, the ions in liquid phase led to molecular congestion in the nanopore and inhibited the liquid outflow (Figure 4g). 

Figure 4. Effect of liquid-solid interaction on the liquid outflow behavior in LN system [27]. (a) Experiment setup (b) P-∆V curves in a single-step test (c) Outflow efficiency of water as functions of Pin and the diameter of nanopores (d) Outflow efficiency as functions of the concentration of electrolytes (e) MD simulation results of outflow efficiency (f) Enhanced hydrophobic confinement as function of nanopore diameter (g) Ion-induced molecular congestion during liquid outflow.


3.2 Liquid-gas interaction. In addition to the liquid-solid interaction effect, the gas phase also significantly affects the nanoscale liquid outflow behavior in LN system. We have examined the liquid outflow efficiency of an LN system containing different amounts of gas molecules through cyclic quasi-static compression tests (Figure 5a). During the first loading cycle, the Pin was exactly the same for all LN systems, indicating that the liquid-solid interaction was not affected by the increased gas molecules. In the second loading cycle, remarkable increase in the effective pore volume Vin was observed with the increasing amount of gas molecules. Therefore, the degree of liquid outflow was significantly improved by the only system variable, the gas content (Figure 5b). The effect of the gas molecules on liquid outflow was due to the gas-enhanced bubble nucleation process (Figure 5c). During the liquid infiltration process, all the gas molecules were fully dissolved in the liquid phase, which resulted in higher gas concentration Cb,0 in nanoconfined liquid phase at the peak pressure. Upon unloading, the gas molecules escaped from the nanopores and quickly saturated the bulk liquid phase according to Henry’s law. Thus, higher Cb,0 led to a higher conserved gas concentration in the nanoconfined liquid phase Cn,t. This gas phase imposed strong interaction with the confined liquid. As system pressure decreased, the confined gas solution became supersaturated and tended to separate from the nanoconfined liquid phase to the vapor bubble phase, releasing the free energy of the system. Consequently, these retained gas in the confined liquid promoted the nucleation process and resulted in higher liquid outflow.

Figure 5. Effect of liquid-gas interaction on the liquid outflow behavior in LN system [28]. (a) Experimental setup (b) Liquid outflow degree as a function of liquid-gas ratio (c) Schematic of the extra gas effect on the liquid outflow behavior.


3.3 Competing liquid-solid interaction (LSI) and liquid-gas interaction (LGI). While both the LSI and LGI can facilitate the liquid outflow in LN system, it remains unclear the individual contribution of LSI and LGI and their competition due to the lack of an effective experimental approach to decouple these two effects. To address this challenge, we evaluated the liquid outflow behavior of a series of nanoporous silica with same pore geometry, interconnections, and surface conditions, but varied nanopore sizes (Figure 6a). The mechanical response of a 6.9 nm pore sized LN system with or without gas molecules were characterized by cyclic compression tests (Figure 6b). The constant Pin in the first loading cycle indicated constant LSI in both systems. The difference in the degree of liquid outflow as shown in the second loading cycle was solely due to the LGI. Thus, the individual contribution of LSI and LGI on liquid outflow was successfully decoupled (Figure 6b). The effect of LSI and LGI in LN systems with changing nanopore sizes were differentiated and plotted in Figure 6c, confirming that as pore size decreased, the contribution of LGI was reduced and the LSI became dominant. The reduced LGI effect can be attributed to the gas oversolubility in the nanoconfined liquid phase. The LGI effect was proportional to the product of retained gas concentration in the nanoconfined liquid (Cn,0 - ΔCn,0) and the Henry's law constant kH,T. In smaller pores, the gas solubility Sn increased in a nearly exponential manner, leading to the exponential decrease of kH,T. This fast-decreasing kH,T dominated the LGI effect and resulted in a much-reduced liquid outflow.

Figure 6. Competition between the liquid-solid and liquid-gas interactions. (a) The LN system used (b) Decouple of the liquid-solid interaction and liquid-gas interaction (c) Effect of LSI and LGI on liquid outflow as a function of pore size (d) The relative change of system parameters as the pore size decreases.


4. Applications of LN materials

In this section, we will show several applications of LN materials by utilizing its excellent energy absorption performance.

4.1 LN-filled thin-walled structures. Thin-walled structures have been widely used as energy absorption devices due to their light weight and low cost. However, the mismatch between the buckling initiation stress and the post-buckling stress dramatically reduces its energy absorption capacity. To tackle this problem, we have employed LN as the filling material in thin-walled tubes (Figure 7a), which can create a perfect liquid-solid interfacial “bonding” between the LN and tube wall and potentially improve the energy absorption capacity [29,30]. It has been found that the specific energy absorption capacity of this composite structure was promoted by 55% under dynamic loading conditions, much higher than that of the best metallic foam-filled thin-walled tubes. This reinforcement effect of LN filler was due to the high energy absorption efficiency of LN material and the liquid-solid interaction between the LN and tube wall. The much-enhanced liquid-solid interaction, as characterized by a strengthening coefficient of 3.8 (Figure 7b), suppressed the inward buckling mode and led to the severe plastic deformation of the tube wall (Figure 7c), contributing to the overall energy absorption capacity. For optimized performance, the infiltration pressure of LN filler should match the tube fracture strength and tube material should be ductile to endure the much-enhanced filler-tube interaction. The LN-filled tube was also found to be inert to structural imperfection due to this intimate filler-tube interaction [31].

Figure 7. Application of LN materials. (a) A typical LN-filled thin-walled tube (b) The increased strengthening coefficient of LN-filled tube, representing the enhanced filler-tube wall interaction [30] (c) Severe plastic deformation of the tube wall resulted from the enhanced filler-tube wall interaction [29] (d) Schematic of one-pot free-radical synthesis of LN-hydrogel (e) SEM images of free-fried LN-Hydrogel [32].


4.2 LN-functionalized hydrogel. Through incorporating the hydrophobic nanoporous particles in the water swollen polymeric network (Figure 7d), we have constructed micro-LN systems in the hydrogel material to improve its energy mitigation capacity (Figure 7e). Upon hydrostatic loading condition, the water molecules in the hydrogel were activated and forced into the nanopores. Due to the ultra-large surface area of nanoporous particles, substantial energy was dissipated as heat during this liquid infiltration process. By adding 1 wt% nanoporous particle, the specific energy absorption capacity of the composite hydrogel under confined conditions was improved by 300%. Such a composite hydrogel system is a new paradigm for designing strong and tough hydrogel materials.


5. Summary and outlook

In summary, we have demonstrated that the feasibility for the LN system to mitigate energy under dynamic loading conditions by elaborating the deformation mechanism and the ultra-fast liquid flow of the LN system. We have also elucidated the driving forces of the spontaneous liquid outflow and decoupled the effects of liquid-solid and liquid-gas interactions. The unprecedented energy mitigation performance of the LN system has the potential to revolutionize the design of thin-walled structures and hydrogels.  

Future research of LN material will include but not limited to the continuous investigation of the liquid infiltration and outflow behaviors and smart and multifunctional devices based on nanofluidics. For example, 

(1) Field-responsive liquid infiltration and outflow behavior for sensing and actuating.

(2) Controlled liquid outflow from nanopores for medical applications such as drug delivery.

(3) Adaptive energy mitigation system at various strain rates. 

(4) Nanoscale platform to probe the mechanical properties of confined liquid [33].

We would like to invite everyone to share your perspectives on this topic and look forward to discussing with you in the comments.




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