1. Characterization of Hydrophobic Nanoporous Particle Liquids for
Energy Absorption
Yi Hsu, Yingtao Liu*
School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman OK 73019
ABSTRACT
Recently, the development of hydrophobic nanoporous technologies has drawn increased attention, especially for the
applications of energy absorption and impact protection. Although significant amount of research has been conducted
to synthesis and characterize materials to protect structures from impact damage, the tradition methods focused on
converting kinetic energy to other forms, such as heat and cell buckling. Due to their high energy absorption efficiency,
hydrophobic nanoporous particle liquids (NPLs) are one of the most attractive impact mitigation materials. During
impact, such particles directly trap liquid molecules inside the non-wetting surface of nanopores in the particles. The
captured impact energy is simply stored temporarily and isolated from the original energy transmission path. In this
paper we will investigate the energy absorption efficiency of combinations of silica nanoporous particles and with
multiple liquids. Inorganic particles, such as nanoporous silica, are characterized using scanning electron microscopy.
Small molecule promoters, such as methanol and ethanol, are introduced to the prepared NPLs. Their effects on the
energy absorption efficiency are studied in this paper. NPLs are prepared by dispersing the studied materials in
deionized water. Energy absorption efficiency of these liquids are experimentally characterized using an Instron
mechanical testing frame and in-house develop stainless steel hydraulic cylinder system.
Keywords: Nanoporous liquids, silica, impact protection, energy absorption
1. INTRODUCTION
Head injuries in sports and military due to low velocity impact are occurring at an epidemic level. The Center for
Disease Control and Prevention estimates that as many as 300,000 sports-related “mild” traumatic brain injuries, also
referred to as concussions, occur in the United State each year, which approximately one third of these occurring in
football [1, 2]. In military, the low velocity impact related head injuries can be more severe and often. For example,
the recent wars in Iraq and Afghanistan resulted in about 320,000 blast related traumatic brain injures, most of which
are categorized as uncomplicated concussions [3]. Therefore, protective equipment and gears are essential for
preventing and reducing the severity of injuries to athletes and soldiers. Although a large number of protective
equipment and gears have been widely adopted, their effectiveness may require further improvement in preventing
severe injuries, especially those that cause long-lasting effect on human life, such as brain injuries.
A number of studies related to new concepts and designs of impact protective materials have been conducted in the
past two decades. Traditional approaches use ballistic fabrics, laminated composites, and ceramics for medium and
high velocity impact protection [4]. Ballistic fabrics have received significant attention due to their military
applications. The factors related to impact protection capabilities include: fabric material properties and structures,
impact energy, projectile geometry, frictions between fibers and yarns, interaction of multiple plies [5]. Roylance et
al. [6] have shown that the majority of the impact energy is transferred to the principal yarns which are in direct contact
with the projectile. Impact energy transfers from kinetic energy to strain energy in this process. Similar to ballistic
fabrics, laminated composites have been used as energy absorber for impact protection [7]. Four different failure
modes including matrix cracks, delamination, fiber breakage, and penetration, may exit when composite laminates are
subject to different impact energy and velocities. Due to the complex nature of failure of composites, these materials
are sensitive to strain rates. The use of ceramics in conjunction with other materials is also an accepted method for
producing light-weight armors for impact protection. Ceramics are considered to be suitable as stiff, hard, and strong
in compression. Of their various properties, the high hardness of ceramics is particularly relevant to their applications
as armor again projectiles with velocity of up to 1200 m/s [8]. Protective helmet and padding for sports equipment
and gears usually use soft foams as the shock-absorbing materials [9]. A hard outer shell may be used, such as the
helmets for American football and ice hockey.

2. Energy-absorbing mechanisms such as plastic deformation and material fracture have been studied for low velocity
impact protection using cellular structures, such as honeycomb and foam materials. One of the typical types is the
hexagonal honeycomb with regular hexagonal walls. Caccese et al. [10] reported an optimal design using honeycomb
materials to mitigate head impacts. Detailed honeycomb geometries, including honeycomb depth, cell wall thickness,
cell size, were simulated using finite element method and optimized using a genetic algorithm. Other materials in
honeycomb structures, such as aluminum [11], polystyrene [12], polypropylene [13], and polyurethane [14] have been
studied for impact energy absorption and protection. Nature materials have been studied for impact protection recently.
Fernandes et al. [15] reported the impact response of agglomerated cork. This natural materials presented an
outstanding performance in material relaxation after impacts, which showed the potential for multiple impact
protections. Garcia-Gonzalez [16] reported the mechanical impact behavior of polyether-ether-ketone (PEEK). Due
to their biocompatibility [17], good energy absorption properties can significantly widen the applications of such
materials from protective equipment and gears to biomedical and implant applications. However, all the energy-
absorbing materials need to depend on cell buckling or the kinetic-heat transformation. The energy-absorbing
capability of such materials is significantly reduced after multiple impacts.
A newly discovered nanoporous particle liquids (NPLs) has shown promise in mitigating impact energy by instantly
trapping liquid molecules inside the non-wetting surface of pores of nanoparticles. Unlike the conventional energy
dissipation process, the captured wave energy is not necessarily converted to other forms of energy, as it is in heat or
cell buckling; but simply stored temporarily and isolated from the original energy transmission path. Thus, the pressure
of the transmitted wave that would reach the target can be lowered significantly. Nanoporous particles, such as silica
[18, 19] and multi-walled carbon nanotubes [19], have been reported as the energy-absorbing materials in NPLs. Kong
et al. [20] first reported the energy absorption behaviors of nanoporous silica particles immersed in aqueous solutions.
Ethanol was used as promoter to adjust the infiltration pressure. Therefore, the energy-absorbing capabilities of the
NPLs was optimized. The energy-absorbing properties under dynamic loading were reported by Surani et al. [21].
The nonwetting nanoporous particles were dispersed in aqueous solutions and tested using a Hopkinson bar. Compared
with the quasi-static loading cases, the NPL showed much higher efficiency to absorb impact energy.
In this paper we report the energy absorption capability of different NPLs prepared using end-capped silica nanoporous
particles and ethanol / methanol promoters. All the prepared NPLs are tested experimentally using a customized
stainless steel cylinder under quasi-static load conditions. The load-unload curves of NPLs present critical information,
such as infiltration pressure. The energy absorbed by the NPLs are calculated using the load-unload curves. The energy
absorption capabilities of different NPLs can be compared to optimize the formula of NPLs for impact protection.
The remainder of this article is organized as follows. In section 2, the materials and experimental setup are introduced
in details. In section 3, the material characterization results and the effects of promoters on infiltration pressure and
energy absorption capabilities are discussed. The conclusions of this article is presented in section 4.
2. MATERIALS AND EXPERIMENTS
2.1 Materials
Unless otherwise stated, all the following listed materials and reagents were used as received. Silica gel 100 C(8)-
reversed phase, ethyl alcohol (≥ 99.5), anhydrous methanol (99.8%), deionized water were from Sigma Aldrich.
According to the information provided by the supplier, the silica gel employed in this paper has an average pore size
of 9 nm and the particle size of 40-63 um.
2.2 Preparation of silica based NPL
In this paper all the NPLs tested in each experiment are 10 ml with 0.3 gram of silica nanoporous particles. First, all
the silica particles are kept in a vacuum oven under vacuum at 100 ºC for 24 hours to remove the trace of moisture in
the materials. Second, the measured silica particles is added to the measured deionized water and stirred manually for
5 minutes. Then the promoter, such as ethanol or methanol, is measured and added to the solution. After mixing
manually for 5 more minutes, all the solution is transferred to the stainless steel cylinder. All the air is removed before
sealing the cylinder. The inner diameter of the cylinder is 0.5 inch and the outer diameter of the cylinder is 1.5 inch.
NPLs with different concentration of methanol and ethanol promoters are tested. First, pure deionized water with only
silica particles is prepared and tested as the references. Then NPLs with ethanol concentration of 10%, 20%, and 30%
are prepared and tested. Finally, NPLs with methanol concentration of 10%, 20%, and 30% are prepared and tested.

3. 2.3 Compression test of prepared NPL
The prepared NPL and stainless steel cylinder is tested using an Instron mechanical testing system under quasi-static
compression load with a load rate of 1 mm/min, as shown in Fig. 1. Experimental data including force, displacement,
and time are recorded during the experiment. The pressure inside the cylinder is calculated using the equation:
𝑃 =
𝐹
𝐴
(1)
where P is the pressure inside the cylinder, F is the forced recorded by the load cell, and A is the size of piston in the
cylinder. A full load-unload cycle is conducted in each experiment. The pressure variation and volume change are
calculated accordingly.
Figure 1. Experimental Setup and stainless steel cylinder used during testing.
3. RESULTS AND DISCUSSION
The relationship of pressure variation and volume change per gram of silica particles is shown in Fig. 2. For the
NPL with only deionized water and silica particles, the system responds almost linearly when the pressure applied
is low. Once the pressure passes the infiltration pressure of 15 MPa, the slop of the pressure curve is bend,
resulting in larger volume change. The linear system response is due to the bulk modulus of the solution. Limited
amount of water molecules are pushed into the large pores in silica particles. The compression of air in silica
pores is the main reason of volume change. Once the pressure passes the infiltration pressure, substantial amount
of water molecules are pushed into the small pores in silica particles. These water molecules are temporarily
trapped in the pores. Therefore, the slop of pressure starts reducing, as shown in Fig. 2. During the unloading
process, the system responses almost linearly again. Since the water has been trapped in small pores, only water
molecules in large pores can be released to the solution. Therefore, we observe volume reduction of the NPLs
after experiments.
Promoters, such as ethanol and methanol, can significantly reduce the infiltration pressure of the NPLs. By adding
different amount of ethanol, it is noted that the infiltration pressure of NPLs can reduce from around 20 MPa to
less than 1 MPa. In addition, the plateau of the pressure curves move slightly to the larger volume change range,
indicating more molecules are pushed into the pores in silica particle during the loading process. All the unloading
process still shows similar linear change, as shown in Fig. 2. It proves the release of molecules from large pores
in silica only. The trapped water and ethanol molecules need external energy stimulation before naturally released
to the solution again.

4. Methanol is the second type of promoter studied in this paper. As shown in Fig. 3, NPLs with methanol promoters
show similar trend. When the concentration of methanol increases, the infiltration pressure reduces accordingly.
In addition, more volume change is observed. Since the molecule size of methanol is smaller than that of ethanol,
it is relatively easy to push more methanol molecules to the pores in silica particles. The NPLs with methanol
systems also respond linear during the unloading process, which indicate the release of only water and methanol
molecules from large pores in silica particles.
Figure 2. Load-unload cure of the NPL with ethanol as promoter.
Figure 3. Load-unload cure of the NPL with methanol as promoter.
Comparing the system performance using ethanol and methanol promoters, it is noted that the infiltration pressure of
NPLs with different promoters changes differently. As shown in Fig. 4, the infiltration pressures reduce significantly
when ethanol is used as promoter. As the concentration increased to 30%, the NPL with ethanol has an infiltration
pressure of less than 1 MPa. However, the NPLs with methanol do not show such a significant reduce of infiltration
pressure. The significant reduce of infiltration pressure for NPLs with ethanol is considered to be related to the surface
energy of ethanol and silica pores. More detailed analyses are needed to demonstrate this assumption.
Figure 4. The infiltration pressure of different NPLs with ethanol or methanol promoters

5. The amount of energy absorbed by NPLs is one of the most critical parameter characterized in this email. As shown in
Figs. 5 and 6, it is noted that both NPLs with ethanol and methanol promoters have the maximum energy absorption with
10% centration. However, due to the significant reduce of infiltration for ethanol based NPLs, the energy absorption
capabilities is reduced quickly as the concentration increases. The NPLs with methanol as promoters do not show such a
significant reduce. Based on the application, the optimal concentration of promoter can be selected based on this study.
Figure 5. Energy absorption capability of NPL with ethanol promoters
Figure 6. Energy absorption capability of NPL with methanol promoters
4. CONCLUSIONS
This paper reported an experimental characterization approach to understand the performance of NPLs using silica
nanoporous particles and ethanol / methanol promoters. Both NPLs with ethanol and methanol promoters show good
energy absorption capabilities under a quasi-static load conditions. At the low pressure range, the pressure increases
almost linearly until it reaches the infiltration pressure. Once the pressure passes infiltration pressure, a substantial
amount of water and promoter molecules are pushed into the pores of silica nanoporous particles, resulting in the non-
linear performance of the system. The system shows linear responses again during the unloading process due to the
molecule release only from large pores in the particles. Although both ethanol and methanol can significantly impact
on the infiltration pressure, the methanol promoter does not effect the infiltration pressure of NPLs as significant as
ethanol.
REFERENCES
[1] Thunnan, D.J., C.M. Branche, and J.E. Sniezek, The epidemiology of sports-related traumatic brain injuries
in the United States: recent developments. The Journal of head trauma rehabilitation, 1998. 13(2): p. 1-8.
[2] Powell, J.W. and K.D. Barber-Foss, Traumatic brain injury in high school athletes. Jama, 1999. 282(10): p.
958-963.
[3] Tanielian, T., L.H. Haycox, T.L. Schell, G.N. Marshall, M.A. Burnam, C. Eibner, B.R. Karney, L.S.
Meredith, J.S. Ringel, and M.E. Vaiana, Invisible Wounds of War. Summary and Recommendations for
Addressing Psychological and Cognitive Injuries. 2008, DTIC Document.

6. [4] Tabiei, A. and G. Nilakantan, Ballistic impact of dry woven fabric composites: a review. Applied Mechanics
Reviews, 2008. 61(1): p. 010801.
[5] Cheeseman, B.A. and T.A. Bogetti, Ballistic impact into fabric and compliant composite laminates.
Composite structures, 2003. 61(1): p. 161-173.
[6] Roylance, D., Stress wave propagation in fibres: effect of crossovers. Fibre Science and Technology, 1980.
13(5): p. 385-395.
[7] Belingardi, G. and R. Vadori, Low velocity impact tests of laminate glass-fiber-epoxy matrix composite
material plates. International Journal of Impact Engineering, 2002. 27(2): p. 213-229.
[8] Sadanandan, S. and J. Hetherington, Characterisation of ceramic/steel and ceramic/aluminium armours
subjected to oblique impact. International journal of impact engineering, 1997. 19(9): p. 811-819.
[9] Wilson, B.D., Protective headgear in rugby union. Sports Medicine, 1998. 25(5): p. 333-337.
[10] Caccese, V., J.R. Ferguson, and M.A. Edgecomb, Optimal design of honeycomb material used to mitigate
head impact. Composite structures, 2013. 100: p. 404-412.
[11] Xie, S. and H. Zhou, Analysis and optimisation of parameters influencing the out-of-plane energy absorption
of an aluminium honeycomb. Thin-Walled Structures, 2015. 89: p. 169-177.
[12] Di Landro, L., G. Sala, and D. Olivieri, Deformation mechanisms and energy absorption of polystyrene foams
for protective helmets. Polymer testing, 2002. 21(2): p. 217-228.
[13] Avalle, M., G. Belingardi, and R. Montanini, Characterization of polymeric structural foams under
compressive impact loading by means of energy-absorption diagram. International Journal of Impact
Engineering, 2001. 25(5): p. 455-472.
[14] Seidi, M., M. Hajiaghamemar, J. Ferguson, and V. Caccese, Injury Mitigation Performance of a Head
Protection Wear with Polyurethane Honeycomb. 2015, SAE Technical Paper.
[15] Fernandes, F., R. Pascoal, and R.A. de Sousa, Modelling impact response of agglomerated cork. Materials
& Design, 2014. 58: p. 499-507.
[16] Garcia-Gonzalez, D., A. Rusinek, T. Jankowiak, and A. Arias, Mechanical impact behavior of polyether–
ether–ketone (PEEK). Composite Structures, 2015. 124: p. 88-99.
[17] Horak, Z., D. Pokorný, P. Fulin, M. Slouf, D. Jahoda, and A. Sosna, [Polyetheretherketone (PEEK). Part I:
prospects for use in orthopaedics and traumatology]. Acta chirurgiae orthopaedicae et traumatologiae
Cechoslovaca, 2009. 77(6): p. 463-469.
[18] Han, A. and Y. Qiao, Pressure-induced infiltration of aqueous solutions of multiple promoters in a
nanoporous silica. Journal of the American Chemical Society, 2006. 128(32): p. 10348-10349.
[19] Han, A. and Y. Qiao, Controlling infiltration pressure of a nanoporous silica gel via surface treatment.
Chemistry Letters, 2007. 36(7): p. 882-883.
[20] Kong, X., F.B. Surani, and Y. Qiao, Effects of addition of ethanol on the infiltration pressure of a mesoporous
silica. Journal of materials research, 2005. 20(04): p. 1042-1045.
[21] Surani, F.B., X. Kong, D.B. Panchal, and Y. Qiao, Energy absorption of a nanoporous system subjected to
dynamic loadings. Applied Physics Letters, 2005. 87(16): p. 163111.
[22] http://www.sigmaaldrich.com/catalog/product/sial/60759?lang=en&region=US