Separation of substances is central to many industrial and medical processes ranging from wastewater treatment and purification to medical diagnostics. Conventional solid-based membranes allow particles below a critical size to pass through a membrane pore while inhibiting the passage of particles larger than that critical size; membranes that are capable of showing reversed behavior, that is, the passage of large particles and inhibition of small ones, are unusual in conventional engineering applications. Inspired by endocytosis and the self-healing properties of liquids, we show that free-standing membranes composed entirely of liquid can be designed to retain particles smaller than a critical size given the particle inertial properties. We further demonstrate that these membranes can be used for previously unachievable applications, including serving as particle barriers that allow macroscopic device access through the membrane (for example, open surgery) or as selective membranes inhibiting gas/vapor passage while allowing solids to pass through them (for example, waste/odor management).

Here, we show that free-standing membranes composed purely of liquids can achieve particle separation regimes that cannot be attained by conventional membrane technologies ( Fig. 1 , B and C). Specifically, by tuning the membrane surface tension and geometric parameters, we demonstrate that one can design a membrane that can retain particles smaller than a critical size based on the particle inertial properties. Further, we have demonstrated that the unique properties of liquid membranes allow applications that were previously unachievable by conventional membrane technologies, including entrapment of microscopic entities while allowing the passage of macroscopic objects, and selective gas-solid separation where the membrane allows solids to pass through while inhibiting gas passage.

( A ) Schematics showing the materials architectural differences between solid membranes, liquid-infused porous materials [for example, slippery liquid-infused porous surfaces (SLIPS) ( 5 )], and the liquid membrane presented in the current work. ( B ) Conventional solid membranes use porous geometries to allow small particles to pass through while mechanically inhibiting the passage of large particles. ( C ) Liquid membranes rely on entirely different mechanisms for particle separation and allow reversed separation behavior: Small particles can be retained, while large ones pass through the membrane.

As displayed in nature, membranes that allow large particles to pass while retaining small ones must be dynamically reconfigurable and self-healing—properties commonly exhibited by liquids. While liquids have many unique materials properties, membrane engineering efforts have predominantly focused on solid-based materials. In recent years, the concept of incorporating liquids into solid-based materials has led to breakthrough surface technologies ( 5 – 9 ). For example, the incorporation of stable liquid layers into porous solids allows self-healing, robust liquid repellency ( 5 ), anti-biofouling ( 6 ), anti-icing ( 7 ), and even gating ( 9 ) properties. Despite the unique materials properties of liquids (for example, rapid self-healing), the concept of using membranes composed entirely of liquids as functional materials has remained unexplored ( Fig. 1A ).

For centuries, particle separation has been a process of great significance. Today, its importance spans across several fields ranging from medical diagnostics ( 1 ) to wastewater treatment and water desalination ( 2 ). Some of the simplest separation techniques are sieving and filtration—processes that rely on membranes that allow certain particles to pass through them while preventing the passage of others. Conventional membranes are porous and allow particles smaller than a typical pore size to pass through the pores while retaining those larger than the pore size. Membranes that allow relatively large particles to pass while retaining smaller ones, however, are counterintuitive and uncommon. While unusual in human practice, membranes with these capabilities are readily found in nature. Cells, for example, are encased by a phospholipid bilayer composed of amphiphilic molecules that can dynamically reconfigure themselves ( 3 , 4 ). This property, in conjunction with other biological mechanisms, makes possible the engulfment of large particles without fluid exchange as exemplified by endocytosis.

RESULTS

Design principles Liquid membranes consist of a stabilized liquid material that can be as simple as a two-component system [for example, water and surfactant—a soap film (10–12)] or a complex multicomponent system designed for a specific application. The simplest stabilized liquid membrane can be prepared by mixing deionized water with varying concentrations of surfactant [for example, sodium dodecyl sulfate (SDS)]. Subsequently, a solid ring can be used to support a free-standing liquid membrane. By varying the concentration of SDS, we can tune the film surface tension from ~35 to ~72 mN/m. When two spheres with identical materials properties but of different sizes are released from a fixed height H onto a liquid membrane with a surface tension γ, one will notice that, at certain values of H, the larger sphere will pass through the membrane and the smaller sphere will be retained within the membrane (Fig. 2A and movie S1) (11, 12). On the basis of this experimental observation, we can categorize the particle-membrane interactions in two different regimes: (i) the particle retention regime and (ii) the particle “pass-through” regime. Fig. 2 Particle separation demonstration and membrane design. (A) This image is an overlay of four time-lapse images extracted from a video capturing two PTFE beads (one small and one large) falling into a liquid membrane at the same time from the same drop height. Tweezers used to hold the beads here were only shown for the image before the beads were released for clarity. (B) Data from 718 independent bead drop experiments used to determine the criteria for retention versus pass-through (each marker represents an individual bead drop event, and the error bars indicate the possible errors resulting from the measurements of physical parameters in Eq. 3). Scale bar, 1 cm. To experimentally determine the mechanisms that dictate whether a particle will pass through or remain in the film, we systematically dropped beads into a liquid membrane of a given surface tension from different heights and recorded whether the bead was retained in or passed through the membrane. Specifically, we dropped relatively smooth beads (root mean square roughness, ξ < 2.5 μm) into a stabilized liquid membrane of a given surface tension and radius from heights H ranging from 0.5 to 15 cm (tables S1 to S3). Typical particle impact velocities u b were <2 m/s. We repeated this experiment using liquid membranes with different surface tensions (35 mN/m < γ < 72 mN/m). To test for the effect of particle geometry on retention/pass-through, we conducted the same set of experiments for beads and membranes of different radii (355 μm< R b <4.4 mm and 3 mm< R f <6 cm, respectively). Further testing was carried out using beads composed of different materials [that is, glass, polystyrene (PS), and polytetrafluoroethylene (PTFE)] to investigate the effect of surface chemistry on the particle-membrane interaction. To gain physical insights into the particle separation mechanism, we compared the magnitudes of the kinetic energy ( ) of the beads at impact to the other forms of energy, such as the maximum increase in film surface energy due to stretching (E S ) (12) and energy dissipation (E diss ) due to film pinning (E P ) at the bead boundary (section S2 and figs. S1 and S2) (1) Note that, in our particular experiments, the capillary number Ca was small (that is, Ca ~ 10−2), indicating that the viscous effects were less significant than the surface tension effects and were therefore neglected. In addition, it has been shown that the films will form a catenoid shape (13) when slow-moving particles impact liquid films (that is, Weber number, We < 3200; our particles impacted the membranes at <2 m/s). Therefore, the maximum change in surface energy (E S ) due to the film stretching was approximated to be the difference between the maximum area the film can stretch and the area of the flat film of outer radius R f (that is, the radius of the liquid membrane) and inner radius R b (that is, the radius of the spherical impacting particle; section S2). This change in surface energy can be approximated as (2)where . Furthermore, we estimated the energy loss due to pinning (E P ) based on our experimental parameters and found this term to be negligible in our experiments. Therefore, E* can be reduced to the following equation for our experiments (3)where is a geometry term. Conceptually, Eq. 3 is the ratio of energy converted to surface energy and the kinetic energy at impact. It is important to note that E* accounts for shape deformation of a liquid membrane that is not captured by the conventional Weber number—a dimensionless number typically representing the relative importance of the kinetic energy of an impacting liquid droplet and its surface energy (14). On the basis of 718 independent bead dropping experiments, we generally observe that particles are retained when E* > ~ 1 and pass through when E* < ~ 1 for all particle surface chemistries used here (Fig. 2B), provided that the weight F g of the bead does not exceed the capillary force the film can exert on the bead (F γ ) (that is, F g < F γ ; we note that Eq. 3 may not be applicable when aggregation of smaller particles occurs, creating an effectively large particle, which could lead to F g > F γ ). That is, there is an E* value that separates the particle retention and particle pass-through regimes and that describes the conditions under which a particle will pass through or be blocked by the membrane. This further highlights the fact that the particle separation physics of a liquid membrane are different from those of a solid membrane. Therefore, E* (that is, Eq. 3) can be used as a simple criterion to categorize the particle separation regimes of the liquid film for smooth beads with negligible dissipation effects.

Potential applications I: Insect and particle barrier From a materials design perspective, the above criterion allows us to design a membrane (that is, γ and R f ) that can retain particles smaller than a critical size given the inertial properties of the impacting particles (that is, ρ b and u b ; movie S2 and fig. S3). This capability can also be extended from simple particles to living organisms. For example, the typical speed of certain air particulates such as pollen or dust (with densities <2 g/cm3) is <1 m/s; using these values, we predict that objects of size <1 mm can be retained in membranes (35 mN/m) designed in our experiments (R f = 1.5 cm). In this example, micro-/nanoscopic particles and contaminates (for example, pollen), as well as certain slow-moving, disease-carrying insects (for example, mosquitoes and gnats) would not pass through the liquid membrane (Fig. 3A). We have further verified this prediction by dropping a number of relevant insects (that is, fruit fly, housefly, and mosquito) at their typical locomotion speeds at impact (Fig. 3B and movie S3). Note that dead insects were used in these experiments to ensure that the impact speed is near the cruising speed of their live counterparts (table S5). To further demonstrate the effectiveness of these liquid membranes in retaining live flying insects, we allowed live fruit flies (wild-type Drosophila melanogaster Canton Special) to interact with a liquid membrane. This demonstration showed that liquid membranes can effectively prevent the passage of flying fruit flies (Fig. 3C and movie S4). Fig. 3 Liquid membranes as selective microorganism and particulate barrier. (A) This plot shows the type of organisms that theoretically can pass through (or be retained by) a specified liquid membrane (with a radius of 1.5 cm and surface tension of ~35 mN/m) according to the value of E* based on the characteristic size, locomotion speed, and mass or density of the organism of interests. (B) Demonstrations of retention of fruit flies (Drosophila hydei), houseflies (Musca domestica), and mosquitoes [Culicidae (Diptera)] by the liquid membranes at impact speeds of ~0.5, ~1.1, and ~0.9 m/s, respectively (see movie S3). Here, dead insects are used in an effort to control the impact velocity of the insect. Each panel represents an overlay of multiple images from one video to show insect position over time. (C) Time series of a live fruit fly flying into a liquid membrane (see movie S4). In the left panel, the short arrows point to fruit flies that have already been trapped in the membrane, and the long arrow points to the fruit fly of interest. In the next two panels, we see the fruit fly of interest flying up into the membrane, where it is retained in the last panel. Scale bars, 1 cm.

Potential applications II: Self-cleaning, nonfouling membrane for continuous particle separation In addition to the unique size selectivity of liquid membranes, their mobile liquid interface offers unique capabilities that cannot be readily accomplished by any conventional synthetic membranes, including in-membrane object maneuverability (Fig. 4A and movie S5) and transport of retained particles by external forces (Fig. 4B and movie S6). These unique aspects of liquid membranes can be used to design separation membranes that resist fouling issues common for many solid-based membranes. Specifically, our liquid membranes can resolve the local membrane fouling issue in two separate ways. First, liquid membranes allow in-membrane object transport through external forces (for example, gravity), which allow contaminates to be transported away from the region of separation. Second, aggregates of the collected contaminates can be removed from the membrane once the weight of the aggregates exceeds the capillary force supported by the liquid membrane (Fig. 4C and movie S7). These unique mechanisms allow the liquid membranes to perform continuous separation without fouling. Fig. 4 Potential use of liquid membranes in nonfouling particle separation and surgery. (A) Dynamic reconfigurability: Unlike those in solid membranes, objects embedded in the liquid membranes can move freely in the plane of the membrane due to the mobility of the liquid molecules (see movie S5). (B) Particle transport: Particles retained in the film can also move within the plane of the liquid membrane, allowing them to be transported away if needed (see movie S6). Note also that these particular liquid membranes are transparent, allowing them to be used in applications requiring through-film visibility. (C) Self-cleaning of liquid membranes: Here, a tilted liquid membrane passively removes contaminates (that is, small sand particles) from the separation region by gravity, allowing the large particle to be collected in the left petri dish. The small particles are collected downstream, forming a growing aggregate that will later fall from the membrane into the petri dish on the right when the weight of the aggregates exceeds the capillary force exerted by the liquid membrane (“aggregate removal”; see movie S7). (D) Simulated surgery: We demonstrated that the liquid membrane can block contaminants during simulated surgical procedures without inhibiting visibility or in-film maneuverability and can passively and continuously collect and remove contaminants (see movie S8). (E) Plot showing the retention of various amounts of contaminants on a liquid membrane. Note that, in all the trials, no measurable amount of contaminant leaked through the membranes and, therefore, no data are shown for the red data bar representing “mass passed through.” The contaminant was different quantities of fluorescent powder [the same as that used in (D)] sprinkled from a drop height of ~1 cm. Scale bars, 1 cm.

Potential applications III: Surgical film In addition, the simple yet unique capabilities of the free-standing liquid membranes could lend them the ability to provide out-of-the-box solutions to various problems, such as blocking contaminants for open surgery in regions where a dust-free space for safe surgical care is limited (15), or other applications involving blockage of small objects while allowing the passage of large devices. As a proof-of-concept demonstration of such an application, we showed that the liquid membrane can block contaminants during simulated surgical procedures without inhibiting visibility or maneuverability and can collect and remove contaminants (Fig. 4D and movie S8). In our demonstration, we were able to manipulate surgical tools within the membrane and pass bovine flesh from the simulated surgical opening through the membrane. Meanwhile, particles introduced to the membranes were trapped and diverted to the membrane edge due to the mobility of the liquid interface. In addition, we have shown quantitatively that these liquid membranes can successfully prevent contaminants from passing through them (Fig. 4E).