(A) Schematics showing the materials architectural differences between solid membranes, liquid-infused porous materials [for example, slippery liquid-infused porous surfaces (SLIPS) ()], 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.

(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 ). Scale bar, 1 cm.

(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.

(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.

(A) A liquid film can serve as a gas diffusion barrier while allowing macroscopic objects to pass through (for example, odor management; see movie S9). (B) Schematic of the experimental setup in assessing the ability of a liquid membrane (barrier) to prevent the passage of fog. (C) Quantitative measurements showing that a liquid membrane can be as effective as a solid glass barrier in blocking the fog passage. (D) Gas sequestration experimental setup. Here, a hexane-filled test tube was covered with either parafilm or a liquid membrane. (E) The measured sensor output (signal voltage V_s normalized by the maximum voltage in ambient conditions, V_s,max) over time when exposed to ambient (black circles) and hexane vapor (red circles) and when covered by either a parafilm sheet (green circles) or a liquid membrane (blue circles). The liquid membrane consists of a 7:3 ratio of deionized water to glycerol by volume and 8.5 mM SDS.

(A) Time sequence of images showing the mechanical perturbation of a liquid membrane by a smooth glass rod for >3 hours. Probing took place at a period of 3.55 s over 3042 cycles (see movie S10), and liquid was replenished at a rate of ~1 to 2 ml/min. We note that the glass rod is hydrophilic, so some liquid from the membrane wets its surface as it passes into the membrane repeatedly. Scale bar, 1 cm. (B) Plot showing the mechanical perturbation cycle of the liquid membrane. The experiment was set up to acquire images at various points in the cycles (shown in red open circles). Images were acquired at a rate of 1 frame/s. Note that A is the vertical location of the probe tip, and A₀ is the amplitude of the probing cycle.