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Search results for: “droplet”

  • Droplet on a Plucked String

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    The Mystery of the Binary Droplet

    What goes on inside an evaporating droplet made up of more than one fluid? This is a perennially fascinating question with lots of permutations. In this one, researchers observed water-poor spots forming around the edges of an evaporating drop, almost as if the two chemicals within the drop are physically separating from one another (scientifically speaking, “undergoing phase separation“). To find out if this was really the case, they put particles into the drop and observed their behavior as the drop evaporated. What they found is that this is a flow behavior, not a phase one. The high concentration of hexanediol near the edge of the drop changes the value of surface tension between the center and edge of the drop. And that change is non-monotonic, meaning that there’s a minimum in the surface tension partway along the drop’s radius. That surface tension minimum is what creates the separated regions of flow. (Video and image credit: P. Dekker et al.; research pre-print: C. Diddens et al.)

  • Evaporating Binary Droplet

  • Predicting Droplet Sizes

    Predicting Droplet Sizes

    Squeeze a bottle of cleaning spray, and the nozzle transforms a liquid jet into a spray of droplets. These droplets come in many sizes, and predicting them is difficult because the droplets’ size distribution depends on the details of how their parent liquid broke up. Shown above is a simplified experimental version of this, beginning with a jet of air striking a spherical water droplet on the far left. In less than 3 milliseconds, the droplet has flattened into a pancake shape. In another 4 milliseconds, the pancake has ballooned into a shape called a bag, made up of a thin, curved water sheet surrounded by a thicker rim. A mere 10 milliseconds after the jet and drop first meet, the liquid is now a spray of smaller droplets.

    Researchers have found that the sizes of these final droplets depend on the balance between the airflow and the drop’s surface tension; these two factors determine how the drop breaks up, whether that’s rim first, bag first, or due to a collision between the bag and rim. (Image credit: I. Jackiw et al.; via APS Physics)

  • How Water Droplets Charge Up

    How Water Droplets Charge Up

    Rubbing a balloon on your hair can build a significant electrical charge. Water droplets have the same issue when they slide across a hydrophobic, electrically-insulated surface. A new study models why these charges build up and tests the model both experimentally and through simulation. They focused their theory on three effects that determine how much charge builds up. The first is a two-way chemical reaction that continuously creates charge at the interface, with positive charge building in the drop. Secondly, the drop’s contact angle with the surface sets how many protons can build up at the contact line, thereby affecting the electrical field they generate. And, finally, fluid motion at the rear of the drop deflects protons upward, shifting the electrical field. In particular, their model predicts that the higher contact angles of hydrophobic surfaces should increase charge build-up and faster sliding velocities should slow charge build-up, both of which agree with experiments.

    The model should help researchers understand various charging scenarios, like those found on self-cleaning surfaces, in inkjet printing, and in semiconductor manufacturing. In the last scenario, rinsing semiconductor wafers in ultrapure water can build up charges in the kilovolt range, which is enough to damage the product. (Image credit: D. Carlson; research credit: A. Ratschow et al.; via APS Physics)

  • “Dew Point” Deposits Droplets

    “Dew Point” Deposits Droplets

    Artist Lily Clark loves to work in water. One of her recent sculptures, “Dew Point,” uses superhydrophobic ceramic to grow and manipulate water droplets over and over and over. Droplets coalesce in four corners until they grow large enough for gravity to pull them into a circular depression. Given their limited contact with the ceramic, the falling water droplets zip and slide on their way to a return slit in the center of the piece. You can see more of the action in the video below. Personally, I’m reminded of coins falling into a collection box! (Video credit: L. Turczan; artwork by: L. Clark; via Colossal)

  • Magnetic Droplet Crystals

  • Droplet Medusa

    Droplet Medusa

    Vibration is one method for breaking a drop into smaller droplets, a process known as atomization. Here, researchers simulate this break-up process for a drop in microgravity. Waves crisscrossing the surface create localized craters and jets, making the drop resemble the Greek mythological figure of Medusa. With enough vibrational amplitude, the jets stretch to point of breaking, releasing daughter droplets. (Image and research credit: D. Panda et al.)

  • Linking Size and Origin in Droplets

    Linking Size and Origin in Droplets

    Respiratory diseases like measles, flu, tuberculosis, and COVID-19 are all transmitted by droplets. Some are tiny and airborne, capable of traveling long distances. Other drops are larger and only capable of traveling short distances. A new review paper consolidates what we know about these droplets and categorizes them by size and origin.

    It turns out that a droplet’s size can tell us where it originated in the body. The largest type of droplets come from our mouths, lips, and tongues. Some form from filaments of saliva that stretch across our mouths and burst during exhalation. Others originate in our nasal passages where a sneeze can destabilize the mucus film there. These types of droplets are best suited to transmitting diseases that reside in the upper respiratory tract. Coughing, sneezing, singing, and speaking all produce these droplets, but breathing does not.

    In contrast, the smallest classes of droplets come from the bronchial passages of the lungs, where films form after exhalation closes a passage. When we inhale again, the passage reopens, the film breaks up, and tiny droplets flow further into the lungs before getting exhaled. Breathing alone is enough to create and spread these tiny droplets, which are well-suited to spreading diseases that reside deep in the lungs, like tuberculosis.

    In between these extremes are medium-sized droplets created from movement around our vocal cords. The formation mechanism for these droplets is least understood, but they are connected to breathing, coughing, speaking, singing, and so on.

    Ultimately, understanding the mechanics of disease transmission is about knowing how to best prevent transmission. Knowing the size of droplets responsible for transmission lets us prioritize responses that work. For example, if large droplets are the primary transmission mechanism, loose-fitting masks and face masks will stop the spread. But for smaller droplets, ventilation measures and well-fitted N-95 respirators are the better choice. (Image credit: Anton; research credit: M. Pöhlker et al.; via APS Physics)

  • Scooting Droplets

    Scooting Droplets

    As a child, I always loved watching rain on the windows as I rode in the car. Hemispherical droplets got stretched by the wind flowing over them. But they never stretched smoothly; instead they seemed to shiver and shake unevenly. A recent study looks at a similar situation: drops of glycerin forced to slide along a horizontal surface under the force of the wind. Like the drops on my parents’ car, the glycerin gets stretched out into an elongated oval. Surface waves develop atop the drop and move downstream. The drops, the authors observe, move a bit like a crawling caterpillar, pilling up and smoothing out as they move. (Image credit: rain – A. Alves, experiment – A. Chahine et al.; research credit: A. Chahine et al.; via APS Physics)

    This series of images shows an elongated droplet subjected to airflow moving from left to right. Waves form on the drop and move downstream in a fashion similar to a caterpillar crawling.
    This series of images shows an elongated droplet subjected to airflow moving from left to right. Waves form on the drop and move downstream in a fashion similar to a caterpillar crawling.

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