Latest / The Robotics Podcast with Fexingo: Autonomous Systems, Industrial Robots, and Hardware / Why Robot Hands Cant Handle Wet Objects
Transcript
- Lucas: So we've spent a lot of episodes on robot hands and the things they can't do — soft objects, small objects, irregular shapes. But there's one variable that makes all of those problems worse, and it's something we've barely touched on. Luna: Water. Lucas: Exactly. Moisture. Lubrication. Any kind of liquid film on an object's surface. It sounds trivial, but it's one of the most stubborn problems in robotic manipulation right now. Luna: I remember a researcher saying that water is basically kryptonite for most grippers. Why is that? Lucas: Well, think about how robot grippers work. The dominant approach is friction-based. You've got two fingers, or three, or a compliant pad, and you apply enough normal force to generate friction. But water reduces the coefficient of friction dramatically — sometimes by 40, 50 percent or more. Luna: So the gripper has to squeeze harder to get the same grip. But squeezing harder can damage a fragile object, right? Lucas: Exactly. That's the tension. A wet glass bottle on a production line: the gripper has to apply more force to keep it from slipping, but too much force and you crack the glass. And the problem isn't just friction. Surface tension plays a role too. A thin film of water creates capillary adhesion, which actually adds a small attractive force — but it's inconsistent and changes rapidly as the liquid evaporates or gets squeezed out. Luna: So it's not just less friction — it's a moving target. The grip conditions change second by second. Lucas: Right. And that's a nightmare for control systems that assume stable contact parameters. There's a great paper out of Carnegie Mellon from last year that measured grip stability on wet, dry, and oily surfaces. On dry surfaces, the failure rate for a standard parallel-jaw gripper was under 2 percent. On wet surfaces, it jumped to 23 percent. And on oily surfaces — think machine tool lubricant — it was over 40 percent. Luna: That's huge. And it's not just manufacturing. Food processing must be even worse. Lucas: Absolutely. Food is the extreme case. You've got wet chicken, wet fish, wet lettuce — all with varying surface textures, deformability, and biofilms. A lot of food processing plants still use human hands precisely because robots can't handle the combination of wetness and variability. Luna: I've seen videos of those salmon filleting robots — they're fast but they still lose a lot of yield because the fish slips. And that's expensive. Lucas: That's the example I was going to bring up. Marel, the Icelandic food processing equipment company, has been working on this for years. Their latest generation uses a combination of needle grippers — tiny needles that penetrate the skin slightly — and vacuum suction, but even then, wet scales reduce suction seal effectiveness. They get maybe 85 percent success on wet fish versus 97 percent on dry. Luna: So what are the approaches that are actually working? What's the state of the art? Lucas: There are a few directions. One is biomimetic — looking at how animals handle wet objects. Tree frogs, for example, have hexagonal toe pads that drain water through channels, maintaining contact. Researchers at Bristol have made synthetic versions using microstructured elastomers that wick moisture away from the contact interface. Luna: So it's not just about adding more force — it's about designing the surface itself to manage the liquid. Lucas: Exactly. Another approach is tactile sensing combined with fast feedback control. If the gripper can detect micro-slip in real time — before the object actually drops — it can adjust force continuously. Companies like OnRobot have sensors that measure vibration and shear forces. But the challenge is that water can interfere with the sensor readings themselves, especially capacitive sensors. Luna: Water is conductive, so it messes with capacitance. That's a real problem. Lucas: Right. So some teams are moving to optical tactile sensors — like the GelSight sensors from MIT — which use a camera and a reflective gel. They're not affected by moisture, but they're bulkier and more expensive. Luna: What about soft robotics? I remember Soft Robotics Inc. makes these grippers that kind of wrap around objects. Do those work better wet? Lucas: They do, actually. Soft grippers — the ones that use pneumatic actuation to curl around an object — have a natural advantage because they conform to the shape and create a larger contact area. So even with reduced friction, the total grip force is spread out. But they still struggle with very slippery or wet objects because the gripper material itself is typically silicone, which can be slippery against wet surfaces. Luna: So they need a different material. Maybe something with microspikes or directional friction. Lucas: Yes. That's exactly what some research groups are doing. There's a team at Harvard that developed a gripper with microscopic angled spines — like a gecko's foot but for wet surfaces. The spines catch on microscopic irregularities on the object surface, even when there's a liquid film. They've demonstrated picking up wet glass beads and even slippery fish scales. Luna: That's incredible. But is it production-ready? Can you run that for millions of cycles in a food plant? Lucas: Not yet. The spines wear down after a few thousand cycles. So there's a durability problem. But it shows the direction we're heading: away from simple friction-based models and toward mechanical interlocking and active fluid management. Luna: It's almost like we're rediscovering that water is a really complex material to interact with. Which is obvious if you think about it — but robotics has been avoiding it for decades. Lucas: Right. And the market pressure is growing. The global food processing robotics market is projected to grow at about 12 percent compound annual growth rate through 2030. A lot of that growth depends on solving the wet handling problem. Same with pharmaceuticals — wet tablet handling, wet vial gripping. Luna: So it's not just a niche research problem. It's a bottleneck for entire industries. Lucas: Exactly. And that's why you're seeing more venture funding going into gripper startups that focus on novel materials and sensing. Companies like Grabit and RightHand Robotics are investing heavily in this. Luna: It's one of those problems where the solution probably won't be one magic material — it'll be a combination of surface design, sensing, and control working together. Lucas: I think that's right. And speaking of combinations — there's a really interesting approach coming out of the University of Tokyo where they're using ultrasonic vibrations to create a thin air gap between the gripper and the wet surface. The vibrations expel the water film and then the gripper can make direct contact. Luna: So it's like a mini sonic shower that clears the water away just before gripping. That's clever. Lucas: It works in the lab. They got a 90 percent success rate on wet polypropylene — which is notoriously slippery. But scaling the ultrasonic transducer and making it fast enough for high-speed pick and place is challenging. Luna: Seems like every promising solution has a trade-off. But that's what makes this field interesting. Lucas: It really is. And for anyone listening who's fascinated by these kinds of engineering challenges — the kind where the physical world doesn't cooperate with our clean models — we hope this show gives you something to think about. And if you'd like to help us keep making episodes like this, ad-free and focused on the details, listeners like you are what make it possible. You can support the show at buy me a coffee dot com slash fexingo. Luna: Yeah, it's a small way to keep these deep dives coming. And we really appreciate it. Lucas: So back to the wet handling problem — I think the next big breakthrough will come when someone combines a microstructured surface with real-time slip detection and a control algorithm that predicts the moisture dynamics. That's the holy grail. Luna: And when that happens, we'll do a whole episode on it. Until then, it's a reminder that even the most mundane things — like water — can be the hardest to engineer. Lucas: Absolutely. Thanks for listening.