## 2 EvoBot  
In order to reach a reconfigurable and extendible liquid handling robot, we have employed a modular design building on research in the field of modular robotics [13]. A modular approach allows us to encapsulate complexity while providing a simple plug and play interface for users. Furthermore, this allows non-expert users to configure the robot for a wide range of experiments. To further extend  
Figure 1: The EvoBot liquid handling robot. (**a**) The robot is prepared to perform experiments with Petri dishes and reagents on the experimental layer and four modules in the head; (**b**) A close-up of the movable head equipped with two different syringe modules (5 m and 1 mL sizes). the versatility of the robot, it has two different levels of modularity: layers and modules (see Figure 2). Thus, the robot consists of one structural frame with horizontal sockets, where different kinds of layers are attached by using a cam lever mechanism. These layers can be moved up and down in a few seconds. In its basic configuration, the robot has three layers: an actuation, an experimental, and an observation layer. This default configuration can be easily changed if required by the experiment. The actuation layer has a head, which holds modules and can be moved in the horizontal plane by using two stepper motors. The head has 17 sockets where modules can be plugged in, but only 11 sockets can be populated at the same time as all sockets overlap the adjacent ones. Most of the modules that are plugged into the head are designed to perform an action on the experiments, but it is also possible to add sensor modules including cameras or scanners. The experimental layer supports the objects of the experiment such as Petri dishes, microscope slides, or tubes. It consists of a transparent part laser cut from an acrylic sheet, which is held in place by aluminium profiles organised in a rectangle. The acrylic part can be customised according to experiments. In fact, there usually are holes to easily dispose liquid waste into a container below the experimental surface. The observation layer is optional, and it can be the same as the actuation layer. This layer is below the transparent experimental layer, and any modules or devices placed here cannot get in physical contact with the contents of the experiment vessels. Therefore, most modules plugged into this layer are used to sense or observe the ongoing experiments. In addition, for some experiments the sensor does not need to move, consequently the observation layer does not require actuation. As an example, we often use a statically mounted webcam instead of a fully actuated observation layer. However, the robot was designed in order to be able to use an active head in this layer. One possible application for an active observation layer is to move a microscope. Specifically, a user was interested in studying spiking droplets in a similar experiment as the one reported in Section 5.5. In this case, they wanted to track moving oil droplets in a Petri dish. As the microscope has a very short field of view, the head  
Figure 2: Overview of modularity in EvoBot. Users can decide the layers that they need for their experiments and attach modules with specific functionality to the layers. of the observation layer could move the microscope module to observe the oil water interface of the droplet, even if the droplet was moving. The second level of modularity is provided by modules that can be plugged into the layers. These modules add new functionality to the system. Currently, we have implemented three different kinds of modules: syringe modules, a pump-based dispensing module, and a heavy payload module. Their designs are shown in Figure 3.  
The syringe module moves liquids with precision and is usually the most used module in experiments. The module can move the syringe up and down in addition to the movement of the plunger to dispense or aspirate liquids.