<div class="container">
  <button class="button">
    <span>Liquid effect</span>
  </button>
</div>

// Reset
* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

// Variables
$buttonSize: '400px';

.container {
  align-items: center;
  background-color: #000;
  display: flex;
  justify-content: center;
  min-height: 100vh;
}

.button {
  background-color: #d0d;
  border: 1px solid #fff;
  box-shadow: inset 0 0 35px rgba(0, 0, 0, .5);
  color: #fff;
  height: calc(#{$buttonSize} / 4);
  overflow: hidden;
  position: relative;
  text-transform: uppercase;
  width: $buttonSize;
  
  // Text
  span {
    font-size: 16px;
    letter-spacing: .5em;
    padding: 0 2em;
    position: relative;
    font-weight: bold;
    text-shadow: 0 2px 2px rgba(0, 0, 0, .5);
    z-index: 2;
  }
  
  &:before,
  &:after {
    content: '';
    height: calc(#{$buttonSize} * 2);
    left: 50%;
    position: absolute;
    top: -85%;
    transform: translate(-50%, -85%);
    transition: top .5s;
    width: calc(#{$buttonSize} * 2);
  }
  
  &:before {
    animation: liquid 8s linear infinite;
    background-color: rgba(0, 0, 0, 1);
    border-radius: 45%;
  }
  
  &:after {
    animation: liquid 10s linear infinite;
    background-color: rgba(0, 0, 0, .5);
    border-radius: 40%;
  }
  
  &:hover {
    &:before,
    &:after { top: -120%; }
  }
}

@keyframes liquid {
  0% { transform: translate(-50%, -85%) rotate(0deg); }
  100% { transform: translate(-50%, -85%) rotate(360deg); }
}